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The XSB System Version 2.1 Volume 1: Programmer`s Manual

1. Redo Fail 8 debug_ctl leash PortList Allows the user to specify at which ports the tracer should stop and prompt the user for direction PortList must be a list of port names i e a sublist of Call Exit Redo Fail Only ports that are show n can be leash ed 9 debug_ctl hide PredArityPairList The list must be of the form P1 A1 P2 A2 ie each either must specify a predicate arity pair Each predicate on the list will become non traceable That is during the trace each such predicate will be treated as an black box procedure and trace will not go into it 10 debug_ctl unhide PredArityPairList If the list is a predicate arity list every predicate on that list will become traceable again Items in the list can contain variables For instance debug_ctl unhide _ 2 will make all 2 ary that were previously made untraceable traceable again As a special case if PredArityPairList is a variable all predicates previously placed on the untraceable list will be taken off 11 debug_ctl hidden List This returns the list of predicates that the user said should not be traced 7 2 Low Level Tracing XSB also provides a facility for low level tracing of execution This can be activated by invoking the emulator with the T option see Section 3 5 or through the predicate trace O It causes trace information to be printed out at every call including those to system trap handlers The v
2. Y 1 Cs 2927104 Ret ret 3 no get_calls p 1 3 Cs Ret Cs 2927152 Ret ret 1 Cs 2927104 Ret ret 3 no get_call Skeleton Subgoal_Structure_Pointer Return Skeleton Tabling This predicate binds Subgoal_Structure Pointer and Return_Skeleton only if Skeleton is a variant i e identical up to variable renaming of some entry in the subgoal table It may be compared to the previous predicate get_cal1 3 which unifies Skeleton with entries in the subgoal trie Repeating the example from get_calls 3 calls to get_ca11 3 will act as follows get_call p X Y Cs Ret no get_call p 1 Y Cs Ret Y _864620 CHAPTER 6 STANDARD PREDICATES 114 Cs 2927104 Ret ret _864620 no get_call p Y 3 Cs Ret Y _864792 Cs 2927152 Ret ret _864792 no get_call p 1 3 Cs Ret no get_calls_for_table Skeleton Call Tabling Succeeds whenever Skeleton unfies with an entry in the subgoal table When this predicate suceeds it returns a variant of the call entry with which Skeleton unifies This predicate does not provide any information about whether a table is complete Use table_state to inquire about a table s state Calls to get_calls_for_table 2 for the example in get_calls 3 would act as follows get_calls_for_table p X Y Call X _646608 Y _646436 Call p 1 _646724 X _646608 Y _646436 Call p _646720 3 no get_calls_for_table p
3. a X b X a 1 user compiled cpu time used 0 3 seconds user loaded yes assert a 3 yes listing a A compiled a 3 yes Predicate listing 0O always succeeds The query listing is just a notational shorthand for the query listing X listing Predicate_Indicator If Predicate_Indicator is a variable then listing 1 is equivalent to listing 0 If Predicate_Indicator is an atom then listing 1 lists the dynamic clauses for all predicates of that name found in module usermod of the database The argument Predicate_Indicator can also be a predicate indicator of the form Name Arity in which case only the clauses for the specified predicate are listed Finally it is possible for Predicate_Indicator to be a list of predicate indicators and or atoms e g listing foo 2 bar blah 4 If Predicate_Indicator is not a variable an atom or a predicate indicator or list of predicate indicators of the form Name Arity predicate listing 1 will simply fail In future releases of XSB we intend to allow the user to specify a predicate indicator of the form Module Name Arity as argument of listing 1 xsb_configuration Feature Name Value Succeeds iff the current value of the XSB feature Feature_Name is Value This predicate provides information on a wide variety of features related to how XSB was built including the compiler used the compiler and loader flags the machine and OS o
4. fmt_write Format args A1 A2 Similar to formatted write in C The semantics of the arguments is the same as for fmt_read 4 fmt_write_string String Format args A1 A2 Like fmt_write 2 but the output string is placed in String 6 3 Convenience These predicates are standard and often self explanatory so they are described only briefly true Always succeeds otherwise Same as true 0 fail Always fails CHAPTER 6 STANDARD PREDICATES 69 X Y Defined as if by the clause Z Z i e X and Y are unified Xx Y Y Succeeds if X and Y are not unifiable fails if X and Y are unifiable It is thus equivalent to X Y 6 4 Negation and Control 212 0 Cut discard all choice points made since the parent goal started execution Cuts across tabled predicates are not valid The compiler checks for such cuts although whether the scope of a cut includes a tabled predicate is undecidable in the presence of meta predicates like ca11 1 Further discussion of conditions allowing cuts and of their actions can be found in Section 5 1 fail_if P If the goal P has a solution fails otherwise it succeeds Equivalently it is true iff call P see section 6 8 is false Argument P must be ground for sound negation as failure although no runtime checks are made by the system The standard predicate fail_if 1 is compiled by the XSB compiler Exceptions instantiation_error P is not instantiated type_err
5. ti F A ti all and ti_off F A ti_off all are specified ti_off F A ti off_all takes precedence Note that unification factoring may have no effect when a predicate is well indexed to begin with For example unification factoring has no effect on the following program p a c X q X p b c X r X even though the two clauses have c 0 in common The user may examine the results of the transformation by using the ti_dump compiler option see Section 3 8 2 Unification factoring was once called transformational indexing hence the abbreviation ti in the compiler direc tives CHAPTER 3 SYSTEM DESCRIPTION 35 Other Directives XSB has other directives not found in other Prolog systems hilog atom atoMn Declares symbols atom through atom as HiLog symbols The hilog declaration should appear before any use of the symbols See Chapter 4 for a purpose of this declaration 1ldoption Options This directive is only recognized in the header file H file of a foreign module See the chapter Foreign Language Interface in Volume 2 for its explanation compiler_options OptionsList Indicates that the compiler options in the list Options List should be used to compile this file This must appear at the beginning of the file Thes options will override any others including those given in the compilation command The options may be optionally prefixed with or to indicate that they sho
6. C 2 In cmplib asm asm_inst builtin compile flatten inprog pre_cond preproc singlton spec timod tp_comp tp_index tp_var varproc writeasm consult dbcmpl file_op standard x_interp asm_opt config listutil printres suppltab tp_cond tpinline curr_sym dcg _qsc hilogsym std_sbp xsb_read asmpass2 cp_opt parse gensym symtab tp_eval tprog 159 dcg debugger loader string xsb_tok auxlry cutcheck peephole sanity tabdef tp goal useinfer Bibliography 1 J Alferes C Damasio and L Pereira SLX a top down derivation procedure for programs 10 11 12 with explicit negation In M Bruynooghe editor International Logic Programming Symp pages 424 439 1994 J Alferes C Damasio and L Pereira A logic programming system for non monotonic rea soning Journal of Automated Reasoning 1995 F Banchilhon D Maier Y Sagiv and J Ullman Magic sets and other strange ways to implement logic programs In PODS ACM 1986 C Beeri and R Ramakrishnan On the power of magic J Logic Programming 10 3 255 299 1991 D Boulanger Fine grained goal directed declarative analysis of logic programs Proceedings of the International Workshop on Verification Model Checking and Abstract Interpretation 1997 Available through http www dsi unive it bossi VMCAI html W Chen M Kifer and D Warren HiLog A foundation for higher order logic programming J Logic Programming
7. ically provides full compilation of HiLog predicates As a result most uses of HiLog execute at essentially the speed of compiled Prolog For more information about the compilation scheme for HiLog employed in XSB see 39 HiLog can also be used with tabling so that the program above can also be written as table closure _ _ _ closure R X Y R X Y closure R X Y closure R X Z R Z Y A further goal of XSB is to provide in implementation engine for both logic programming and for data oriented applications such as in memory deductive database queries and data mining 36 One prerequisite for this functionality is the ability to load a large amount of data very quickly We have taken care to code in C a compiler for asserted clauses The result is that the speed of asserting and retracting code is faster in XSB than in any other Prolog system of which we are aware At the same time because asserted code is compiled into SLG WAM code the speed of executing asserted code in XSB is faster than that of many other Prologs as well We note however that XSB does not follow the semantics of assert specified in 27 Data oriented applications may also require indices other than Prolog s first argument indexing XSB offers a variety of indexing techniques for asserted code Clauses can be indexed on a groups of arguments or on alternative arguments For instance the executable directive index p 4 3 2 1 specifies indexes on t
8. if necessary and the source file is not too big compile the module using predicate compile 2 with the options specified Loading load the object code of the module into memory Importing import all the exported predicates of that module to the current working module usermod Query Execution execute any queries that the module may contain There are two steps to name resolution determination of the proper directory prefix and determination of the proper extension When FileName is absolute i e in UNIX contains a slash determination of the proper directory prefix is straightforward However the user may also enter a name without any directory prefix In this case the directory prefix is a directory in the dynamic loader path see section 3 4 where the source file exists Once the directory prefix is determined the file name is checked for an extension If there is no extension the loader first checks for a file in the directory with the P extension or c for foreign modules before searching for a file without the extension Note that since directories in the dynamic loader path are searched in a predetermined order see section 3 4 if the same file name appears in more than one of these directories the compiler will consult the first one it encounters Compilation is performed if the update date of the the source file P is later than that of the the object file 0 and if the source file is not larger than the defau
9. maintains more detailed statistical information T print a trace of each called predicate v version print the version and configuration information about XSB h help print this help message nobanner don t show the XSB banner on startup quietload don t show the module loaded messages noprompt don t show prompt for non interactive use The order in which these options appear makes no difference i Brings up the XSB interpreter This is the normal use and because of this use of this option is optional and is only kept for backwards compatibility 1 Forces the interpreter to print unbound variables as letters as opposed to the default setting which prints variables as memory locations prefixed with an underscore For example starting XSB s interpreter with this option will print the following 7 Y X Z 3 W foo X Z Y A X A Z 3 W foo A 3 as opposed to something like the following 7 Y X Z 3 W foo X Z _10073976 _10073976 3 foo _10073976 3 N gt lt li n used in conjunction with the i option to indicate that the usual read eval print top loop is not to be entered but instead will interface to a calling C program See the chapter Calling XSB from C in Volume 2 for details d Produces a disassembled dump of byte_code_file to stdout and exits c size Allocates initial size Kbytes of space to the trail choice point stack area The trail stack gr
10. 123 john 500 X kostis sofia X Y Z YCW f a b c d map double h map P A B C Like a functor in Prolog a functor in HiLog can be characterized by its name and its arity which is the number of arguments this functor is applied to For example the compound term whose principal functor is map P of arity 2 and which has arguments L1 and L2 is written as map P Li L2 CHAPTER 4 SYNTAX 39 As in Prolog when we need to refer explicitly to a functor we will normally denote it by the form Name Arity Thus in the previous example the functor map P of arity 2 is denoted by map P 2 Note that a functor of arity 0 is represented as an atom In Prolog a compound term of the form p ty ta t is usually pictured as a tree in which every node contains the name p of the functor of the term and has exactly k children each one of which is the root of the tree of terms t1 to tg For example the compound term s np kostis vp v loves np sofia would be pictured as the following tree l v np kostis loves sofia The principal functor of this term is s 2 Its two arguments are also compound terms In illustra tion the principal functor of the second argument is vp 2 Likewise any external representation of a HiLog compound term t t1 t2 tk can be pictured as a tree in which every node contains the tree representation of the name t of the functor of the term and has exactly k chi
11. 25 specialisation 28 configuration 5 control 69 cut 93 124 125 128 debugger 119 ports 119 definite clause grammars 124 directives 166 Compiler 30 indexing 33 modes 30 tabling 31 emulator command line options 18 errors undefined predicate 15 garbage collection 7 22 GPP 24 gpp 26 grammars definite clause 124 high level tracing 119 indexing directives 33 transformational 34 inlines Compiler 35 invoking the Compiler 24 load search path 16 Local Scheduling 62 low level tracing 122 memory management 7 22 mode analysis compiler options 27 modes directives 30 negation stable models 59 stratified 52 unstratified 54 notational conventions 4 options command line arguments 18 Compiler 25 predicate indicator 94 program state of 93 INDEX scheduling strategy 7 set_dcg style 1 129 sets bags 84 specialisation Compiler 28 compiler options 27 stable models 58 stacks default sizes 18 expanding 18 state of the program 93 tabled aggregation 60 tabling aggregate predicates 86 answer completion 57 compiler options 27 complete evaluation 53 conditional answers 54 cuts 49 directives 31 dynamic predicates 51 table inspection predicates 112 term indicator 94 terms comparison of 90 trace options 119 tracing high level 119 low level 122 unification factoring compiler options 27 well founded semantics 57
12. Like Prolog the form X is a special notation for lists see Section 4 1 6 while the form X is just syntactic sugar for the term X Examples of HiLog atoms are h foo I am also a HiLog atom 4 1 4 Variables Variables may be written as any sequence of alphanumeric characters including _ beginning with either a capital letter or _ For example X Hilog Vari _3 _List If a variable is referred to only once in a clause it does not need to be named and may be written as an anonymous variable represented by a single underscore character _ Any number of anonymous variables may appear in a HiLog clause all of these variables are read as distinct variables Anonymous variables are not special at runtime 4 1 5 Compound Terms Like in Prolog the structured data objects of HiLog are compound terms or structures The external representation of a HiLog compound term comprises a functor called the principal functor or the name of the compound term and a sequence of one or more terms called arguments Unlike Prolog where the functor of a term must be an atom in HiLog the functor of a compound term can be any valid HiLog term This includes numbers atoms variables or even compound terms Thus since in HiLog a compound term is just a term followed by any finite number of arguments all the following are valid external representations of HiLog compound terms foo bar prolog a X hilog X
13. Thus nothing is seen until control comes back to that procedure either at the Success or the Failure port q Quasi skip This is like Skip except that it does not mask out spy points S Verbose skip Similar to Skip mode but trace continues to be printed The user is prompted again when the current call terminates with success or failure This can be used to obtain a full trace to the point where an error occurred or for code profiling See more about profiling below e Exit Causes immediate exit from XSB back to the operating system Other standard predicates that are useful in debugging are spy Preds where Preds is a spy specification or a list of such specifications and must be instantiated This predicate sets spy points conditional or unconditional on predicates A spy specification can be of several forms Most simply it is a term of the form P N where P is a predicate name and N its arity Optionally only a predicate name can be provided in which case it refers to all predicates of any arity currently defined in usermod It may optionally may be prefixed by a module name e g ModName P N Again if the arity is omitted the specification refers to all predicates of any arity with the given name currently defined in the given module A spy specification may also indicate a conditional spy point A conditional spy specification is a Prolog rule the head indicating the predicate to spy and the body CHAPTER 7 DEBUGGING
14. after being initially delayed the answer can become unconditional If simpl s turns out to be true the answer should be removed it is false In fact it is this last case that occurs in Exercise 5 3 5 The answer simp1_p X tnot simpl_s is derived and returned to the user XSB does not currently print out the delay list The answr is then removed through simplification so that when the query is re executed the answer does not appear We will examine in detail how to alter the XSB interface so that evaluation of the well founded semantics need not be confusing It is worthwhile to note that the behavior just described is uncommon Version 2 1 of XSB handles dynamically stratified programs through delaying negative literals when it becomes necessary to look to their right in a clause and then simplifying away the delayed literals when and if their truth value becomes known However to ensure efficiency literals are never delayed unless the engine determines them to not to be stratified under the LRD stratified evaluation method When Conditional Answers are Needed A good Prolog programmer uses the order of literals in the body of a clause to make her program more efficient However as seen in the previous section delaying can break the order that literals are evaluated within the body of a clause It then becomes natural to ask if any guarantees can be made that XSB is not delaying literals unnecessarily Such a guarantee can in
15. and finally the quote character As explained below APPENDIX A GPP GENERIC PREPROCESSOR 139 these sequences should be supplied using the syntax of C strings they must start with a non alphanumeric character and in the first five strings special matching sequences can be used see below If the argument corresponding to the quote character is the empty string that functionality is disabled For meta macro specifications there are only 7 parameters as the argument reference sequence and quote character are shared with the user macro syntax The structure of a comment string is the following it must start with a sequence of characters matching the given comment string start sequence and always ends at the first occurrence of the comment string end sequence unless it is preceded by an odd number of occurrences of the string quote character if such a character has been specified In certain cases comment strings can be specified to enable macro evaluation inside the comment string in that case if a quote character has been defined for macros it can be used as well to prevent the comment string from ending with the difference that the macro quote character is always removed from output whereas the string quote character is always output Also note that under certain circumstances a comment string specification can be disabled in which case the comment string start sequence is simply ignored Finally it is possible to specify a strin
16. otherwise it fails As a notational convenience the user can supply SystemCall in the form of a list something currently not possible for she11 2 For example the call shell echo HOME CHAPTER 6 STANDARD PREDICATES 111 will output in the current output stream of XSB the name of the user s home directory while the call File test c shell cc c File will call the C compiler to compile the file test c Note that in UNIX systems since she11 1 is executed by forking off a shell process it cannot be used for example to change the working directory of the interpreter For that reason the standard predicate cd 1 described below should be used shell SystemCall Result Calls the operating system with the atom SystemCall as argument and returns the result of the call in Result In comparison with she11 1 this predicate always succeeds even if the SystemCall cannot be successfully executed ls Under UNIX this command lists in the current output stream the files in the system s current directory if it can do so If so it succeeds It is the same as shel1 1s F 0 cd Dir Under UNIX and Windows this predicate changes the interpreter s working directory to Dir If the directory specified does not exist or is not a directory or the user does not have execute permission for that directory predicate cd 1 simply fails raising no permission error Exceptions instantiation_error Dir is not
17. p when the second p in this query is called all simplification on p will have been performed However this query will succeed if p is true or undefined Exercise 5 3 6 Write a predicate wfs_call Tpred Val such that if Tpred is a ground call to a tabled predicate wfs_ca11 Tpred Val calls Tpred and unifies Val with the truth value of Tpred under the well founded semantics How would you modify w s_ca11 Tpred Val so that it properly handled cases in which Tpred is non ground Trouble in Paradise Answer Completion The engine for XSB performs both Prolog style and answer resolution along with delay and simplification What it does not do is to perform an operation called answer completion which is needed in certain pathological programs Exercise 5 3 7 Consider the following program table p 1 r 0 s 0 ac_p X ac_p X ac_p X tnot ac_s ac_s tnot ac_r ac_s ac_p X ac_r tnot ac_s ac_r Using either the predicate from Exercise 5 8 6 or some other method determine the truth value of ac_p X What should the value be hint what is the value of ac_s 1 CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 58 For certain programs XSB will delay a literal such as ac_p X that it will not be able to later simplify away In such a case an operation called answer completion is needed to remove the clause p X p X Without answer completion XSB may consider some answers to be undefine
18. representation is h a h b is a notational shorthand for the term apply h a h b Notice that even though the two occurrences of h refer to the same symbol only the one where h appears in a functor position is encoded with the special functor apply n n gt 1 The encoding of HiLog terms is performed based upon the existing declarations of hilog symbols These declarations see section 3 8 4 determine whether an atom that appears in a functor position of an external representation of a HiLog term denotes a functor or the first argument of a set of special functors apply The actual encoding is as follows e The encoding of any variable or parameter symbol atom or number that does not appear in a functor position is the variable or the symbol itself CHAPTER 4 SYNTAX 42 e The encoding of any compound term t where the functor fis an atom that is not one of the hilog symbols as a result of a previous hilog declaration is the compound term that has fas functor and has as arguments the encoding of the arguments of term t Note that the arity of the compound term that results from the encoding of t is the same as that of t e The encoding of any compound term t where the functor fis either not an atom or is an atom that is a hilog symbol is a compound term that has apply as functor has first argument the encoding of fand the rest of its arguments are obtained by encoding of the arguments of termt Note that in this case the ari
19. 1 use table_once 1 instead Exceptions The same as call1 1 table_once X table_once 1 is a weaker form of once 1 suitable for situations in which a single solution is desired for a subcomputation that may involve a call to a tabled predicate table_once Pred succeeds only once even if there are many solutions to the subgoal Pred However it does not cut over the subcomputation started by the subgoal Pred thereby ensuring the correct evaluation of tabled subgoals 6 9 Information about the State of the Program In XSB various aspects of the program state information about predicates modules clauses and their object files can all be inspected in ways similar to many Prolog systems However because the CHAPTER 6 STANDARD PREDICATES 94 atom based module system of XSB may associate structures with particular modules predicates are provided to inspect these elements as well The following descriptions of state predicates use the terms predicate indicator term indicator and current module to mean the following e By predicate indicator we mean a compound term of the form M F A or simply F A When the predicate indicator is fully instantiated M and F are atoms representing the module name and the functor of the predicate respectively and A is a non negative integer representing its arity Example usermod append 3 e By term indicator we mean a predicate or function symbol of arity N followed by a sequence of N vari
20. 1 2 Call Call p 1 _646676 Call p _646672 3 no The second example backtracks through all entries in the table since only skeletal information is used from the first argument Exception instantiation_error First argument is not instantiated table_error First argument is not a tabled predicate get_returns Subgoal_Structure Pointer Return_Skeleton Tabling Backtracks through the answer trie for the subgoal whose subgoal structure is pointed CHAPTER 6 STANDARD PREDICATES 115 to by Subgoal_Structure Pointer and instantiates the Return_Skeleton with the bindings corresponding to the return One way of accessing subgoals and answers of the example in in get_calls 3 is as follows get_calls p Y 3 Cs Ret get_returns Cs Ret Y 2 Cs 2561656 Ret ret 2 Y 1 Cs 2561656 Ret ret 1 Y 1 Cs 2559032 Ret ret 3 Y 1 Cs 2559032 Ret ret 3 no get_returns Subgoal_Structure_Pointer Return_Skeleton LeafNode Tabling Same as above except for the fact that the third argument is bound to the leaf node corresponding to the return in the return trie get_returns for_call Call Return Tabling Succeeds whenever there is a table entry for a variant of Call If this is the case Return is instantiated with an entry for the table corresponding to call and successive entries are returned by backtracking If there are no answers in the table or no table entry for Call the goal
21. 10 10 11 CONTENTS ii 3 8 2 CompilerOptions ite god di a A A aei a a BE 25 38 30 Specialisation i a ias e et Wan he A a wee aa ee ad 28 38A Compiler Directives oa a EP aw ee a hoy ees 30 3 00 Inline Predicates ar cee cos to a Behe eet te en wl Boas oe Es Be ae 35 4 Syntax 36 A E A A A att 36 AAT A A A A iat Ee ae 36 4 1 2 Floating point Numbers 0 020 002 0202 00 37 ALSO LAOS stu A hee atk GO had wos ak A A A 37 ATA Variables rita Ge oe Waa wn ae Base Gok Ree ROE Ca dee aoa ey 38 AAS Compound Terms ri TT we we Acad ae kee Pet ee ee a SY 38 ALTO Listas a teed oe ex Ge oki Saad ee E Caos dees Boa SY 39 4 2 From HiLog to Prolog 02 020000000000 00 41 43d Operators it A A Moses tant wa as PEs 42 5 Using Tabling in XSB A Tutorial Introduction 46 5 1 XSB asa Prolog SysteM ee 46 5 2 Tabling in Definite Programs ee 47 5 3 Stratified Normal Programs 2 0 0 2 ee 52 5 3 1 Non stratified Programs soaa 54 5 3 2 On Beyond Zebra Implementing Other Semantics for Non stratified Programs 58 5 4 Tabled Aggregation ee 60 SAL Local Evaluation i ikea s Gilkey Be eee hee es Gy Soe SoS 62 6 Standard Predicates 63 6 1 Inputvand Output ess ci tie Bp wee Gee A E BGO WAS E ai 63 6 0 1 File Handling ss sc ia poe ah wee a ee A a eae ho eR ee 63 612 Character IO 2 fared Se AP oe Bee eee A and 65 6 1 3 Term T O a E oc e Bk Be AA Poa SE ted 66 627 S
22. 121 indicating conditions under which to spy For example to spy the predicate p 2 when the first argument is not a variable one would write spy p X nonvar X Notice that the parentheses around the rule are necessary The body may be empty i e the rule may just be a fact The head of a rule may also be prefixed using with a module name One should not put both conditional and unconditional spy points on the same predicate nospy Preds where Preds is a spy specification or a list of such specifications and must be instantiated at the time of call What constitutes a spy specification is described above under spy nospy removes spy points on the specified predicates If a specification is given in the form of a fact all conditional spy points whose heads match that fact are removed debug Turns on debugging mode This causes subsequent execution of predicates with trace or spy points to be traced and is a no op if there are no such predicates The predicates trace 1 and spy 1 cause debugging mode to be turned on automatically nodebug Turns off debugging mode This causes trace and spy points to be ignored debugging Displays information about whether debug mode is on or not and lists predicates that have trace points or spy points set on them debug_ct1 option value debug ct1 2 performs debugger control functions as described below These commands can be entered before starting a trace or inside the trace The
23. 152 gt urf is urf APPLY test foo gt foo is not urf A 9 Author Denis Auroux e mail aurouxQmath polytechnique fr Please send me e mail for any comments questions or suggestions Many thanks to Michael Kifer for valuable feedback and for prompting me to go beyond version 1 0 Appendix B Standard Predicates and Functions B 1 List of Standard Predicates abolish Name Arity abolish Name Arity abolish_all_tables abolish table_call Term abolish_table_pred Pred abort abort Message analyze_table Pred arg Index Term Arg arg0 Index Term Arg assert Clause asserta Clause assertz Clause atom Term atomic Term atom_codes Atom CharList bagof Elem Goal Bag break C Listi Token List2 call Term callable Term cd Dir clause Head Body close FileName compare Res Termi Term2 compile Module compile Module Options compound Term 153 APPENDIX B STANDARD PREDICATES AND FUNCTIONS 154 consult Module consult Module Options copy term Term Copy cputime Time current_atom Atom current_functor Functor current _functor Functor Term current_input File current_module Module current_module Module File current_op Precedence Type Name current_output File current_predicate Predicate current_predicate Predicate Term debug debugging edit File erase Reference expand_term Term Expanded_Term fail fail_if Goal f
24. 2 Y 3 Z tnot p 1 3 CHAPTER 6 STANDARD PREDICATES 117 no delete_return Subgoal_ Structure Pointer tLeafNode Tabling Deletes a return from the return trie The LeafNode parameter should be obtained from a call to get_returns 3 abolish_all_tables Tabling abolish_all_tables 0 abolishes all tables presently in the system Predicates which have been declared tabled remain tabled but information about calls and returns is deleted In Version 2 1 abolish_all_tables1 0 reclaims used space The predicate always succeeds Note that incomplete tables are abolished automatically by the system on exceptions and when the interpreter level is resumed In these cases the user does not need to abolish tables to maintain correctness abolish_table_pred PredSpecification or Skeleton Tabling Predicate abolish_table_pred 1 abolishes tables for all calls to the predicate denoted by PredSpecification as in path 2 or by Skeleton as in path X Y The predicate remains tabled but information about its calls and their returns is removed from the system In Version 2 1 abolish table_pred 1 reclaims the space used by the calls to this predicate and their answers Note that incomplete tables are abolished automatically by the system on exceptions and when the interpreter level is resumed In these cases the user does not need to abolish tables to maintain correctness Exception instantiation_error Argument is a variable type_error
25. Also note that different file names that is names which do not unify represent different input streams even if these different file names correspond to the same file Exceptions permission error File F is directory or file is not readable 63 CHAPTER 6 STANDARD PREDICATES 64 instantiation error F is not instantiated at the time of call existence_error File F does not exist seeing F F is unified with the name of the current input stream This is exactly the same with predicate current_input 1 described in section 6 9 and it is only provided for upwards compatibility reasons seen Closes the current input stream Current input reverts to stream userin the standard input tell F Makes file F the current output stream e If there is an open output stream associated with F and that was opened previously by tell 1 then that stream is made the current output stream e Otherwise the specified file is opened for output and made the current output stream If the file does not exist it is created Also note that different file names that is names which do not unify represent different output streams even if these different file names correspond to the same file Exceptions permission_error File F does not have write permission or is a directory instantiation_error F is uninstantiated telling F F is unified with the name of the current output stream This predicate is exactly t
26. Every stable model of a program contains the well founded partial model as a submodel As a result the XSB can be used to evaluate stable model semantics through the residual program to which we now turn The Residual Program Given a program P and query Q the residual program for Q and P consists of all conditional and unconditional answers created in the complete evaluation of Q Exercise 5 3 8 Consider the following program table ppgte_p 0 ppgte_q 0 ppgte_r 0 ppgte_s 0 ppgte_t 0 ppgte_u 0 ppgte_v 0 ppgte_p ppgte_q ppgte_p ppgte_r ppgte_q ppgte_s ppgte_r ppgte_u ppgte_q ppgte_t ppgte_r ppgte_v ppgte_s ppgte_w ppgte_u undefined CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 59 ppgte_t ppgte_x ppgte_v undefined ppgte_w ppgte 1 ppgte_x ppgte 0 ppgte_w undefined ppgte_x undefined ppgte 0 table undefined 0 undefined tnot undefined Write a routine that uses get_residual 2 to print out the residual program for the query ppgte p fail Try altering the tabling declarations in particular by making ppgte q 0 ppgte_r 0 ppgte_s 0 and ppgte_t 0 non tabled What effect does altering the tabling declarations have on the residual program When XSB returns a conditional answer to a literal L it does not propagate the delay list of the conditional answer but rather delays L itself even if L does not occur in a negative loop This has the advantage of ensuring that
27. HiLog term is successfully read that is in the presense of syntax errors read 1 does not fail but continues reading terms until a term with no syntax errors is encountered If a call to read Term causes the end of the current input stream to be reached variable Term is unified with the term end_of file In that case further calls to read 1 for the same input stream will cause an error failure Exceptions existence_error end of file is reached before the current term is read read Stream Term read 2 has the same behavior as read 1 but the input stream is explicitly designated using the first argument write Term The HiLog term Term is written to the current output stream according to the operator declarations in force Any uninstantiated subterm of term Term is written as an anonymous variable an underscore followed by a non negative integer All proper HiLog terms HiLog terms which are not also Prolog terms are not written in their internal Prolog representation Predicate write 1 always succeeds without producing an error The HiLog terms that are output by write 1 cannot in general be read back using read 1 This happens for two reasons e The atoms appearing in term Term are not quoted In that case the user must use writeq 1 or write_canonical 1 described below which quote around atoms whenever necessary e The output of write 1 is not terminated by a full stop therefore if the user wants the term to be accepted
28. This definition would work but it is slightly less efficient than the previous definition since it is known that bagMax is deterministic bagMin SetPred 7Arg HiLog Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate bagMin returns with Arg bound to the minimum element under the Prolog term ordering of the set SetPred To use this predicate it must be imported from aggregs and you must give the following definitions in the main module usermod hilog minimum minimum X Y Z X lt Y gt Z X Z Y These decarations are necessary because of a current limitation in how HiLog predicates can be used This requirement will be lifted in a future release With this definition bagMin 2 can be and is defined as bagMin Call Var bagReduce Call Var minimum zz zz where structures are the largest elements in the term ordering bagSum SetPred Arg HiLog Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate bagSum returns with Arg bound to the sum of the elements of the set SetPred To use this predicate it must be imported from aggregs and you must give the following definitions in the main module usermod hilog sum sum X Y Z Z is X Y These decarations are necessary because of a current limitation in how HiLog predicates can be used This requirement will be lifted in a future release With this definition bagSum 2 can be and is defined
29. a unary predicate that is true of all salaries or rather is the name of a bag of all salaries It is a bag since it may contain the same salary multiple times XSB provides a predicate bagSum which can be used to sum up the elements in a named bag So given the definition of the HiLog predicate salaries 1 above we can get the sum of all the salaries with bagSum salaries TotalSals The first argument to bagSum is the name of a bag and the second is bound to the sum of the elements in the bag We can also do a group by to get total salaries within departments as follows We define a parameterized predicate sals Dept to be the bag of salaries of employees in department Dept as follows sals Dept Sal employee _Name Dept Sal This rule says that Sal is in the bag named sals Dept if there is an employee with some name who works in department Dept and has salary Sal Now with this definition we can define a predicate deptPayro11 2 that associates with each department the sum of all the salaries of employees in that department deptPayroll Dept Payroll bagSum sals Dept Payrol1 XSB provides analogous aggregate operators described below to compute the minimum max imum count and average of a bag respectively These predicates are all defined using a more basic predicate bagReduce 4 bagReduce SetPred Arg 0p Id HiLog Tabling filterReduce SetPred Arg 0p Id Tabling SetPred must be a HiLog set
30. applied to in the HiLog term is Arity The first of these cases corresponds to the usual behaviour of Prolog s functor 3 while the second is the extension of functor 3 to handle HiLog terms Like the Prolog s functor 3 predicate hilog functor 3 can be used in either of the following two ways 1 If Term is initially instantiated then e If Term is a Prolog compound term F and Arity are unified with the name and arity of its principal functor respectively e If Term is an atom or a number F is unified with Term and Arity is unified with 0 e If Term is any other HiLog term F and Arity are unified with the name and the number of arguments that F is applied to Note that in this case F may still be uninstantiated 2 If Term is initially uninstantiated then at least Arity must be instantiated and e If Arity is an integer in the range 1 255 then Term becomes instantiated to the most general Prolog or HiLog term having the specified F and Arity as name and number of arguments F is applied to respectively The variables appearing as arguments are all unique e If Arity is 0 then F must be a Prolog or HiLog constant and it is unified with Term Note that in this case F cannot be a compound term e If Arity is anything else then hilog functor 3 aborts In other words the standard predicate hilog_functor 3 either decomposes a given HiLog term into its name and arity or given an arity and possibly a name constructs
31. are compiled as tabled predicates and SLG evaluation is used instead suppl_table The intention of this option is to direct the system to table for efficiency rather than termination When specified the compiler uses tabling to ensure that no predicate will depend on more than three tables or EDB facts as specified by the declaration edb of Section 3 8 4 The action of suppl_table is independent of that of auto_table in that a predicate tabled by one will not necessarily be tabled by the other During compilation suppl_table occurs after auto_table and uses table declarations generated by it if any spec_repr When specified the compiler performs specialisation of partially instantiated calls by replacing their selected clauses with the representative of these clauses i e it performs folding whenever possible We note in general the code replacement operation is not always sound i e there are cases when the original and the residual program are not computationally equivalent The compiler checks for sufficient but not necessary conditions that guarantee computational equivalence If these conditions are not met specialisation is not performed for the violating calls spec_off When specified the compiler does not perform specialisation of partially instantiated calls unfold_off When specified singleton sets optimisations are not performed during specialisation This option is necessary in Version 2 1 for the specialisation of table
32. compiler directives are recognized in Version 2 1 of XSB Mode Declarations The XSB compiler accepts mode declarations of the form mode ModeAnnot Mode Annot where each Mode Annot is a mode annotation a term indicator whose arguments are elements of the set From Version 1 4 1 on mode directives are used by the compiler for tabling directives a use which differs from the standard use of modes in Prolog systems See Section 3 8 4 for detailed examples Mode annotations have the following meaning This argument is an input to the predicate In every invocation of the predicate the argument position must contain a non variable term This term may not necessisarily be ground but the predicate is guaranteed not to alter this argument mode see assert This argument is an output of the predicate In every invocation of the predicate the argument position will always be a variable as opposed to the annotation below This variable is unified with the value returned by the predicate We note that Prolog does not enforce the requirement that output arguments should be variables however output unification is not very common in practice mode cputime This argument is either e An output argument of the predicate for which a non variable value may be supplied for this argument position If such a value is supplied the result in this position is unified with the supplied supplied value
33. delayed literals are not propagated exponentially through conditional answers Stable Models Stable models are one of the most popular semantics for non stratified programs The intuition behind the stable model semantics for a ground program P can be seen as follows Each negative literal notL in P is treated as a special kind of atom called an assumption To compute the stable model a guess is made about whether each assumption is true or false creating an assumption set A Once an assumption set is given negative literals do not need to be evaluated as in the well founded semantics rather an evaluation treats a negative literal as an atom that succeeds or fails depending on whether it is true or false in A Example 5 3 1 Consider the simple non stratified program writes_manual terry writes manual kostis has_time terry writes_manual kostis writes_manual terry has time kostis has_time terry has_time kostis there are two stable models of this program in one writes_manual terry is true and in another writes_manual kostis is true In the Well Founded model neither of these literals is true The residual program for the above program is writes_manual terry writes manual kostis writes manual kostis writes_manual terry has_time terry has_time kostis CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 60 Computing stable models is an intractable problem meaning that any algorith
34. derived a check is made against the table for S to see if the answer is there If the answer isn t in the table for S the answer is added and scheduled to be returned to all instances where S has been called if the answer is already in the table the evaluation simply fails and backtracks to generate more answers CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 48 Predicates can be declared tabled in a variety of ways A common form is the compiler directive table pi ni p n where p is a predicate symbol and n is an integer representing the arity of p This directive can be added to the file containing the predicate to be tabled and then to compile the file Exercise 5 2 1 Unless otherwise noted the file table_eramples P in the directory XSB_DIR examples contains all code for the running examples in this section Consult the file into XSB and type the query path 1 Y and continue hitting semi colons until you have exhausted all answers Type the query again Can you guess why the order of answers is different Now type abolish_all_tables and retry the path query Exercise 5 2 2 If you are curious try rewriting the path query as it would be written in Prolog Will it now terminate for the provided edge 2 relation Remember in XSB you can always hit lt ctr1 gt C if you go into an infinite loop The return of answers in tabling aids in filtering out redundant computations indeed it is this prop
35. directory The xsb xsbrc P file which is automatically con sulted by the XSB interpreter might look like the following assert library_directory assert library_directory assert library_directory my_friend assert library_directory usr lib sbprolog After loading the module of the above example the current working directory is searched first as opposed to the default action of searching it last Also XSB s system library directories lib syslib and cmplib will now be searched after searching the user s my_friend s and the usr lib sbprolog directory In fact XSB also uses library_directory 1 for internal purposes For instance before the user s xsb xsbrc P is consulted XSB puts the packages directory and the directory xsb config CONFIGURATION on the library search path The directory xsb config CONFIGURATION is used to store user libraries that are machine or OS dependent CONFIGURATION for a machine is something that CHAPTER 3 SYSTEM DESCRIPTION 17 looks like sparc sun solaris2 6 or pc 1inux gnu and is selected by XSB automatically at run time Note that the file xsb xsbrc P is not limited to setting the library search path In fact arbitrary Prolog code can go there We emphasize that in the presense of a xsb xsbrc P file it is the user s responsibility to avoid module name clashes with modules in XSB s system library directories Such name c
36. fact be made using the concept of dynamic stratification 31 Without going into the formalism of dynamic stratification we note that a program is dynamically stratified CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 57 if and only if it has a two valued model It is also known that computation of queries to dynamically stratified programs is not possible under any fixed strategy for selecting literals within the body of a clause In other words some mechanism for breaking the fixed order literal selection strategy must be used such as delaying However by redefining dynamic stratification to use an arbitrary fixed order literal selection strategy such as the left to right strategy of Prolog a new kind of stratification is characterized called Left to Right Dynamic Stratification or LRD stratification LRD stratified is not as powerful as dynamic stratification but is more powerful than other fixed order stratification methods and 1t can be shown that for ground programs XSB delays only when programs are not LRD stratified In the language of 38 XSB is delay minimal Programming in the Well founded Semantics XSB delays literals for non LRD stratified programs and later simplifies them away But how can the programmer determine when all sim plification has been done One method is to use local evaluation discussed below in Section 5 4 1 A second method is to make a top level call for a predicate p as follows p fail
37. fails This predicate creates fresh variables for the return rather than unifying them with variables in the first argument An explicit unification of a call with its return can be done if so desired Example Let us continue from the example in get_calls_for_table 3 get_returns_for_call p 1 X Return X _646412 Return p 1 _646628 X _646412 Return p 1 2 X _646412 Return p 1 3 no CHAPTER 6 STANDARD PREDICATES 116 get_returns_for_call p X Y Return no get_returns_for_call p 1 2 Return no Exception instantiation_error First argument is not instantiated table_error First argument is not a tabled predicate get_residual Skeleton 7Delay_list Tabling get_residual 2 is used to backtrack through answers that unify with a skeleton These answers may in fact come from any call that unifies with Skeleton Since the Delay_list of an answer consists of those literals whose truth value is unknown in the well founded model of the program see Section 5 get_residual 2 can be useful when extensions of the well founded model are desired Consider the program For the following program and table table p 2 p 1 2 p 1 3 tnot p 2 3 p 2 3 tnot p 1 3 calls to get_residual 3 will act as follows get_residual p X Y List X 1 Y 2 z 1 X 1 Y 3 Z tnot p 2 3 X 2 Y 3 Z tnot p 1 3 X 1 Y 3 Z tnot p 2 3 X
38. files contain SLG WAM instructions that can be executed by the emulator These byte coded instructions are machine independent so usually no installation procedure is needed for the byte code files The directory packages contains the various applications written in XSB which are not part of the system per se You must already be familiar with the build directory which is what you must have used to build XSB This directory contains XSB configuration scripts The directory etc contains miscellaneous files used by XSB The directory docs contains this manual in WT X dvi and Postscipt format and the directory examples contains sample programs to demonstrate various features of XSB 3 3 The Module system of XSB XSB has been designed as a module oriented Prolog system Modules provide a small step towards logic programming in the large that facilitates large programs or projects to be put together from components which can be developed compiled and tested separately Also module systems enforce the principle of information hiding and can provide a basis for data abstraction CHAPTER 3 SYSTEM DESCRIPTION 14 The module system of XSB unlike the module systems of most other Prolog systems is atom based Briefly the main difference between atom based module systems and predicate based ones is that in an atom based module system any symbol in a module can be imported exported or be a local symbol as opposed to the predicate based one
39. form requires an extra set of parentheses assert Head Body Default first argument indexing asserta Clause If the index specification for the preicate is not tries this predicate adds a dynamic clause Clause to the database before any other clauses for the same predicate currently in the database If the index specification for the predicate is trie the clause is asserted arbitrarily within the trie and a warning message sent to stderr assertz Clause If the index specification for the predicate is not tries this predicate adds a dynamic clause Clause to the database after any other clauses for the same predicate currently in the database If the index specification for the predicate is trie the clause is asserted arbitrarily within the trie and a warning message sent to stderr retract Clause removes through backtracking all clauses in the database that match with Clause Clause must be of one of the forms Head or Head Body Note that because of the precedence of 2 using the second form requires an extra set of parentheses retract Head Body Space is reclaimed when a clause is retracted retractall Head removes every clause in the database whose head matches with Head The predicate whose clauses have been retracted retains the dynamic property contrast this behavior with that of predicates abolish 1 2 below Predicate retractall 1 is determinate and always succeeds The term Head is not furthe
40. from Module Here PredList can either be a Prolog list or a comma list The import 1 can also be used as a directive in a source module see section 3 3 We provide a sample session for compiling dynamically loading and querying a user defined module named quick_sort For this example we assume that quick_sort is a file in the current working directory and contains the definitions of the predicates concat 3 and qsort 2 both of which are exported compile quick_sort Compiling quick_sort quick_sort compiled cpu time used 1 439 seconds yes import concat 3 qsort 2 from quick_sort yes concat 1 3 2 L qsort L S L 1 3 2 S 1 2 3 yes The standard predicate import 1 does not load the module containing the imported predicates but simply informs the system where it can find the definition of the predicate when and if the predicate is called 3 5 Command Line Arguments There are several command line options for the emulator The general synopsis is CHAPTER 3 SYSTEM DESCRIPTION 19 xsb xsb xsb xsb xsb xsb xsb flags 1 i flags n flags module flags B boot_module D cmd_loop_driver t e goal flags B module_to_disassemble d h v help version nobanner quietload noprompt memory management flags c tcpsize m glsize o complsize u pdlsize r g gc_type miscellaneous flags s T module Module to execute after
41. ignored under certain circumstances see below e s strl Un specify comments or strings Identical to c e infile Specify an input file from which gpp reads its input If no input file is specified input is read from standard input A 4 Syntax Specification The syntax of a macro call is the following it must start with a sequence of characters matching the macro start sequence as specified in the current mode followed immediately by the name of the macro which must be a valid identifier i e a sequence of letters digits or underscores _ The macro name must be followed by a short macro end sequence if the macro has no arguments or by a sequence of arguments initiated by an argument start sequence The various arguments are then separated by an argument separator and the macro ends with a long macro end sequence In all cases the parameters of the current context i e the arguments passed to the body being evaluated can be referred to by using an argument reference sequence followed by a digit between 1 and 9 Macro parameters may alternately be named see below Furthermore to avoid interference between the gpp syntax and the contents of the input file a quote character is provided The quote character can be used to prevent the interpretation of a macro call comment or string as anything but plain text The quote character protects the following character and always gets removed during evaluation Two consecut
42. in argument 2 of 2 Aborting X foolY Error Argument 2 of 2 is not a proper list Aborting CHAPTER 6 STANDARD PREDICATES 80 Exceptions instantiation_error Argument 2 of 2 is not a proper list type error Head of argument 2 of 2 is not an atom or number Term F ArgList HiLog When Term is a Prolog term this predicate behaves exactly like the Prolog 2 How ever when Term is a proper HiLog term 2 succeeds unifying F to its HiLog functor and ArgList to the list of the arguments to which this HiLog functor is applied Like 2 the use of 2 can nearly always be avoided by using the more efficient predi cates hilog functor 3 and hilog_arg 3 The behaviour of 2 on HiLog terms is as follows e If initially Term is uninstantiated then the list in the second argument of 2 must be instantiated to a proper list list of determinate length whose head can be any Prolog or HiLog term e If the arguments of 2 are both uninstantiated or if the second of them is not what is expected 2 aborts producing an appropriate error message Examples p a b c L p a b c En h a b c L h a b c E map p a b L map p a b E T X a bl apply X a b H T 2 2 apply 2 2 H T atb 2 T apply atb 2 T 31X Error Argument 2 of 2 is no
43. instantiated at the time of call type_error Dir is not an atom edit Module Provided that the environment variable EDITOR has been set and the system is executing under UNIX a call to edit foo will call the default editor on the file named foo P The argument to edit 1 should be instantiated it can be an absolute or a relative file name but it should not contain the suffix P Users can also set their preferred options of calling the default editor by setting an environment variable named EDITOR_OPTIONS Examples of possible uses of predicate edit 1 are 1 If the environment variables have been set as follows setenv EDITOR usr ucb vi setenv EDITOR_OPTIONS 1 a call like edit foo will call the vi editor in the mode where left and right parentheses and curly brackets are checked for balance for the file foo P in the current working directory CHAPTER 6 STANDARD PREDICATES 112 2 If on the other hand they have been set as follows setenv EDITOR usr local bin emacs setenv EDITOR_OPTIONS r a call like edit foo will call the emacs editor in reverse video for the file foo P in user s home directory 6 12 Tabling In XSB tables are designed so that they can be used transparently by computations However it is often useful to be able to explicitly inspect a table or to alter its state In the following predicates which are provided for this purpose Skeleton refers to information a
44. load it into memory is to type the query my_file where my_file is the name of the file or preferably the name of the module obtained from the file name by deleting the suffix P To find more about the module system of XSB see Section 3 3 If you are eccentric or you don t know how to use an editor you can also compile and load predicates input directly from the terminal by using the command user A CTRL d or the atom end_of_file followed by a period terminates the input stream 2 5 Sample XSB Programs If for some reason you don t feel like writing your own XSB programs there are several sample XSB programs in the directory XSB_DIR examples All contain source code The entry predicates of all the programs in that directory are given the names demo 0 which prints out results and go 0 which does not print results Hence a sample session might look like the actual times shown below may vary and some extra information is given using comments after the character my_favourite_prompt gt cd XSB_DIR examples my_favourite_prompt gt XSB_DIR bin xsb XSB Version 2 0 Gouden Carolus of June 27 1999 i586 pc linux gnu mode optimal engine slg wam scheduling batched queens queens loaded yes demo 7 ee output from queens program Time used 0 4810 sec yes statistics memory total 1906488 bytes 203452 in use 1703036 free permanent space 202552 bytes This co
45. local sym SYMm import sym sym from module where sym has the form functor arity If the user does not want to use modules he can simply bypass the module system by not supplying any export declarations Such exportless files non modules will be loaded into the module usermod which is the working module of the XSB interpreter Currently the module name is stored in its byte code file which means that if the byte code file is renamed the module name is not altered and hence may cause confusion to the user and or the system So it is advisable that the user not rename byte code files generated for modules by the XSB compiler However byte code files generated for non modules can be safely renamed We will try to fix the problem described above in future releases In order to understand the semantics of modules the user should keep in mind that in a module oriented system the name of each symbol is identified as if it were prefixed with its module name Operator symbols can be exported as any other symbols but their precedence must be redeclared in the importing module CHAPTER 3 SYSTEM DESCRIPTION 15 hence two symbols of the same functor arity but different module prefixes are distinct symbols Currently the following set of rules is used to determine the module prefix of a symbol e Every predicate symbol appearing in a module i e that appears as the head of some clause is assumed to be local to that m
46. modules In this case the names of the source files should have the extension c and a P file must not exist A header file with extension H must be present for a foreign language module see the chapter Foreign Language Interface in Volume 2 3 8 2 Compiler Options Compiler options can be set in three ways from a global list of options see set_global_compiler_options 1 from the compilation command see compile 2 and consult 2 and from a directive in the file to be compiled see compiler directive compiler_options 1 set_global_compiler_options 0ptionsList OptionsList is a list of compiler options described below Each can optionally be prefixed by or indicating that the option is to be turned on or off respectively No prefix turns the option on This evaluable predicate sets the global compiler options in the way indicated These options will be used in any subsequent compilation unless reset by another call to this predicate or overridden by options provided in the compile invocation or overridden by CHAPTER 3 SYSTEM DESCRIPTION 26 options in the file to be compiled The following options are currently recognized by the compiler optimize When specified the compiler tries to optimize the object code In Version 2 1 this option optimizes predicate calls among other features so execution may be considerably faster for recursive loops However due to the nature of the optimizations the user may not be abl
47. move XSB there 2 Make sure that after you have obtained XSB by anonymous ftp using the binary option or from the web you have uncompressed it by following the instructions found in the file README 3 Note that after you uncompress and untar the XSB tar file a subdirectory XSB will be tacked on to the current directory All XSB files will be located in that subdirectory In the rest of this manual let us use XSB_DIR to refer to this subdirectory Note the orig inal directory structure of XSB must be maintained namely the directory XSB_DIR should contain all the subdirectories and files that came with the distribution In particular the following directories are required for XSB to work emu syslib cmplib lib packages build and etc 4 Change directory to XSB_DIR build and then run these commands configure makexsb This is it In addition it is now possible to install XSB in a shared directory e g usr local for everyone to use In this situation you should use the following sequence of commands CHAPTER 2 GETTING STARTED WITH XSB 6 configure prefix SHARED_XSB makexsb makexsb install where SHARED_XSB denotes the shared directory where XSB is installed In all cases XSB can be run using the script XSB_DIR bin xsb However if XSB is installed in a central location the script for general use is lt central installation directory gt lt xsb version gt bin xsb Important The XSB executable de
48. number of subgoals are encountered in query evaluation 2 each of these subgoals have a finite number of answers Indeed it can be proven that for any program with the bounded term depth property roughly where all terms generated in a program have a maximum depth SLG computation will terminate These programs include the important class of Datalog programs Variant and Subsumptive Tabling The above description gives the general idea of how tabling affects definite programs but is imprecise on certain points In XSB a subgoal subgoals S2 can use a table from S if S is a variant of S2 that is if S and S2 are the same up to variable renaming Other tabling strategies may allow S2 to use the table of S if S2 not more general than S1 or S2 is subsumed by S1 Example 5 2 1 The terms p f Y X 1 and p Z U 1 are variants but p Y X 1 and p Z Z 1 are not In fact the former subsumes the latter Just as a subsumption or variance relation can be used to decide when one subgoal can use the table of another the two relations can be used to determine when an answer should be returned In XSB s engine a derived answer A will be considered new and returned to a subgoal S only if A is not a variant of some other previously derived answer for S In Version 2 1 of XSB subgoal subsumption is not supported although work on an engine that includes subgoal subsumption is nearing completion Answer subsumption however can be flexibly
49. of this manual Several predicates perform tabled aggregation besides filterReduce 4 One of these is the predicate filterPO1 Pred Preference_structure Partial_order Analoguosly to filterReduce1 4 if Pred is an n ary predicate filterP0 4 forms a n 1 ary predicate Pred1 whose last argument is Preference_structure and whose functor and all other arguments are determined by Pred filterP0O Pred Preference_structure Partial_order then calls Predi and for each re turn of Pred1 fails if there is some answer already in the table for filterP01 4 such that the first n arguments of Pred in the tabled answer unify with the first n arguments of Pred in the return and whose preference structure last argument is preferred to that of the return A case study in the use of filterP0 4 to construct preference logic grammars can be found in 10 CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 62 5 4 1 Local Evaluation For the shortest path example simply failing until a minimal answer was derived and then returning that solution was an effective technique for computing the shortest path However this approach will not always work As we have seen in Exercise 5 2 6 programs can consist of sets of mutually recursive predicates and in principle these sets can be arbitrarily large If these computations are to use tabled aggregation the approach taken by filterReduce 4 will not suffice To see this we make the notion of mutual recursion more
50. on SB Prolog primarily designed and written by Saumya Debray David S Warren and Jiyang Xu Thanks are also due to Weidong Chen for his work on Prolog clause indexing for SB Prolog and to Richard O Keefe who contributed the Prolog code for the Prolog reader and the C code for the tokenizer Contents 1 Introduction 2 Getting Started with XSB 2 1 Installing XSB under UNIX e 2 1 1 Possible Installation Problems a 2 2 Installing XSB under Windows e 2 2 1 Using Cygnus Software s CygWin32 e e e 2 2 2 Using Microsoft Visual C e 2 9 Invoking XO Bay ut is a A SA a ao 2 4 Compiling XSB programs saaa ee 2 5 Sample XSB Programs 2 0 ee e 2 6 Exitine SB Mya dne a Hh ty a ld ie A ind ee ah etic te ah A 3 System Description 3 1 Entering and Exiting XSB 1 ee 3 2 The System and its Directories o 3 3 The Module system of XSB 2 e 3 4 The Dynamic Loader and its Search Path o o e 3 4 1 Changing the Default Search Path and the Packaging System 3 4 2 Dynamically loading predicates in the interpreter 3 5 Command Line Arguments 2 2 0 0 ce ee 3 6 Memory Management 2 2 0 0 2 ee ee 3 7 Compiling and Consulting 3 8 The Compilers egos 4 ead A See th Ba a a ee gt ce 3 8 1 Invoking the Compiler 0 0 0 0 0 0000000020 0048
51. programmed as discussed in Section 5 4 Cuts and Tabling Tabling integrates well with most Prolog functionality even for non pure Prolog predicates Meta logical predicates like var 1 and predicates with side effects like read 1 and write 1 can be used freely in tabled predicates as long as 1t is remembered that only the first call to a goal will execute program clauses the rest will look up answers from a table The use of cuts with tabling is more problematic as can be seen from the following exercise Exercise 5 2 4 Consider the program table cut_p 1 cut_q 1 cut_r 0 cut_s 0 cut_p X cut_q X cut_r cut_r cut_s cut_s cut_q _ cut_q 1 cut_q 2 once Term call Term We also note that the library subsumes contains routines for checking variance and subsumption CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 50 What solutions are derived for the goal p X Suppose that cut p 1 were rewritten as pi X q1 X once r1 How should this cut over a table affect the answers generated for cut p 1 What happens if you rewrite p 1 in this way and compile it in XSB The solution Version 2 1 of XSB takes to the problem posed in Exercise 5 2 4 is to check whether a tabled predicate statically lies in the scope of a cut at compile time If so the compilation is aborted However cuts are allowed within tabled predicates subject as always to the restriction that the scope of a cut cannot include a ca
52. s use filterP0 3 acts as bagP0 3 with the single difference that it does not depend on HiLog so that filterP0 3 will be more robust especially when XSB s module system is used filterPO0 HtPred 0rder Tabling filterPO Pred 0Order succeds only for a solution Pred of Pred for which there is no solution Predn to Pred such that Order Predn Pred Example For the following program table p 2 b 1 2 p 1 3 b 1 1 prefer b X X b X Y X Y the query filterPO b X Y will succeed only with the binding X 1 Y 1 bagMax SetPred Arg HiLog Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate bagMax returns with Arg bound to the maximum element under the Prolog term ordering of the set SetPred To use this predicate it must be imported from aggregs and you must give the following definitions in the main module usermod hilog maximum maximum X Y Z X lt Y gt Z Y Z X These decarations are necessary because of a current limitation in how HiLog predicates can be used This requirement will be lifted in a future release With this definition bagMax 2 can be and is defined as follows bagMax Call Var bagReduce Call Var maximum _ CHAPTER 6 STANDARD PREDICATES 89 Where variables are minimal in the term ordering Another possible definition of bagMax 2 would be hilog 1t 1t X Y X Y bagMax Call Var bagP0 Call Var lt
53. second When the tracer prompts for input the user may enter a return or a single character followed by a return with the following meanings c lt CR gt Creep Causes the system to single step to the next port i e either the entry to a traced predicate called by the executed clause or the success or failure exit from that clause 119 CHAPTER 7 DEBUGGING 120 a Abort Causes execution to abort and control to return to the top level interpreter b Break Calls the evaluable predicate break thus invoking recursively a new incarnation of the system interpreter The command prompt at break level n is nN The user may return to the previous break level by entering the system end of file character e g ctr1 D or typing in the atom end_of file or to the top level interpreter by typing in abort f Fail Causes execution to fail thus transferring control to the Fail port of the current execution h Help Displays the table of debugging options 1 Leap Causes the system to resume running the program only stopping when a spy point is reached or the program terminates This allows the user to follow the execution at a higher level than exhaustive tracing n Nodebug Turns off debug mode K Retry fail Transfers to the Call port of the current goal Note however that side effects such as database modifications etc are not undone s Skip Causes tracing to be turned off for the entire execution of the procedure
54. that is any terms with principal functor 1 that are not directives like the ones described in Section 3 8 are executed in the order that they are encountered 3 8 The Compiler The XSB compiler translates XSB source files into byte code object files It is written entirely in Prolog Both the sources and the byte code for the compiler can be found in the XSB system directory cmplib Prior to compiling XSB filters the programs through GPP a preprocessor written by Denis Auroux auroux math polytechnique fr This preprocessor maintains high degree of compatibility with the C preprocessor but is more suitable for processing Prolog programs The preprocessor is invoked with the compiler option xpp_on as described below The various features of GPP are described in Appendix A XSB also allows the programmer to use preprocessors other than GPP However the modules that come with XSB distribution require GPP This is explained below see xpp_on compiler option The following sections describe the various aspects of the compiler in more detail 3 8 1 Invoking the Compiler The compiler is invoked directly at the interpreter level or in a program through the Prolog predicates compile 1 2 The general forms of predicate compile 2 are compile File 0ptionList compile FileList OptionList and at the time of the call both of its arguments should be ground The second form allows the user to supply a proper list of file names
55. the module as usual and then using the d module 0 command line option of the XSB emulator see Section 3 5 index_off When specified the compiler does not generate indices for the predicates compiled 3 8 3 Specialisation From Version 1 4 0 on the XSB compiler automatically performs specialisation of partially instan tiated calls Specialisation can be thought as a source level program transformation of a program to a residual program in which partially instantiated calls to predicates in the original program are replaced with calls to specialised versions of these predicates The expectation from this process is that the calls in the residual program can be executed more efficiently that their non specialised counterparts This expectation is justified mainly because of the following two basic properties of the specialisation algorithm Compile time Clause Selection The specialised calls of the residual program directly select at compile time a subset containing only the clauses that the corresponding calls of the original program would otherwise have to examine during their execution at run time By doing so laying down unnecessary choice points is at least partly avoided and so is the need to select clauses through some sort of indexing CHAPTER 3 SYSTEM DESCRIPTION 29 Factoring of Common Subterms Non variable subterms of partially instantiated calls that are common with subterms in the heads of the selected clauses are factored
56. the standard predicate atom_codes 2 should be used instead The syntax for numbers that is accepted by name 2 is exactly the one which read 1 accepts Predicate name 2 is provided for backwards compatibility It is advisable that new programs use the predicates atom_codes 2 and number_codes 2 described below In Version 2 1 predicate name 2 is not yet implemented for converting from a real number to its character list representation and if the representation of a real is provided as CharList it will be converted to an atom If both of the arguments of name 2 are uninstantiated or CharList is not a proper list of ASCII characters name 2 will abort and an error message will be sent to the standard error stream Examples name Foo L L 70 111 111 name L 91 93 EP i name 431 L L 52 51 49 name X 102 111 111 X foo CHAPTER 6 STANDARD PREDICATES 82 name X X name X Foo X Foo name X 52 51 49 X 431 name X 45 48 50 49 51 integer X X 213 name 3 14 L Error Predicate name 2 for reals is not implemented yet Aborting Exceptions instantiation_error Both arguments are uninstantiated or argument 2 of name 2 contains a variable or is not a proper list type_error Constant is not a variable an atom or a number range_error CharList is not a list of ASCII characters implementation_error Constant
57. this document intended to be the user manual reflects the current status Version 2 1 of our system While we have taken great effort to create a robust and efficient system we would like to emphasize that XSB is also a research system and is to some degree experimental When the research features of XSB tabling HiLog and Indexing Techniques are discussed in this manual we also cite documents where they are fully explained All of these documents can be found via the world wide web or anonymous ftp from www ftp cs sunysb edu the same host from which XSB can be obtained While some of Version 2 1 is subject to change in future releases we will try to be as upward compatible as possible We would also like to hear from experienced users of our system about features they would like us to include We do try to accomodate serious users of XSB whenever we can Finally we must mention that the use of undocumented features is not supported and at the user s own risk Chapter 2 Getting Started with XSB This section describes the steps needed to install XSB under UNIX and under Windows 2 1 Installing XSB under UNIX If you are installing on a UNIX platform the version of XSB that you received may not include all the object code files so that an installation will be necessary The easiest way to install XSB is to use the following procedure 1 Decide in which directory in your file system you want to install XSB and copy or
58. this progam should be that the barber shaves the mayor but the case of the barber is trickier If we conclude that the barber does not have himself our meaning does not reflect the first rule in the program If we conclude that the barber does shave himself we have reached that conclusion using information beyond what is provided in the progra The well founded semantics does not treatshaves barber barber as either true or false but as undefined Prolog of course would enter an infinite loop XSB s treatment of negation is discussed further in Chapter 5 The second important extension in XSB is support of HiLog programming 6 39 HiLog allows a form of higher order programming in which predicate symbols can be variable or structured For example definition and execution of generic predicates like this generic transitive closure relation are allowed closure R X Y R X Y closure R X Y R X Z closure R Z Y where closure R 2 is syntactically a second order predicate which given any relation R returns its transitive closure relation closure R With XSB support is provided for reading and writing CHAPTER 1 INTRODUCTION 3 HiLog terms converting them to or from internal format as necessary see Section 4 2 Special meta logical standard predicates see Section 6 5 are also provided for inspection and handling of HiLog terms Unlike earlier versions of XSB prior to version 1 3 1 the current version automat
59. to a list of terminals by the time of the call then the goal corresponds to parsing List as a phrase of type Phrase otherwise if List is unbound then the grammar is being used for generation phrase Phrase List Rest This predicate is true iff the segment between the start of list List and the start of list Rest can be parsed as a phrase i e sequence of terminals of type Phrase In other words if the search for phrase Phrase is started at the beginning of list List then Rest is what remains unparsed after Phrase has been found Again Phrase can be any term which would be accepted as a nonterminal of the grammar or in general any grammar rule body and must be instantiated to a nonvariable term at the time of the call otherwise an error message is sent to the standard error stream and the predicate fails Predicate phrase 3 is the analogue of ca11 1 for grammar rule bodies and provides a se mantics for variables in the bodies of grammar rules A variable X in a grammar rule body is treated as though phrase X appeared instead X would expand into a call to phrase X L R for some lists L and R expand_term Term1 Term2 This predicate is used to transform terms that appear in a Prolog program before the program CHAPTER 8 DEFINITE CLAUSE GRAMMARS 128 is compiled or consulted The standard transformation performed by expand_term 2 is that when Termi is a grammar rule then Term2 is the corresponding Prolog clause otherwis
60. y This acts in a similar way to define but the second argument y is evaluated immediately Since user macro definitions are also evaluated each time they are called this means that the macro y will undergo two successive evaluations The usefulness of defeval is considerable as it is the only way to evaluate something more than once which can be needed e g to force evaluation of the arguments of a meta macro that normally doesn t perform any evalu ation However since all argument references evaluated at define time are understood as the arguments of the body in which the macro is being defined and not as the arguments of the macro itself usually one has to use the quote character to prevent immediate evaluation of argument references e undef x This removes any existing definition of the user macro 2 e ifdef x This begins a conditional block Everything that follows is evaluated only if the identifier z is defined until either a else or a endif statement is reached Note however that the commented text is still scanned thoroughly so its syntax must be valid It is in particular APPENDIX A GPP GENERIC PREPROCESSOR 143 legal to have the else or endif statement ending the conditional block appear as only the result of a user macro expansion and not explicitly in the input e ifndef x This begins a conditional block Everything that follows is evaluated only if the identifier z is not defined e ifeq xy This begins a
61. zero will return the ASCII code of the single character after the apostrophe for example 0 A 65 4 1 2 Floating point Numbers A HiLog floating point number consists of a sequence of digits with an embedded decimal point optionally preceded by a minus sign and optionally followed by an exponent consisting of uppercase or lowercase E and a signed base 10 integer Using these rules examples of HiLog floating point numbers are 1 0 34 56 817 3E12 0 0314e26 2 0E 1 Note that in any case there must be at least one digit before and one digit after the decimal point 4 1 3 Atoms A HiLog atom is identified by its name which is a sequence of up to 1000 characters other than the null character Just like a Prolog atom a HiLog atom can be written in any of the following forms e Any sequence of alphanumeric characters including _ starting with a lowercase letter e Any sequence from the following set of characters except of the sequence which begins a comment x x N 7 lt gt 7 3 7 E e Any sequence of characters delimited by single quotes such as sofaki sey Aa gt _ op If the single quote character is to be included in the sequence it must be written twice For example 2 don t 333 CHAPTER 4 SYNTAX 38 e Any of the following bog 0 Q Note that the bracket pairs are special While and are atoms and gt are not
62. 15 3 187 230 1993 W Chen T Swift and D Warren Efficient top down computation of queries under the well founded semantics J Logic Programming 24 3 161 199 September 1995 W Chen and D S Warren Tabled Evaluation with Delaying for General Logic Programs Journal of the ACM 43 1 20 74 January 1996 M Codish B Demoen and K Sagonas Semantics based program analysis for logic based lan guages using XSB Springer International Journal of Software Tools for Technology Transfer 2 1 29 45 Nov 1998 B Cui T Swift and D S Warren A case study in using preference logic grammars for knowledge representation Available at http www cs sunysb edu tswift 1998 S Dawson C R Ramakrishnan I V Ramakrishnan K Sagonas S Skiena T Swift and D S Warren Unification factoring for efficient execution of logic programs In Proc of the 22nd Symposium on Principles of Programming Languages pages 247 258 ACM 1995 S Dawson C R Ramakrishnan and D S Warren Practical program analysis using general purpose logic programming systems a case study In ACM PLDI pages 117 126 May 1996 160 BIBLIOGRAPHY 161 13 14 15 16 17 18 19 1201 191 199 193 24 25 126 197 S Debray SB Prolog System Version 3 0 A User Manual 1988 B Demoen and K Sagonas CAT the Copying Approach to Tabling In C Palamidessi H Glaser and K Meinke editors Pr
63. 2 3 yes callable _ _1 yes CHAPTER 6 STANDARD PREDICATES 74 callable _ a yes callable 3 14 no proper_hilog X HiLog Succeeds if X is a proper HiLog term otherwise it fails Examples In this example and the rest of the examples of this section we assume that h is the only parameter symbol that has been declared a HiLog symbol proper_hilog X no proper_hilog foo a f b A no proper_hilog X a b c yes proper_hilog 3 6 2 4 yes proper_hilog h no proper_hilog a d e X a c yes proper_hilog a a X a yes functor Term Functor Arity Succeeds if the functor of the Prolog term Term is Functor and the arity number of argu ments of Term is Arity Functor can be used in either the following two ways 1 If Term is initially instantiated then e If Term is a compound term Functor and Arity are unified with the name and arity of its principal functor respectively e If Term is an atom or a number Functor is unified with Term and Arity is unified with 0 2 If Term is initially uninstantiated then either both Functor and Arity must be instan tiated or Functor is instantiated to a number and e If Arity is an integer in the range 1 255 then Term becomes instantiated to the most general Prolog term having the specified Functor and Arity as principal functor and number of arguments respectively The variables appearing as arg
64. 6 7 Comparison The evaluable predicates described in this section are meta logical They are used to compare and order terms rather than to evaluate or process them They treat uninstantiated variables as objects with values which may be compared and they never instantiate those variables Each of these predicates simply succeeds or fails there is no side effect substitution or error condition associated with them The predicates described in this section should not be used when what the user really wants is arithmetic comparison predicates or unification predicates see section 6 3 The predicates described take into account a standard total ordering of terms which has as follows variables Q lt floating point numbers Q lt integers Q lt atoms Q lt compound terms Within each one of the categories the ordering is as follows CHAPTER 6 STANDARD PREDICATES 91 e variables are put in a standard order roughly the oldest first the order is not related to the names of variables Also note that two anonymous variables are not identical terms Unfortunately in the current implementation of our system Version 2 1 variables tend to move rather quickly as a result of unification and thus the ordering may not continue to hold if the variables get unified to some other variables We intend to ameliorate this bug in future releases e floating point numbers and integers are put in numeric order from oo to 00 Note that a
65. Argument is not a predicate specification of a callable term table_error Argument is not a specification of a tabled predicate abolish_table_call Call Tabling Predicate abolish_table_call 1 abolishes tables for a particular call to a predicate denoted by Call The predicate remains tabled and information about all other tables remains intact In Version 2 1 abolish_table_call 1 does not reclaim used space Note that incomplete tables are abolished automatically by the system on exceptions and when the interpreter level is resumed In these cases the user does not need to abolish tables to maintain correctness Example Continuing the example started in the description of predicate get_calls_for_table 3 the call abolish_table_call p 1 X would produce the table Gall Returns fae p 1 3 p 2 3 Exception CHAPTER 6 STANDARD PREDICATES 118 table_error Argument 1 is not a callable predicate table_state Call State Tabling If the first argument is a valid Term indicator table_state 1 unifies state with one of the set not_yet_called complete incomplete undef The meaning of these atoms is defined as not_yet_called iff the predicate corresponding to Call has been declared tabled but there is no table entry for call complete iff the table entry for Call contains all solutions incomplete iff the table entry for Call may not contain all solutions undef iff the predicate corresponding to Call has not be
66. CHAPTER 6 STANDARD PREDICATES 86 L a X b Y _922820 L a b abolish_all_tables p X tfindall Y p Y L X b Y _922820 L b a X a Y _922820 L b al no Exceptions Same as predicate finda11 3 see above Also table_error Upon execution Goal is not a subgoal of a tabled predicate tbagof X Goal List tsetof X Goal List Tabling The standard predicates tbagof 3 and tsetof 3 provide tabled versions of bagof 3 and setof 3 in a similar manner to the way in which tfindal1 3 provides a tabled version of findall 3 X Goal The system recognises this as meaning there exists an X such that Goal is true and treats it as equivalent to call Goal The use of this explicit existential quantifier outside predicates setof 3 and bagof 3 constructs is superfluous 6 6 1 Tabling Aggregate Predicates HiLog provides an elegant way to introduce aggregate operations into XSB HiLog allows a user to define named and parameterized sets or bags For example say we have a simple database like predicate employee Name Dept Sal which contains a tuple for each employee in our concern and contains the employee s name department and salary From this predicate we can construct a set or bag really that contains all the salaries of employees in the relation hilog salaries salaries Sal employee _Name _Dept Sal CHAPTER 6 STANDARD PREDICATES 87 So salaries is the name of
67. L CHAPTER 6 STANDARD PREDICATES 84 sl i 49 50 51 number_codes N 49 50 51 integer N N 123 number_codes 31 4e 10 L L 51 46 49 51 57 57 57 55 69 43 49 48 number_codes N 314e 8 N 3 14e 10 number_codes foo L Type error in argument 1 of number_codes 2 number expected but something else found Aborting Exceptions instantiation_error Both arguments are uninstantiated or argument 2 is not a proper list or it contains a variable type_error Number is not a variable or a number range_error CharList is not a list of ASCII characters number_chars Number CharAtomList Like number_codes but the list returned or input is a list of characters as atoms rather than ASCII codes For instance number_chars 123 X binds X to the list 1 2 gt 3 instead of 49 50 51 number_digits Number DigitList Like number_chars but the list returned or input is a list of digits as numbers rather than ASCII codes for floats the atom or and e will also be present in the list For instance number_digits 123 X binds X to the list 1 2 3 instead of 1 2 3 and number_digits 123 45 X binds X to 1 2 3 4 5 0 0 e 0 2 6 6 All Solutions and Aggregate Predicates Often there are many solutions to a problem and it is necessary somehow to compare these solutions with one another The most general way of doing this is to c
68. Ramakrishnan I Ramakrishnan S Smolka T Swift and D Warren Efficient model checking using tabled resolution In Proceedings of CAV 97 1997 P Rao I Ramakrishnan K Sagonas T Swift and D Warren Efficient table access mecha nisms for logic programs Journal of Logic Programming 38 1 31 54 Jan 1999 M Rules Diagnostica Technical report Medicine Rules 1999 http medicinerules com K Sagonas and T Swift An abstract machine for tabled execution of fixed order stratified logic programs ACM TOPLAS 20 3 586 635 May 1998 K Sagonas T Swift and D Warren XSB as an efficient deductive database engine In Proc of SIGMOD 1994 Conference ACM 1994 K Sagonas T Swift and D Warren An abstract machine for computing the well founded semantics In Joint International Conference and Symposium on Logic Programming pages 274 289 1996 K Sagonas T Swift and D S Warren The limits of fixed order computation Theoretical Computer Science 2000 To apperar Preliminary version apeared in International Workshop on Logic and Databases LNCS K Sagonas and D Warren Efficient execution of HiLog in WAM based Prolog implemen tations In L Sterling editor Proceedings of the 12th International Conference on Logic Programming pages 349 363 MIT Press June 1995 H Seki On the power of Alexandrer templates In Proc of 8th PODS pages 150 159 ACM 1989 T Swift A new formulation of tabled resolutio
69. SB SHARED_XSB bin xsb and indeed using this command invokes XSB s top level interpreter which is the usual way of using XSB Version 2 1 of XSB can also directly execute object code files from the command line interface Suppose you have a top level routine go in a file foo P that you would like to run from the UNIX or Windows command line As long as foo P contains a directive go and foo has been compiled to an object file foo 0 then XSB_DIR bin xsb B foo 0 will execute go loading the appropriate files as needed In fact the command XSB_DIR bin xsb is equivalent to the command XSB_DIR bin xsb B XSB_DIR syslib loader 0 There are several ways to exit XSB A user may issue the command halt or end_of_file or simply type CTRL d at the XSB prompt To interrupt XSB while it is executing a query strike CTRL c 12 CHAPTER 3 SYSTEM DESCRIPTION 13 3 2 The System and its Directories The XSB system when installed resides in a single directory that contains the following subdirec tories 1 build 2 docs 3 emu 4 etc 5 examples 6 cmplib 7 lib 8 packages 9 syslib The directory emu contains the source and object code for the XSB emulator which is written in C The directories syslib cmplib and 1ib contain source and object code for the basic Prolog libraries the compiler and the extended Prolog libraries respectively All the source programs are written in XSB and all object byte code
70. The XSB System Version 2 1 Volume 1 Programmers Manual E Konstantinos Sagonas Terrance Swift David S Warren Juliana Freire Prasad Rao with contributions from Steve Dawson Michael Kifer November 1999 Credits Day to day care and feeding of XSB including bug fixes ports and configuration man agement has been done by Kostis Sagonas David Warren Terrance Swift Prasad Rao Steve Dawson Juliana Freire Baoqiu Cui and Michael Kifer In Version 2 1 the core engine development of the SLG WAM has been mainly imple mented by Terrance Swift Kostis Sagonas Prasad Rao and Juliana Freire The break down roughly was that Terrance Swift wrote the initial tabling engine and builtins Prasad Rao wrote the trie based table access routines and Kostis Sagonas implemented most of tabled negation Juliana Freire revised the table scheduling mechanism starting from Version 1 5 0 to create a more efficient engine and implemented the engine for local evaluation Starting from XSB Version 2 0 XSB includes another tabling engine CHAT which was designed and developed by Kostis Sagonas and Bart Demoen CHAT supports heap garbage collection both based on a mark amp slide and on a mark amp copy algorithm which was developed and implemented by Bart Demoen and Kostis Sagonas Memory expansion code for WAM stacks was written by Ernie Johnson and Bart De moen while memory management code for CHAT areas was written
71. The predicate fails if this unification fails If a Any parallelisation directives parallel are simply ignored by the compiler but do not result in syntax errors to enhance compatibility with various other earlier versions of PSB Prolog The most common uses of mode declarations in Prolog systems are to reduce the size of compiled code or to speed up a predicate s execution CHAPTER 3 SYSTEM DESCRIPTION 31 variable term is supplied the predicate succeeds and the output variable is unified with the return value mode H e An input output argument position of a predicate that has only side effects usually by further instantiating that argument The symbol is used to denote the symbol that cannot be entered from the keyboard This argument does not fall into any of the above categories Typical cases would be the following e An argument that can be used both as input and as output but usually not with both uses at the same time mode functor e An input argument where the term supplied can be a variable so that the argument can not be annotated as or is instantiated to a term which itself contains uninstantiated variables but the predicate is guaranteed not to bind any of these variables mode var write We try to follow these mode annotation conventions throughout this manual Finally we warn the user that mode declarations can be error prone and since errors in mode
72. X mode define countdown it 1 1 def ine loop countdown else Done define loop endif loop eval 1 1 countdown 10 The following is an unfortunately very weak attempt at implementing functional abstraction in gpp in standard mode Understanding this example and why it can t be made much simpler is an exercise left to the curious reader mode string NN define ASIS x x define SILENT x ASISO define EVAL x f v SILENT mode string QQQ MSSM TAN defeval TEMPO x defeval TEMP1 define TEMP2 TEMPO f TEMP1 TEMP2 v APPENDIX A GPP GENERIC PREPROCESSOR define LAMBDA x f v SILENT ifneq v O define TEMP3 a b c EVAL a b c else define TEMP3 a b c LAMBDA a b endif TEMP3 x f v define EVALAMBDA x y SILENT defeval TEMP4 x defeval TEMP5 y define APPLY f v SILENT defeval TEMP6 ASIS EVA f TEMP6 EVAL TEMP4 TEMPS v This yields the following results LAMBDA z z z gt LAMBDA z z z LAMBDA z z z 2 gt 2 2 define f LAMBDA y yx y f gt LAMBDA y y y APPLY f blah gt blah blah APPLY LAMBDA t t t t t gt t t t t LAMBDA x APPLY f x x urf gt urfturf urfturf APPLY APPLY LAMBDA x LAMBDA y x y fo00 bar gt foo bar define test LAMBDA y ifeq y urf y is urf else y is not urf endif APPLY test urf 151 APPENDIX A GPP GENERIC PREPROCESSOR
73. XSB starts up Module should have no suffixes no directory part and the file module O must be on the library search path boot_module This is a developer s option The B flags tells XSB which bootstraping module to use instead of the standard loader The loader must be specified using its full pathname and boot_module D must exist module_to_disassemble This is a developer s option The d flag tells XSB to act as a disassembler The B flag specifies the module to disassemble cmd_loop_driver The top level command loop driver to be used instead of the standard one Usually needed when XSB is run as a server i bring up the XSB interpreter e goal evaluate goal when XSB starts up l the interpreter prints unbound variables using letters n used when calling XSB from C B specify the boot module to use in lieu of the standard loader D Sets top level command loop driver to replace the default t trace execution at the SLG WAM instruction level for this to work build XSB with the debug option d disassemble the loader and exit c N allocate N KB for the trail choice point stack m N allocate N KB for the local global stack o N allocate N KB for the SLG completion stackof u N allocate N KB for the SLG unification table copy stack r turn off automatic stack expansion g gc_type choose the garbage collection none sliding or copying CHAPTER 3 SYSTEM DESCRIPTION 20 S
74. ables enclosed in parentheses if N is greater than zero A term indicator may optionally be prefixed by the module name thus it can be of the form M Term Example usermod append _ e A module M becomes a current i e known module as soon as it is loaded in the system or when another module that is loaded in the system imports some predicates from module M Note that due to the dynamic loading of XSB a module can be current even if it has not been loaded and that some predicates of that module may not be defined In fact a module can be current even if it does not exist This situation occurs when a predicate is improperly imported from a non existent module Despite this a module can never lose the property of being current current_input Stream Succeeds iff stream Stream is the current input stream or procedurally unifies Stream with the current input stream There are no error conditions for this predicate current_output Stream Succeeds iff stream Stream is the current output stream or procedurally unifies Stream with the current output stream There are no error conditions for this predicate current_module Module The standard predicate current _module 1 allows the user to check whether a given module is current or to generate through backtracking all currently known modules Succeeds iff Module is one of the modules in the database This includes both user modules and system modules Note that predicat
75. ace character terminating a macro call or a comment string is left in the input stream for further processing If the argument is off or 0 this feature is disabled e mode charset id op par string Specify the character sets to be used for matching the o O and i special sequences The first argument must be one of id the set matched by i op the set matched by o or par the set matched by O in addition to the one matched by lo string is a C string which lists all characters to put in the set It may contain only the special matching sequences a A b B and the other sequences and the negated sequences are not allowed When a is found inbetween two non special characters this adds all characters inbetween e g A Z corresponds to all uppercase characters To have in the matched set either put it in first or last position or place it next to a x sequence A 7 Examples Here is a basic self explanatory example in standard or cpp mode APPENDIX A GPP GENERIC PREPROCESSOR 146 define FOO This is define BAR a message define concat 1 2 concat FOO BAR ifeq concat foo bar foo bar This is output ttelse This is not output endif Using argument naming the concat macro could alternately be defined as define concat x y x y In TeX mode and using argument naming the same example becomes define F00 This is define BAR a message define concat x
76. age directory do not need to have the same name but it is convenient to follow the above naming schema 4 Now if you need to invoke the foobar application you can simply type foobar at the XSB prompt This is because both and my_lib foobar have already been automatically added to the library search path 5 If your application files export many predicates you can simplify the use of your package by having my_1ib foobar P import all these predicates renaming them and then exporting them This provides a uniform interface to the foobar module since all the package predicates are can now be imported from just one module foobar CHAPTER 3 SYSTEM DESCRIPTION 18 In addition to adding the appropriate directory to the library search path the predicate bootstrap_userpackage 3 also adds information to the predicate package_configuration 3 so that other applications could query the information about loaded packages Packages can also be unloaded using the predicate unload_package 1 For instance unload_package foobar removes the directory my_lib foobar from the library search path and deletes the associated information from package_configuration 3 3 4 2 Dynamically loading predicates in the interpreter Modules are usually loaded into an environment when they are consulted see section 3 7 Specific predicates from a module can also be imported into the run time environment through the standard predicate import PredList
77. ans of an indexing directive This is a directive of the form index Functor Arity IndexArg indicating that an index should be created for predicate Functor Arity on its IndexArg argu ment One may also use the form index Functor Arity IndexArg HashTableSize which allows further specification of the size of the hash table to use for indexing this predicate if it is a dynamic i e asserted predicate For predicates that are dynamically loaded this directive can be used to specify indexing on more than one argument or indexing on a combination of arguments see its description on page 106 For a compiled predicate the size of the hash table is computed automatically so HashTableSize is ignored All of the values Functor Arity IndexArg and possibly HashTableSize should be ground in the directive More specifically Functor should be an atom Arity an integer in the range 0 255 and IndexArg an integer between 0 and Arity If IndexArg is equal to 0 then no index is created for that predicate An index directive may be placed anywhere in the file containing the predicate it refers to As an example if we wished to create an index on the third argument of predicate foo 5 the compiler directive would be CHAPTER 3 SYSTEM DESCRIPTION 34 index foo 5 3 Unification Factoring When the clause heads of a predicate have portions of arguments common to several clauses indexing on the principal functor of one argu
78. as bagSum Call Var bagReduce Call Var sum 0 CHAPTER 6 STANDARD PREDICATES 90 bagCount SetPred Arg HiLog Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate bagCount returns with Arg bound to the count i e number of elements of the set SetPred To use this predicate it must be imported from aggregs and you must give the following definitions in the main module usermod hilog successor successor X _Y Z Z is X 1 These decarations are necessary because of a current limitation in how HiLog predicates can be used This requirement will be lifted in a future release With this definition bagCount 2 can be and is defined as bagCount Call Var bagReduce Call Var successor 0 bagAvg SetPred Arg HiLog Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate bagAvg returns with Arg bound to the average i e mean of elements of the set SetPred To use this predicate it must be imported from aggregs and you must give the following definitions in the main module usermod hilog sumcount sumcount S C X S1 C1 S1 is S X C1 is C 1 These decarations are necessary because of a current limitation in how HiLog predicates can be used This requirement will be lifted in a future release With this definition bagAvg 2 can be and is defined as bagAvg Call Avg bagReduce Call Sum Count sumcount 0 0 Avg is Sum Count
79. as input to read 1 the terminating full stop must be explicitly sent to the current output stream Predicate write 1 treats terms of the form VAR N specially it writes A if N 0 B if N 1 Z if N 25 A1 if N 26 etc Terms of this form are generated by numbervars 1 3 described in the section Library Utilities in Volume 2 write Stream Term write 2 has the same behavior as write 1 but the output stream is explicitly designated using the first argument writeln Term writeln Term can be defined as write Term nl CHAPTER 6 STANDARD PREDICATES 67 writeln Stream Term writeln Term can be defined as write Stream Term nl Stream display Term The HiLog term Term is displayed on the terminal standard output stream according to the operator declarations in force In other words display 1 is similar to write 1 but the result is always written on userout Like write 1 display 1 always succeeds without producing an error After returning from a call to this predicate the current output stream remains unchanged write_prolog Term HiLog This predicate acts as does write 1 except that any HiLog term Term is written as a Prolog term write_prolog 1 outputs Term according to the operator declarations in force Because of this it differs from write_canonical 1 described below despite the fact that both predicates write HiLog terms as Prolog terms write_prolog Stream Term HiL
80. as the parameter for compile 1 2 In this case the compiler will compile all the files in FileList with the compiler options specified in OptionList but see Section 3 8 2 below for the precise details compile Files is just a notational shorthand for the query compile Files The standard predicates consult 1 2 call compile 1 if necessary Argument File can be any syntactically valid UNIX or Windows file name in the form of a Prolog atom but the user can also supply a module name The list of compiler options OptionList if specified should be a proper Prolog list i e a term of the form option optiong option J CHAPTER 3 SYSTEM DESCRIPTION 25 where option is one of the options described in Section 3 8 2 The source file name corresponding to a given module is obtained by concatenating a directory prefix and the extension P or c to the module name The directory prefix must be in the dynamic loader path see Section 3 4 Note that these directories are searched in a predetermined order see Section 3 4 so if a module with the same name appears in more than one of the directories searched the compiler will compile the first one it encounters In such a case the user can override the search order by providing an absolute path name If File contains no extension an attempt is made to compile the file File P or File c before trying compiling the file with name File We recommend use o
81. ate Module Functor Arity This succeeds once for every predicate that is loaded in XSB s database To find the predicates having arity 3 that are loaded in the global modules of the system use current_predicate Functor 3 while to find all predicates loaded in the global modules of the system regardless of their arity use current_predicate Predicate current_predicate Name Term_Indicator Succeeds iff Term_Indicator is the most general term corresponding to one of the predicates having functor Name that are defined and loaded in a particular module in the database The module can be either system or user defined Or procedurally current_predicate 2 unifies Name with the name of a loaded predicate and Term_Indicator with the most general term corresponding to that predicate The flavours of this predicate are analogous to those of current_predicate 1 and behave according to whether Term_Indicator has one of the following two forms 1 Module Term 2 Term module is assumed to be global or usermod If Term_Indicator is uninstantiated then this predicate succeeds only for global modules Like current_predicate 1 only predicates that have a property in the following set The only limitation is that blah must indeed be a module in the sense that it exports at least one symbol CHAPTER 6 STANDARD PREDICATES 98 loaded dynamic foreign see predicate_property 2 below are reported For example if pr
82. atom_codes 2 atom expected but something else found Aborting atom_codes X 52 300 49 Range error in argument 2 of atom_codes 2 ASCII code expected but 300 found Aborting Exceptions instantiation_error Both arguments are uninstantiated or argument 2 is not a proper list or it contains a variable type_error Atom is not a variable or an atom range_error CharList is not a list of ASCII characters atom_chars Number CharAtomList Like atom_codes but the list returned or input is a list of characters as atoms rather than ASCII codes For instance atom_chars abc X binds X to the list a b c instead of 97 98 99 number_codes Number CharCodeList The standard predicate number_codes 2 performs the conversion between a number and its character list representation If Number is supplied and is a number CharList is unified with a list of ASCII codes comprising the printed representation of that Number If on the other hand Number is a variable then CharList must be a proper list of ASCII character codes that corresponds to the correct syntax of a number either integer or float In that case Number is instantiated to that number otherwise number_codes 2 will simply fail If both of the arguments of number_codes 2 are uninstantiated or CharList is not a proper list of ASCII characters number_codes 2 aborts and an error message will be sent to the standard error stream Examples number_codes 123
83. b c arg 2 HiLog A A 105 108 111 103 arg 2 atbtc A Azc arg 3 X a b c A CHAPTER 6 STANDARD PREDICATES 78 X _595820 A b arg 2 map f a b A Aza arg 1 map f a b A A map f arg 1 atb foo bar A A atb arg0 Index Term Argument Unifies Argument with the Index argument of Term if Index gt 0 or with the functor of Term if Index 0 hilog_arg Index Term Argument HiLog If Term is a Prolog term it has the same behaviour as arg 3 but if Term is a proper HiLog term hilog_arg 3 unifies Argument with the Index 1 argument of the Prolog representation of Term Semantically Argument is the Index argument to which the HiLog functor of Term is applied The arguments of the Term are numbered from 1 upwards An atomic term is taken to have O arguments Initially Index must be instantiated to a positive integer and Term to any non variable Prolog or HiLog term If the initial conditions are not satisfied or I is out of range the call quietly fails Note that like arg 3 this predicate does not succeed for Index 0 Examples hilog_arg 2 p a b A A b hilog_arg 2 h a b A A b N hilog_arg 3 X a b c A X _595820 A c hilog_arg 1 map f a b A Aza hilog_arg 2 map f a b A A b hilog_arg 1 atb foo bar A A foo hilog_arg 1 apply foo A A foo hil
84. b_configuration perl_support yes xsb_configuration perl_archlib usr 1lib per15 i386 linux 5 00404 xsb_configuration perl_cc_compiler cc xsb_configuration perl_ccflags Dbool char DHAS_BOOL I usr local include xsb_configuration perl_libs lnsl lndbm lgdbm ldb ldl 1m lc lposix 1crypt xsb_configuration javac usr bin javac Tells where XSB is currently residing can be moved xsb_configuration install_dir InstallDir User home directory Usually HOME If that is null then it would be the directory where XSB is currently residing This is where we expect to find the xsb directory xsb_configuration user_home Home Where XSB invocation script is residing xsb_configuration scriptdir ScriptDir where are cmplib syslib lib packages etc live xsb_configuration cmplibdir CmplibDir CHAPTER 6 STANDARD PREDICATES 103 xsb_configuration libdir LibDir xsb_configuration syslibdir SyslibDir xsb_configuration packagesdir PackDir xsb_configuration etcdir EtcDir architecture and configuration specific directories xsb_configuration config_dir ConfigDir xsb_configuration config_libdir ConfigLibdir site specific directories xsb_configuration site_dir usr local XSB site xsb_configuration site_libdir SiteLibdir site and configuration specific directories xsb_co
85. behavior inside meta macro calls including user macro definitions since these come inside a define meta macro call the second character corresponds to the behavior inside user macro parameters and the third character corresponds to the behavior outside of any macro call Each of these characters can take the following values e i disable the comment string specification e c comment neither evaluated nor output e s string the string and its delimiter sequences are output as is q quoted string the string is output as is without the delimiter sequences e C evaluated comment macros are evaluated but output is discarded e S evaluated string macros are evaluated delimiters are output APPENDIX A GPP GENERIC PREPROCESSOR 140 e Q evaluated quoted string macros are evaluated delimiters are not output Important note any occurrence of a comment string start sequence inside another comment string is always ignored even if macro evaluation is enabled In other words comments strings cannot be nested In particular the Q modifier can be a convenient way of defining a syntax for temporarily disabling all comment and string specifications Syntax specification strings should always be provided as C strings whether they are given as arguments to a mode meta macro call or on the command line of a Unix shell If command line arguments are given via another method than a standard Unix shell then the shell be
86. bout the function and arity of a predicate If p 2 is a predicate its skeleton can be represented as p arg1 arg2 where arg1 and arg can be any instantiation pattern Thus the information derived from the skeletons p 1 2 and p A B would be the same The user should be aware that skeletons which are dynamically created e g by functor 3 are located in usermod cf Section 3 3 In such a case the tabling predicates below may not behave in the desired manner if the tabled predicates themselves have not been imported into usermod get_calls Skeleton Subgoal Structure Pointer Return_Skeleton Tabling Backtracks through the subgoal trie unifying Skeleton with entries in the call trie As it does so get_calls 3 binds Subgoal_Structure Pointer with the pointer to the subgoal structure and binds Return_Skeleton to a term of the form ret n where each of its argu ments corresponds to a free variable in the call Example For the following program and table table p 2 p 1 2 p 1 3 p 1 X p 2 3 calls to get_calls 3 will act as follows get_calls p X Y Cs Ret X _864816 Y 3 Cs 2927152 CHAPTER 6 STANDARD PREDICATES 113 Ret ret _864816 X i Y _864644 Cs 2927104 Ret ret _864644 no get_calls p 1 Y Cs Ret Y 3 Cs 2927152 Ret ret 1 Y _864620 Cs 2927104 Ret ret _864620 no get_calls p Y 3 Cs Ret Y _864792 Cs 2927152 Ret ret _864792
87. by Bart Demoen and Kostis Sagonas Rui Marques improved the trailing of the SLG WAM and rewrote much of the engine to make it compliant with 64 bit architectures Assert and retract code was based on code written by Jiyang Xu and significantly revised by David Warren and Rui Marques Trie assert and retract code was written by Prasad Rao The current version of findall 3 was re written from scratch by Bart Demoen In the XSB complier Kostis Sagonas was responsible for HiLog compilation and associ ated builtins Steve Dawson implemented Unification Factoring The auto_table and suppl_table directives were written by Kostis Sagonas The DCG expansion module was written by Kostis Sagonas The handling of the multifile directive was written by Baoqiu Cui C R Ramakrishnan wrote the mode analyzer for XSB The safety check for tabling within the scope of cuts was written by Kate Dvortsova Michael Kifer rewrote parts of the XSB code to make XSB configurable with GNU s Autoconf Harald Schroepfer helped the XSB group with the Solaris port and Yiorgos Adamopoulos suggested the bits to use for the HP 700 series port Steven Dawson Larry B Daniel and Franklin Chen were responsible for the MkLinux and Solaris x86 ports GPP the source code preprocessor used by XSB was written by Denis Auroux He also wrote the GPP manual reproduced in Appendix A The starting point of XSB in 1990 was PSB Prolog 2 0 by Jiyang Xu PSB Prolog in its turn was based
88. can be invoked by the command XSB_DIR bin xsb if you have installed XSB in your private directory If XSB is instaled in a shared directory e g SHARED_XSB for the entire site UNIX only then you should use SHARED_XSB bin xsb In both cases you will find yourself in the top level interpreter As mentioned above this script automatically detects the system configuration you are running on and will use the right files and executables Of course XSB should have been built for that architecture earlier Under Windows you should invoke XSB by typing XSB_DIR config x86 pc windows bin xsb exe You may want to make an alias such as xsb to the above commands for convenience or you might want to put the directory where the XSB command is found in the PATH environment variable However you should not make hard links to this script or to the XSB executable If you invoke XSB via such a hard link XSB will likely be confused and will not find its libraries That said you can create other scripts and cal the above script from there Most of the standard Prolog predicates are supported by XSB so those of you who consider yourselves champion entomologists can try to test them for bugs now Details are in Chapter 6 CHAPTER 2 GETTING STARTED WITH XSB 10 2 4 Compiling XSB programs All source programs should be in files whose names have the suffix P One of the ways to compile a program from a file in the current directory and
89. ccss o x ccss nyu Xp ccii m n Wwe s LAN wu noun s Ig mou nn e c lt n gt strl str2 Specify comments Any unquoted occurrence of str1 will be interpreted as the beginning of a comment All input up to the first following occurrence of str2 will be discarded This option may be used multiple times to specify different types of comment delimiters The optional parameter lt n gt can be specified to alter the behavior of the comment and e g turn it into a string or make it ignored under certain circumstances see below e c strl Un specify comments or strings The comment string specification whose start sequence is stri is removed This is useful to alter the built in comment specifications of a standard mode e g the cpp compatibility mode e s lt n gt strl str2 c Specify strings Any unquoted occurrence of stri will be interpreted as the beginning of a string All input up to the first following occurrence of str2 will be output as is without any evaluation The delimiters themselves are output If c is non empty its first character is used as a string quote character i e a character whose presence immediately before an occurrence APPENDIX A GPP GENERIC PREPROCESSOR 138 of str2 prevents it from terminating the string The optional parameter lt n gt can be specified to alter the behavior of the string and e g turn it into a comment enable macro evaluation inside the string or make the string specification
90. ception is when the macro definition is empty in which case its arguments are not evaluated Note that gpp temporarily switches back to the mode in which the macro was defined in order to evaluate it so it is perfectly safe to change the operating mode between the time when a macro is defined and the time when it is called Conversely if a user macro wishes to work with the current mode instead of the one that was used to define it it needs to start with a mode restore call and end with a mode save call A user macro may be defined with named arguments see define description below In that case when the macro definition is being evaluated each named parameter causes a temporary virtual user macro definition to be created such a macro may only be called without arguments and simply returns the text of the corresponding argument Note that since macros are evaluated when they are called rather than when they are defined any attempt to call a recursive macro causes undefined behavior except in the very specific case when the macro uses undef to erase itself after finitely many loop iterations APPENDIX A GPP GENERIC PREPROCESSOR 142 Finally a special case occurs when a user macro whose definition does not involve any arguments neither named arguments nor the argument reference sequence is called in a mode where the short user macro end sequence is empty e g cpp or TeX mode In that case it is assumed to be an alias macro its argum
91. conditional block Everything that follows is evaluated only if the results of the evaluations of z and y are identical as character strings Any leading or trailing whitespace is ignored for the comparison Note that in cpp mode any unquoted whitespace character is understood as the end of the first argument so it is necessary to be careful e ifneq x y This begins a conditional block Everything that follows is evaluated only if the results of the evaluations of z and y are not identical even up to leading or trailing whitespace e else This toggles the logical value of the current conditional block What follows is evaluated if and only if the preceding input was commented out e endif This ends a conditional block started by a if meta macro e include file This causes gpp to open the specified file and evaluate its contents inserting the resulting text in the current output All defined user macros are still available in the included file and reciprocally all macros defined in the included file will be available in everything that follows The include file is looked for first in the current directory and then if not found in one of the directories specified by the 7 command line option or usr include if no directory was specified Note that for compatibility reasons it is possible to put the file name between or lt gt Upon including a file gpp immediately saves a copy of the current operating mode onto the
92. d a new answer if it was minimal termination could be ensured The XSB predicate filterReduce Pred Binary_operator Identity Value does just this Exercise 5 4 2 The use of filterReduce 4 can be seen most easily through an example such as the following which uses a closely related predicate filterReduce1 4 shorter_path X Y C filterReducel sp X Y min infinity C sp X Y C shorter_path X Z C1 edge Z Y C2 C is C1 C2 sp X Y C edge X Y C min X Y Y number X min X Y X number Y min One Two Min One gt Two gt Min Two Min One Note that the library predicate filterReduce1 4 is tabled so that neither sp 3 nor shorter_path 3 need be tabled Now try the query shorter_path 1 5 C filterReducel Pred Binary_operator Identity Value forms a new predicate out of Pred and Value to get a new predicate to call Binary_Operator must define a binary function in CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 61 which the first two arguments determine the third Id must be the identity of Binary_operator Value becomes the result of applying Op to all the elements in the table that are variants of Pred In our case when a new answer sp X Y C is derived within filterReduce1 4 the later predicate returns only when C is a shorter path for X and Y than any so far derived While shorter_path 4 terminates it returns non optimal solutions and these solutions can in principle be co
93. d errors may occur due to the fact that the space of the old code is reclaimed and may be used for other purposes e Currently term comparisons lt lt gt and gt do not work for terms that overflow the C recursion stack terms that contain more than 10 000 variables and or function symbols Appendix A GPP Generic Preprocessor Version 2 0 c Denis Auroux 1996 99 http www math polytechnique fr cmat auroux prog gpp html As of version 2 1 XSB uses gpp as a source code preprocessor for Prolog programs This helps maintain consistency between the C and the Prolog parts of XSB through the use of the same h files In addition the use of macros improves the readability of many Prolog programs especially those that deal with low level aspects of XSB Chapter 3 8 explains how gpp is invoked in XSB A 1 Description gpp is a general purpose preprocessor with customizable syntax suitable for a wide range of pre processing tasks Its independence on any programming language makes it much more versatile than cpp while its syntax is lighter and more flexible than that of m4 gpp is targeted at all common preprocessing tasks where cpp is not suitable and where no very sophisticated features are needed In order to be able to process equally efficiently text files or source code in a variety of languages the syntax used by gpp is fully customizable The handling of comments and strings is especially advanced Initially
94. d rather than false It is thus is sound but not complete for terminating programs to the well founded semantics Answer completion is not available for Version 2 1 of XSB as it is expensive and the need for answer completion arises rarely in practice However answer completion will be included at some level in future versions of XSB 5 3 2 On Beyond Zebra Implementing Other Semantics for Non stratified Pro grams The Well founded semantics is not the only semantics for non stratified programs XSB can be used to help implement other semantics that lie in one of two classes 1 Semantics that extend the well founded semantics to include new program constructs or 2 semantics that contain the well founded partial model as a submodel An example of a semantics of class 1 is WFSX 2 which adds explicit or provable negation to the default negation used by the Well founded semantics The addition of explicit negation in WFSX can be useful for modeling problems in domains such as diagnosis and hierarchical reasoning or domains that require updates 26 as logic programs WFSX is embeddable into the well founded semantics and this embedding gives rise to an XSB meta interpreter or more efficiently to the preprocessor described in Section Extended Logic Programs in Volume 2 See 42 for an overview of the process of implementing extensions of the well founded semantics An example of a semantics of class 2 is the stable model semantics
95. d semantics win 1 is undefined Informally true answers in the well founded semantics are those that have a tabled derivation False answers are those for which all possible derivations fail either finitely as in Prolog or by failing positive loops win 1 fits in neither of these cases there is no proof of win 1 yet it does not fail in the sense given above and is thus undefined However this explanation does not account for why undefined answers should be represented as conditional answers or why a query with a conditional answer and its negation should both succeed These features arise from the proof strategy of XSB which we now examine in more detail Exercise 5 3 5 Consider the program table simpl_p 1 simpl_r 0 simpl_s 0 simp1_p X tnot simpl_s simpl_s tnot simpl_r simpl_s simpl_p X simpl_r tnot simpl_s simpl_r Is simpl_p X true for any X Try the query simpl p X be sure to backtrack through all possible answers Now try the query again What could possibly account for this behavior At this point it is worthwhile to examine closely the evaluation of the program in Exercise 5 3 5 The query simpl_p X calls simpl_s and simpl_r and executes the portion of the program shown below in bold simpl_p X tnot simpl s simpl s tnot simpl r simpl_s simpl_p X simpl_r tnot simpl s simplr CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 56 Based on evaluating only
96. d using the executable directives index 3 and index 2 For instance dynamic clauses can be declared to have multiple or joint indexes while this indexing can be either hash based as is typical in Prolog systems or based on tries No matter what kind of indexing is used space is dynamically allo cated when a new clause is asserted and unless specified otherwise released when it is retracted Furthermore the size of any index table expands dynamically as clauses are asserted Consider first dynamic predicates that use traditional hash based indexing XSB asserts WAM code for such clauses leading to execution times similar to compiled code for unit and binary clauses Furthermore tabling can be used with a dynamic predicate by explicitly declaring a predicate to be both dynamic and tabled For clauses that are asserted as WAM code the immediate semantics of dynamic predicates is used not the so called logical semantics of assert retract 27 This means that significant care must be taken when modifying the definition of a predicate which is currently being executed Notice that this makes some operations difficult For example one might try to retract from dynamically asserted predicates p 1 and q 1 exactly their intersection by issuing the following query p X q X retract p X retract q X fail CHAPTER 6 STANDARD PREDICATES 105 Neither retract 1 nor retractall 1 support this behavior due to their techniques f
97. de these strings Also note that the first argument of mode the keyword is never evaluated while the second argument is evaluated except of course for the contents of C strings so that the syntax specification may be obtained as the result of a macro evaluation The available mode commands are e mode save mode push Push the current mode specification onto the mode stack e mode restore mode pop Pop mode specification from the mode stack e mode standard name Select one of the standard modes The only argument must be one of default default mode cpp C cpp mode tex TeX tex mode html HTML html mode prolog Prolog prolog mode The mode name must be given directly not as a C string e mode user sl s9 Specify user macro syntax The 9 arguments all of them C strings are the mode specification for user macros see the U command line option and the section on syntax specification The meta macro specification is not affected APPENDIX A GPP GENERIC PREPROCESSOR 145 e mode meta user sl s7 Specify meta macro syntax Either the only argument is user not as a string and the user macro mode specifications are copied into the meta macro mode specifications or there must be 7 string arguments whose significance is the same as for the M command line option see section on syntax specification MA e mode quote c With no argument or as argument r
98. declarations do not show up while running the predicates interactively unexpected behaviour may be witnessed in compiled code optimised to take modes into account currently not performed by XSB However despite this danger mode annotations can be a good source of documentation since they express the programmer s intention of data flow in the program Tabling Directives Memoization is often necessary to ensure that programs terminate and can be useful as an opti mization strategy as well The underlying engine of XSB is based on SLG a memoization strategy which in our version maintains a table of calls and their answers for each predicate declared as tabled Predicates that are not declared as tabled execute as in Prolog eliminating the expense of tabling when it is unnecessary The simplest way to use tabling is to include the directive auto_table anywhere in the source file auto_table declares predicates tabled so that the program will termi nate To understand precisely how auto_table does this it is necessary to mention a few properties of SLG For programs which have no function symbols or where function symbols always have a limited depth SLG resolution ensures that any query will terminate after it has found all correct answers In the rest of this section we restrict consideration to such programs Obviously not all predicates will need to be tabled for a program to terminate The auto_table compiler directive tab
99. declarations that select only a single chain rule of the predicate spec_dump Generates a module spec file containing the result of specialising partially instantiated calls to predicates defined in the module under compilation The result is in Prolog source code form ti_dump Generates a module ti file containing the result of applying unification factoring to pred icates defined in the module under compilation The result is in Prolog source code form See page 34 for more information on unification factoring ti_long names Used in conjunction with ti_dump generates names for predicates created by uni fication factoring that reflect the clause head factoring done by the transformation modeinfer This option is used to trigger mode analysis For each module compiled the mode analyzer creates a module D file that contains the mode information WARNING Occasionally the analysis itself may take a long time As far as we have seen the analysis times are longer than the rest of the compilation time only when the module CHAPTER 3 SYSTEM DESCRIPTION 28 contains recursive predicates of arity gt 10 If the analysis takes an unusually long time say more than 4 times as long as the rest of the compilation you may want to abort and restart compilation without modeinfer mi_warn During mode analysis the D files corresponding to the imported modules are read in The option mi_warn is used to generate warning messages if these D files a
100. descent parsing while efficiently implementable is known to suffer from several deficiencies 1 its time can be exponential in the size of the input and 2 it may not terminate for certain context free grammars in particular those that are left or doubly recursive By appropriate use of tabling both of these limitations can be overcome With appropriate tabling the resulting parsing algorithm is a variant of Earley s algorithm and of chart parsing algorithms In the simplest cases one needs only to add the directive auto_table see Section 3 8 4 to the source file containing a DCG specification This should generate any necessary table declara tions so that infinite loops are avoided for context free grammars That is with a auto_table declaration left recursive grammars can be correctly processed Of course individual table direc tives may also be used but note that the arity must be specified as two more than that shown in the DCG source to account for the extra arguments added by the expansion However due to our current implementation of structures in tabling there are new inefficiencies that can arise In particular when using the standard list representation of the input string in a DCG there may be a large amount of copying and a great deal of space used What happens is that the input string i e list may be copied into and out of the table many times To avoid this problem the input list can be transformed into a
101. e Term2 is simply Termi unchanged If Termi1 is not of the proper form or Term2 does not unify with its clausal form predicate expand_term 2 simply fails Users may override the standard transformations performed by predicate expand_term 2 by defining their own compile time transformations This can be done by defining clauses for the predicate term_expansion 2 When a term Term1 is read in when a file is being compiled or consulted expand_term 2 calls term_expansion 2 first if the expansion succeeds the transformed term so obtained is used and the standard grammar rule expansion is not tried otherwise if Termi is a grammar rule then it is expanded using dcg 2 otherwise Term1 is used as is Note that predicate term_expansion 2 must be defined in the XSB s default read in module usermod and should be loaded there before the compilation begins C L1 Terminal L2 This predicate generally is of no concern to the user Rather it is used in the transformation of terminal symbols in grammar rules and expresses the fact that L1 is connected to L2 by the terminal Terminal This predicate is needed to avoid problems due to source level transformations in the presence of control primitives such as cuts 0 or if then elses gt 2 and is defined by the single clause C Token Tokens Token Tokens The name C was chosen for this predicate so that another useful name might not be pre empted dcg DCG_ Rule Prol
102. e to trace all calls to predicates in the program Also the Prolog code should be static In other words the user is not allowed to alter the entry point of these compiled predicates by asserting new clauses As expected the compilation phase will also be slightly longer For these reasons the use of the optimize option may not be suitable for the development phase but is recommended once the code has been debugged xpp on Filter the program through a preprocessor before sending it to the XSB compiler By default and for the XSB code itself XSB uses GPP a preprocessor developed by Denis Auroux auroux math polytechnique fr that has high degree of compatibility with the C preprocessor but is more suitable for Prolog syntax In this case the source code can include the usual C preprocessor directives such as define ifdef and include This option can be specified both as a parameter to compile 2 and as part of the compiler_options 1 directive inside the source file See Appendix A for more details on GPP When an include file statement is encountered XSB directs GPP preprocessor to search for the files to include in the directories XSB_DIR emu and XSB_DIR prolog_includes However additional directories can be added to this search path by asserting into the predi cate xpp_include_dir 1 which should be imported from module parse XSB predefines the constant XSB_PROLOG which can be used for conditional compilation For instance yo
103. e OR X Y bitwise AND X bitwise negate XxX gt gt Y logical shift right X lt lt Y logical shift left float X floor X exp X log X logarithm with base e log10 X logarithm with base 10 sqrt X asin X acos X APPENDIX B STANDARD PREDICATES AND FUNCTIONS 158 atan X B 3 List of Standard Operators The following operators are provided with the XSB system op 1200 op 1200 op 1198 op 1150 op 1100 op 1100 op 1100 op 1050 op 1050 op 1050 op 1000 op 900 op 700 op 661 op 600 op 500 op 500 op 400 op 200 xfx fx xfx xfx fy fx xfx fy xfx xfy xfy fy xfx xfy xfy yfx fx y x xfy mom mmm a a 9 a foe ad SeS hilog dynamic multifile index ti ti_off table edb mode export local parallel using import from gt a 1 not spy nospy 5 Oe lt 0 gt is lt lt gt gt 32 2 1 1 MY J tyna mod lt lt gt gt 1 J xi while the following is the list of operators in the Prolog draft standard that are not provided op 1150 fx op 400 yfx op 200 xfx discontiguous rem Appendix C List of Module names C 1 In syslib assert basics db dbclause domain eval machine num_vars tables term_exp xsb_writ
104. e actually needed saving program space for large Prolog applications The delay in the loading is done automatically unlike other systems where it must be explicitly specified for non system libraries When a predicate imported from another module see section 3 3 is called during execution the dynamic loader is invoked automatically if the module is not yet loaded into the system The default action of the dynamic loader is to search for the byte code file of the module first in the system library directories in the order lib syslib and then cmp1ib and finally in the current working directory If the module is found in one of these directories then it will be loaded on a first found basis Otherwise an error message will be displayed on the current output stream reporting that the module was not found In fact XSB loads the compiler and most system modules this way Because of dynamic loading the time it takes to compile a file is slightly longer than usual the first time the compiler is invoked in a session 3 4 1 Changing the Default Search Path and the Packaging System Users are allowed to supply their own library directories and also to override the default search path of the dynamic loader User supplied library directories are searched by the dynamic loader before searching the default library directories The default search path of the dynamic loader can easily be changed by having a file named xsb xsbrc P in the user s home
105. e are transformed into SLG WAM instructions The main differences are that compiled code may be more optimized than interpreted code and that compilation produces an object code file This section describes the actions of the standard predicate consult 1 2 and of reconsult 1 2 which is defined to have the same actions as consult 1 2 consult 1 2 is the most conve nient method for entering rules into XSB s database Though consult comes in many flavors the most general form is consult FileList CompilerOptionList At the time of the call both of its arguments should be instantiated ground FileList is a list of filenames or module names see section 3 3 and CompilerOptionList is a list of options that are to be passed to the compiler when and if it should be invoked For a detailed description of the format and the options that can appear in this list see Section 3 8 If the user wants to consult one module file only she can provide an atom instead of a list for the first argument of consult 2 Furthermore if there isn t any need for special compilation options the following two forms FileName consult FileName are just notational shorthands for consult FileName CHAPTER 3 SYSTEM DESCRIPTION 23 Consulting a module file generally consists of the following five steps which are described in detail in the next paragraphs Name Resolution determine the module to be consulted Compilation
106. e compiled temp dir If you are using the GNU C compiler consult its manual pages to find out how you can change the default tmp directory or how you can use pipes to avoid the use of temporary space during compiling Usually changing the default directory can be done by declaring modifying the TMPDIR environment variable as follows setenv TMPDIR dir CHAPTER 2 GETTING STARTED WITH XSB 8 Missing XSB Object Files When an object O file is missing from the lib directories you can normally run the make command in that directory to restore it instructions for doing so are given in Chapter 2 However to restore an object file in the directories syslib and cmplib one needs to have a separate Prolog compiler accessible such as a separate copy of XSB because the XSB compiler uses most of the files in these two directories and hence will not function when some of them are missing For this reason distributed versions normally include all the object files in syslib and cmplib 2 2 Installing XSB under Windows 2 2 1 Using Cygnus Softwares CygWin32 This is easy just follow the Unix instructions This is the preferred way to run XSB under Windows because this ensures that all features of XSB are available 2 2 2 Using Microsoft Visual C 1 XSB will unpack into a subdirectory named xsb Assuming that you have XSB ZIP in the XSB_DIR directory you can issue the command unzip386 xsb zip which will install XSB in the subdirecto
107. e current_module 1 succeeds for a given module even if that module is not a real module in the sense taht it does not export any predicates There are no error conditions associated with this predicate if its argument does not unify with one of the current modules current_module 1 simply fails current_module Module 0bjectFile Predicate current _module 2 gives the relationship between the modules and their associated object file names The file name ObjectFile must be absolute and always ends with 0 It is possible for a current module to have no associated file name as is the case for modules like usermod and global or for the system to be unable to determine the file name of CHAPTER 6 STANDARD PREDICATES 95 a current module In both cases predicate current_module 1 will succeed for this module while current _module 2 will fail The system is unable to determine the file name of a given module if that module is not in one of the directories of the search path see Section 3 4 Once again there are no error conditions associated with this predicate if the arguments of current_module 2 are not correct or Module has no associated File the predicate will simply fail current_atom Atom_Indicator Generates through backtracking all currently known atoms and unifies each in turn with Atom_Indicator current_functor Predicate_Indicator The standard predicate current_functor 1 can be used to find all the currently
108. e subject of stratification we note that the concepts of stratification also underly XSB s evaluation of tabled findall tfinda11 3 Here the idea is that a program is stratified if it contains no loop through tabled findall See the description of predicate tfindal1 3 on page 85 5 3 1 Non stratified Programs As discussed above in stratified programs facts are either true or false while in non stratified programs facts may also be undefined XSB represents undefined facts as conditional answers Conditional Answers Exercise 5 3 4 Consider the behavior of the win 1 predicate from Exercise 5 3 1 win X move X Y tnot move Y CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 55 when the when the move 2 relation is a cycle Load the file XSB_DIR examplescycleik P into XSB and again type the query win 1 Does the query succeed Try tnot win 1 Now import get_residual 2 via the command import get_residual 2 from tables Can you guess what is happening with this non stratified program The predicate get_residual 2 Section 6 12 unifies its first argument with a tabled subgoal and its second argument with the possibly empty delay list of that subgoal The truth of the subgoal is taken to be conditional on the truth of the elements in the delay list Thus win 1 is conditional on tnot win 2 win 2 in tnot win 3 and so on until win 1023 which is conditional on win 1 From the perspective of the well founde
109. ed but not loaded There are no error conditions associated with this predicate if its argument is not what it should be the predicate simply fails Like current_functor 1 predicate current_predicate 1 comes in two flavours depending on the form of its argument Predicate_Indicator CHAPTER 6 STANDARD PREDICATES 97 1 If Predicate_Indicator has the form Module Functor Arity then the execution of current_predicate 1 unifies the predicate indicator with predicates in all current mod ules user defined system defined and global modules 2 If however Predicate_Indicator is uninstantiated or has the form Functor Arity then current _predicate 1 backtracks only through the predicates loaded in the global modules of the system in other words searches only modules usermod and global Since this is the common use of predicate current_predicate 1 this flavour is only for convenience Note that all the following are equivalent current_predicate Functor Arity current_predicate Predicate current_predicate usermod Predicate current_predicate global Predicate So to backtrack through all of the predicates defined and loaded in module blah regardless of whether blah is a system or a user defined module use current_predicate blah Predicate In this case Predicate will have the form Functor Arity To backtrack through all predicates defined and loaded in any current module use current_predic
110. edicate has the same semantics as setof 3 except that the third argument returns an unordered list that may contain duplicates Exceptions Same as predicate ca11 1 see Section 6 8 findall X Goal List Similar to predicate bagof 3 except that variables in Goal that do not occur in X are treated as existential and alternative lists are not returned for different bindings of such variables This makes finda11 3 deterministic non backtrackable Unlike setof 3 and bagof 3 if Goal is unsatisfiable finda11 3 succeeds binding List to the empty list Exceptions Same as predicate ca11 1 see Section 6 8 tfindall X Goal List Tabling Like finda11 3 tfindal1 3 treats all variables in Goal that do not occur in X as existential However in tfindal1 3 the Goal must be a call to a single tabled predicate tfindal11 3 allows the user to build programs that use stratified aggregation If the table to Goal is incomplete tfinda11 3 suspends until the table has been completed and only then computes List See Chapter 5 for further discussion of tfinda11 3 Like findal1 3 if Goal is unsatisfiable tfinda11 3 succeeds binding List to the empty list Some of the differences between predicates finda11 3 and tfindal1 3 can be seen from the following example user Compiling user table p 1 p a p b user compiled cpu time used 0 639 seconds user loaded yes p X findall Y p Y L ps a _922928
111. edicates foo 1 and foo 3 are defined and loaded into module blah the following query will return current_predicate foo blah Term Term foo _638788 638792 638796 Term fo00 _638788 no If a module is specified current_predicate 2 succeeds only for those predicates which are defined and loaded in that module Unless the module is one of the global modules current _predicate 2 fails for those predicates which are imported into that module On the other hand the goal current_predicate Name Term can be used to backtrack through every predicate that is loaded in the global modules of XSB s database Note that the order of term generation is undetermined Once again there are no error conditions associated with this predicate if its argument is not what it should be the predicate simply fails predicate_property Term_Indicator Property The standard predicate predicate_property 2 can be used to find the properties of any predicate which is visible to a particular module Succeeds iff Term_Indicator is a term indicator for a current predicate whose principal functor is a predicate having Property as one of its properties Or procedurally Property is unified with the currently known properties of the predicate having Term_Indicator as its skeletal specification A brief description of predicate_property 2 is as follows e If Term_Indicator is instantiated then Property is successively unified with the
112. egers is 2 278 1 For MIPS based machines e g Silicon Graphics machines the address space for integers is 276 276 1 For all other machines it is 2 2 1 This restriction can cause unexpected results when numbers are computed The amount of space allowed for floating point numbers is similar for each machine For 64 bit platforms addresses integers and floating point numbers are all stored in 60 bits However as the object code file format is the same as for the 32 bit versions compiled constants are subject to 32 bit limitations e Indexing on floating point numbers does not work since as implemented in XSB the seman tics of floating point unification is murky in the best case Therefore it is advisable that if 131 CHAPTER 9 RESTRICTIONS AND CURRENT KNOWN BUGS 132 9 2 you use floating point numbers in the first argument of a procedure that you explicitly index the predicate in some other argument The XSB compiler cannot distinguish the occurrences of a O ary predicate and a name of a module of an import declaration as two different entities For that reason it fails to characterise the same symbol table entry as both a predicate and a module at the same time As a result of this fact a compiler error is issued and the file is not compiled For that reason we suggest the use of mutually exclusive names for modules and 0 ary predicates though we will try to amend this restriction in fut
113. emoves the quote character specification and disables the quoting functionality With one string argument the first character of the string is taken to be the new quote character The quote character cannot be alphanumeric nor _ and cannot be one of the special matching sequences either 999 e mode comment xxx start end Pe c Add a comment specification Optionally a first argument consisting of three characters not enclosed in can be used to specify a comment string modifier see the section on syntax specification The default modifier is ccc The first two string arguments are used as comment start and end sequences respectively The third string argument is optional and can be used to specify a string quote character if it is the functionality is disabled The fourth string argument is optional and can be used to specify a string delimitation warning character if it is the functionality is disabled e mode string xxx start end Pe Pe Add a string specification Identical to mode comment except that the default modifier is 588 e mode nocomment mode nostring start With no argument remove all comment string specifications With one string argument delete the comment string specification whose start sequence is the argument e mode preservelf on off 1 0 Equivalent to the n command line switch If the argument is on or 1 any newline or whites p
114. en declared tabled Exceptions type error Argument 1 is not a callable predicate Chapter 7 Debugging 7 1 High Level Tracing XSB supports a version of the Byrd four port debugger for debugging Prolog code In this release Version 2 1 it does not work very well when debugging code involving tabled predicates If one only creeps see below the tracing can provide some useful information We do intend that future versions will have more complete debugging help for tabled evaluation To turn on tracing use trace 0 To turn tracing off use notrace 0 When tracing is on the system will print a message each time a predicate is 1 initially entered Call 2 successfully returned from Exit 3 failed back into Redo and 4 completely failed out of Fail At each port a message is printed and the tracer stops and prompts for input See the predicates show 1 and leash 1 described below to modify what is traced and when the user is prompted In addition to single step tracing the user can set spy points to influence how the trac ing debugging works A spy point is set using spy 1 Spy points can be used to cause the system to enter the tracer when a particular predicate is entered Also the tracer allows leaping from spy point to spy point during the debugging process The debugger also has profiling capabilities which can measure the cpu time spent in each call The cpu time is measured only down to 0 0001 th of a
115. ents are first evaluated in the current mode as usual but instead of being passed to the macro definition as parameters which would cause them to be discarded they are actually appended to the macro definition using the syntax rules of the mode in which the macro was defined and the resulting text is evaluated again It is therefore important to note that in the case of a macro alias the arguments actually get evaluated twice in two potentially different modes A 6 Meta macros These macros are always pre defined Their actual calling sequence depends on the current mode here we use cpp like notation e define x y This defines the user macro zas y y can be any valid gpp input and may for example refer to other macros x must be an identifier i e a sequence of alphanumeric characters and unless named arguments are specified If x is already defined the previous definition is overwritten If no second argument is given x will be defined as a macro that outputs nothing Neither z nor y are evaluated the macro definition is only evaluated when it is called not when it is declared It is also possible to name the arguments in a macro definition in that case the argument x should be a user macro call whose arguments are all identifiers These identifiers become available as user macros inside the macro definition these virtual macros must be called without arguments and evaluate to the corresponding macro parameter e defeval x
116. eprocessor 134 ALAS SDe SCrIptlony tsetse ae a ee AN A at ie ear As i ae a 134 A2 SOYA er ica RR 135 ARS OPOSITOR ie 135 AA Syntax Specification vt a att Gok a Ra Oh AA Gee aia 138 Ac Evaluation Rules sorora gia cena ats a A a AA a th ei a e 141 AsO Metas Macros 22s se A a A a AAA Be Gh et a oth I 142 ALC Exam ples gt vite sth hw DIAS AA Vide eh ae a IA A 145 A 8 Advanced Examples us ecnccis asa ee Re ee A 150 CONTENTS AO AMOO a DA A AS A AA Ne dee ee B Standard Predicates and Functions B 1 List of Standard Predicates e B 2 List of Standard Functions o oaa B 3 List of Standard Operators C List of Module names Gil Wn sys bein oie feo hw e a ee Ee Baw Ae Ri a 8 ot al ies 6 2 Un empleen y cet a e a a ae et e a de lv 152 153 153 157 158 Chapter 1 Introduction XSB is a research oriented Logic Programming system for Unix and Windows based systems In addition to providing all the functionality of Prolog XSB contains several features not usually found in Logic Programming systems including e Evaluation according to the Well Founded Semantics 44 through full SLG resolution e A compiled HiLog implementation e A variety of indexing techniques for asserted code along with a novel transformation technique called unification factoring that can improve program speed and indexing for compiled code e A number of interfaces to other software systems such a C Java Perl and Oracle e E
117. ernative way of writing the term 2 1 That is 2 1 represents the data structure LX 2 1 and not the number 3 The addition would only be performed if the structure were passed as an argument to an appropriate procedure such as is 2 However from a practical or a programmer s point of view the existence of operators is highly desirable and clearly handy Prolog syntax allows operators of three kinds infix prefix and postfix An infix operator appears between its two arguments while a prefix operator precedes its single argument and a postfiz operator follows its single argument Each operator has a precedence which is an integer from 1 to 1200 The precedence is used to disambiguate expressions in which the structure of the term denoted is not made explicit through the use of parentheses The general rule is that the operator with the highest precedence is the principal functor Thus if has a higher precedence than then the following atb c at b c are equivalent and both denote the same term a b c Note that in this case the infix form of the term a b c must be written with explicit use of parentheses as in atb c If there are two operators in the expression having the same highest precedence the ambiguity must be resolved from the types and the implied associativity of the operators The possible types for an infix operator are y x xfx xfy Operators of type xfx are no
118. erty which makes tabling terminate for many classes of programs The same generation program furnishes a case of the usefulness of tabling for optimizing a Prolog program Exercise 5 2 3 Jf you are still curious load in the file cy1 P in the XSB_DIR examples directory using the command load_dync cyl P and then type the query same_generation X X fail Now rewrite the same_generation 2 program so that it does not use tabling and retry the same query what happens Be patient or use lt ctr1 gt C The examples stress two differences between tabling and SLD resolution beyond termination properties First that each solution to tabled subgoal is returned only once a property that is helpful not only for path 2 but also for same generation 2 which terminates in Prolog Sec ond because answers are sometimes obtained using program clauses and sometimes using answers answers may be returned in an unaccustomed order CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 49 In the language of tabling the first instance of a tabled subgoal S is called a generator subgoal and is expanded using program clauses as in SLD resolution Prolog Subsequent instances of S are referred to as consuming subgoals and are expanded using answers in the table for S instead of program clauses Because consuming subgoals resolve against unique answers rather than repeat edly against program clauses tabling will terminate whenever 1 a finite
119. f DCG expansion in the presence of cuts have been known for a long time and almost all Prolog implementations expand a DCG clause with a 0 in its body in such a way that its expansion is steadfast and has the intended meaning when called with its second argument bound For that reason almost all Prologs translate the DCG clause aces ge to the clause a A B B A c A B But in our opinion this is just a special case of a 0 being the last goal in the body of a DCG clause 2 Most of the control predicates of XSB need not be enclosed in curly brackets A difference with say Quintus is that predicates not 1 1 or fail_if 1 do not get expanded when encountered in a DCG clause That is the DCG clause a gt true gt X f a not p gets expanded to the clause a A B true A C gt X f a C B not p A B and not to the clause a A B true A C gt X f a C B not p A B that Quintus Prolog expands to However note that all non control but standard predicates for example true 0 and 2 get expanded if they are not enclosed in curly brackets 8 5 Interaction of Definite Clause Grammars and Tabling Tabling can be used in conjunction with Definite Clause Grammars to get the effect of a more complete parsing strategy When Prolog is used to evaluate DCG s the resulting parsing algorithm CHAPTER 8 DEFINITE CLAUSE GRAMMARS 130 is recursive descent Recursive
120. f the extension P for Prolog source file to avoid ambiguity Optionally users can also provide a header file for a module denoted by the module name suffixed by H In such a case the XSB compiler will first read the header file if it exists and then the source file Currently the compiler makes no special treatment of header files They are simply included in the beginning of the corresponding source files and code can in principle be placed in either In future versions of XSB the header files may be used to check interfaces across modules hence it is a good programming practice to restrict header files to declarations alone The result of the compilation an SLG WAM object code file is stored in a filename O but compile 1 2 does not load the object file it creates The standard predicates consult 1 2 and reconsult 1 2 both recompile the source file if needed and load the object file into the system The object file created is always written into the directory where the source file resides the user should therefore have write permission in that directory If desired when compiling a module file clauses and directives can be transformed as they are read This is indeed the case for definite clause grammar rules see Chapter 8 but it can also be done for clauses of any form by providing a definition for predicate term_expansion 2 see Section 8 3 Predicates compile 1 2 can also be used to compile foreign language
121. floating point number is always less than an integer regardless of their numerical values e atoms are put in alphabetical i e ASCII order e compound terms are ordered first by arity then by the name of their principal functor and then by their arguments in a left to right order e lists are compared as ordinary compound terms having arity 2 and functor For example here is a list of terms sorted in increasing standard order X 3 14 9 fie foe fum X X X Y fie 0 2 fie 1 1 The basic predicates for comparison of arbitrary terms are Ti T2 Tests if the terms currently instantiating T1 and T2 are literally identical in particular variables in equivalent positions in the two terms must be identical For example the question 7 X Y fails answers no because X and Y are distinct variables However the question X Y X Y succeeds because the first goal unifies the two variables see section 6 3 Ti T2 Tests if the terms currently instantiating T1 and T2 are not literally identical Ti lt T2 Succeeds if term T1 is before term T2 in the standard order Ti gt T2 Succeeds if term T1 is after term T2 in the standard order Ti lt T2 Succeeds if term T1 is not after term T2 in the standard order Ti gt T2 Succeeds if term T1 is not before term T2 in the standard order CHAPTER 6 STANDARD PREDICATES 92 Some further predicates involving comparison of terms are com
122. foundations of object oriented and frame based languages Journal of the ACM 42 741 843 July 1995 M Kifer and V Subrahmanian Theory of generalized annotated logic programming and its applications J Logic Programming 12 4 335 368 1992 R Larson D S Warren J Freire and K Sagonas Syntactica MIT Press 1995 R Larson D S Warren J Freire K Sagonas and P Gomez Semantica MIT Press 1996 J Leite Logic program updates Technical report Universidade Nova de Lisboa 1997 T Lindholm and R O Keefe Efficient implementation of a defensible semantics for dynamic PROLOG code In Proceedings of the International Conference on Logic Programming pages 21 39 1987 X Liu C R Ramakrishnan and S Smolka Fully local and efficient evaluation of alternating fixed points In TACAS 98 Tools and Algorithms for Construction and Analysis of Systems pages 5 19 Springer Verlag 1998 BIBLIOGRAPHY 162 29 J W Lloyd Foundations of Logic Programming Springer 1984 30 I Niemela and P Simons Efficient implementation of the well founded and stable model 131 32 133 134 135 136 37 38 39 40 ZET 1491 43 44 semantics In Joint International Conference and Symposium on Logic Programming pages 289 303 1996 T Przymusinski Every logic program has a natural stratification and an iterated least fixed point model In PODS pages 11 21 1989 Y Ramakrishna C
123. functor 2 succeeds only for those functors function and predicate symbols which are defined in that module Unless the module is one of the global modules current_functor 2 fails for the predicates which are imported into that module On the other hand the goal current functor Name Term can be used to backtrack through every known term Term in the global modules of XSB s database that has Name as its functor Note that the order of term generation is undetermined Once again there are no error conditions associated with this predicate if its arguments are inappropriate the predicate simply fails current_predicate Predicate_Indicator The predicate current_predicate 1 can be used to find all the predicates that are de fined and loaded in a particular current module The module can be either a Prolog or a foreign module see the Chapter Foreign Language Interface in Volume 2 This predicate succeeds iff Predicate_Indicator is a predicate indicator for one of the procedures both Prolog and foreign language ones that are loaded in the database or that are dynamic Note that this includes procedures both in system and in user defined modules Unlike current_functor 1 which reports all predicates that are somehow known to the database current_predicate 1 reports only those predicates that are either created dynamically for example using assert 1 or loaded in the system I e it excludes those predicates which have been import
124. g various declarations a programmer can use also tabled resolution which allows for a different more declarative programming style than Prolog In this section we discuss the various aspects of tabling and how it is implemented in XSB Our aim in this section is to provide a user with enough information to be able to program productively in XSB It is best to read this tutorial with a copy of XSB handy since much of the information is presented through a series of exercises For the theoretically inclined XSB uses SLG resolution which can compute queries to non floundering normal programs under the well founded semantics 44 and is guaranteed to terminate when these programs have the bounded term depth property This tutorial covers only enough of the theory of tabling to explain how to program in XSB For those interested the web site contain papers covering in detail various aspects of tabling often through the links for individuals involved in XSB An overview of SLG resolution and practical evaluation strategies for it are provided in 8 41 38 19 The engine of XSB the SLG WAM is described in 35 33 18 37 7 14 as it is implemented in Version 2 1 and its performance analyzed Examples of large scale applications that use tabling are overviewed in 42 9 12 5 1 XSB as a Prolog System Before describing how to program using tabling it is perhaps worthwhile reviewing some of the goals of XSB 1 To execute tabled predicates at t
125. g warning character whose presence inside a comment string will cause gpp to output a warning this is useful e g to locate unterminated strings in cpp mode Note that input files are not allowed to contain unterminated comments strings A comment string specification can be declared from within the input file using the mode comment meta macro call or equivalently mode string in which case the number of C strings to be given as arguments to describe the comment string can be anywhere between 2 and 4 the first two arguments mandatory are the start sequence and the end sequence and can make use of the special matching sequences see below They may not start with alphanumeric characters The first character of the third argument if there is one is used as string quote character use an empty string to disable the functionality and the first character of the fourth argument if there is one is used as string warning character specification may also be given from the command line in which case there must be two arguments if using the c option and three if using the s option The behavior of a comment string is specified by a three character modifier string which may be passed as an optional argument either to the c s command line options or to the mode comment mode string meta macros If no modifier string is specified the default value is ccc for comments and sss for strings The first character corresponds to the
126. gpp only understands a minimal set of built in macros called meta macros These meta macros allow the definition of user macros as well as some basic operations forming the core of the preprocessing system including conditional tests arithmetic evaluation and syntax spec ification All user macro definitions are global i e they remain valid until explicitly removed meta macros cannot be redefined With each user macro definition gpp keeps track of the corre sponding syntax specification so that a macro can be safely invoked regardless of any subsequent change in operating mode In addition to macros gpp understands comments and strings whose syntax and behavior can be widely customized to fit any particular purpose Internally comments and strings are the same construction so everything that applies to comments applies to strings as well 134 APPENDIX A GPP GENERIC PREPROCESSOR 135 A 2 Syntax gpp o outfile I include path Dname val zl z x m n CI TI HI PI U M c lt n gt stri str2 c stri s lt n gt stri str2 c infile A 3 Options gpp recognizes the following command line switches and options e h Print a short help message e o outfile Specify a file to which all output should be sent by default everything is sent to standard output e I include path Specify a path where the 4 include meta macro will look for include files if they are not present in the current di
127. hapter 4 Syntax The syntax of XSB is taken from C Prolog with extensions This chapter mainly introduces the extensions The syntax of XSB is that of the HiLog language The syntax of HiLog is a proper superset of the Prolog syntax 4 1 Terms The data objects of the HiLog language are called terms A HiLog term can be constructed from any logical symbol or a term followed by any finite number of arguments In any case a term is either a constant a variable or a compound term A constant is either a number integer or floating point or an atom Constants are definite elementary objects and correspond to proper nouns in natural language 4 1 1 Integers The printed form of an integer in HiLog consists of a sequence of digits optionally preceded by a minus sign These are normally interpreted as base 10 integers It is also possible to enter integers in other bases 2 through 36 this can be done by preceding the digit string by the base in decimal followed by an apostrophe If a base greater than 10 is used the characters A Z or a z are used to stand for digits greater than 9 Using these rules examples of valid integer representations in XSB are 1 3456 95359 9888 16 1FA4 12 A0 20 representing respectively the following integers in decimal base 1 3456 95359 728 8100 120 0 Note that the following 36 CHAPTER 4 SYNTAX 37 525 12 2CF4 37 12 20 23 are not valid integers of XSB A base of 0
128. havior must be emulated i e the surrounding quotes should be removed all occurrences of should be replaced by a single backslash and similarly should be replaced by Sequences like An are recognized by gpp and should be left as is Special sequences matching certain subsets of the character set can be used They are of the form Ax where z is one of b matches any sequence of one or more spaces or TAB characters b is identical to gt w matches any sequence of zero or more spaces or TAB characters e B matches any sequence of one or more spaces tabs or newline characters W matches any sequence of zero or more spaces tabs or newline characters e a an alphabetic character a to z and A to Z A an alphabetic character or a space tab or newline a digit 0 to 9 e i an identifier character The set of matched characters is customizable using the mode charset id command The default setting matches alphanumeric characters and underscores Ca to z A to Z 0 to 9 and _ e t a TAB character e n a newline character e o an operator character The set of matched characters is customizable using the mode charset op command The default setting matches all characters in lt gt amp except in Prolog mode where and are not matched e O an operator character o
129. he outer functor symbol of the third argument or on a combination of the outer function symbol of the second and first arguments If data is expected to be structured within function symbols and is in unit clauses the directive index p 4 trie constructs an indexing trie of the p 4 clauses using a left to right traversal through each clause Representing data in this way allows discrimination of information nested arbitrarily deep within clauses These modes of indexing can be combined index p 4 3 2 1 trie creates alternative trie indices beginning with the third argument and with the second and first argument Using such indexing XSB can efficiently perform intensive analyses of in memory knowledge bases with 1 million or so facts Indexing techniques for asserted code are covered in Section 6 10 For compiled code XSB offers unification factoring which extends clause indexing methods found in functional programming into the logic programming framework Briefly unification fac toring can offer not only complete indexing through non deterministic indexing automata but can also factor elementary unification operations The general technique is described in 11 and the XSB directives needed to use it are covered in Section 3 8 A number of interfaces are available to link XSB to other systems In UNIX systems XSB can be directly linked into C programs in Windows based system XSB can be linked into C programs through a DLL interface On ei
130. he same with predicate current_output 1 described in section 6 9 and it is only provided for upwards compatibility reasons told Closes the current output stream Current output stream reverts to userout the standard output stream open File Mode Stream open 1 creates a stream for the file designated in File and binds Stream to a structure representing that stream Mode can be one of either read to create an input stream or write or append to create an output stream If the mode is write the contents of File are removed and File becomes a record of the output stream If the mode is append the output stream is appended to the contents of File Exceptions read mode permission error File F is directory or file is not readable instantiation_error F is not instantiated at the time of call existence_error File F does not exist CHAPTER 6 STANDARD PREDICATES 65 Exceptions write mode permission error File F does not have write permission or is a directory instantiation error F is uninstantiated close Stream close 1 closes the stream Stream file exists F Succeeds if file F exists F must be instantiated to an atom at the time of the call or an error message is displayed on the standard error stream and the predicate aborts Exceptions instantiation error F is uninstantiated 6 1 2 Character I O nl A new line character is sent to the current output stream nl Stream A new line character i
131. he speed of compiled Prolog 2 To ensure that the speed of compiled Prolog is not slowed significantly by adding the option of tabling 3 To execute that the functionality of Prolog is not compromised by support for tabling 46 CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 47 4 To provide Prolog functionality in tabled predicates whenever it is semantically sensible to do so 5 To provide standard predicates to manipulate tables taken as objects in themselves Goals 1 and 2 are addressed by XSBs engine which in Version 2 1 is based on a memory copying version of a virtual machine called the SLG WAM The overhead for SLD resolution using this machine is negligible Thus when XSB is used simply as a Prolog system i e no tabling is used it is reasonably competitive with other Prolog implementations based on a WAM emulator written in C or assembly For example XSB Version 1 6 is about two to three times slower than Quintus 3 1 1 or emulated SICStus Prolog 3 1 Goals 3 4 and 5 have been nearly met but there are a few instances in which interaction of tabling with a Prolog construct has been accomplished or is perhaps impossibe Accordingly we discuss these instances throughout this chapter XSB is still under development however so that future versions may support more transparent mixing of Prolog and tabled code e g allowing tabled predicates in the scope of A 1 or adding Prolog functionality to tabled predica
132. ile _exists File findall Elem Goal List float Term functor Term Functor Arity get Char get0 Char get_call Skeleton Subgoal Structure Pointer Return_Skeleton get_calls Skeleton Subgoal_ Structure Pointer Return_Skeleton get_calls_for_table Skeleton Call Empty get_resudual Call Return get_returns Skeleton Delay_list get_returns_for_call Call Return halt hilog Symbol hilog arg Index Term Arg hilog functor Term Functor Arity hilog op Precedence Type Name hilog symbol Symbol import PredList from Module index Predicate ArgNo HashSize instance Ref Instance integer Term is Result Expression is_absolute_filename Path APPENDIX B STANDARD PREDICATES AND FUNCTIONS 155 is_list Term is_charlist Term is_charlist Term Size keysort Input Output listing listing Predicate load_dyn Module load_dync Module ls module_property Module Property name Term CharList nl nl Stream nodebug nonvar Term nospy Predicate List not Goal notrace number Term number_codes Number Character_list once Goal op Precedence Format Operator open Stream otherwise phrase Phrase List phrase Phrase List Remains predicate_property Predicate Property print Term Currently the same as write 1 proper_hilog Term put Char read Term read Stream Term read_canonical Term real Term reconsult Module reconsult Module Option
133. in the form of tries e Current use of the above specified memory areas allocated in use free e Information about process cpu and clock time Additionally if the emulator is invoked with the s option see Section 3 5 information is printed out about e Maximum use of the memory areas The s option slows down the emulator by about 10 Example statistics memory total 1873737 bytes 171317 in use 1702420 free permanent space 169801 bytes glob loc space 786432 bytes 1080 in use 785352 free global 152 bytes local 928 bytes trail cp space 786432 bytes 436 in use 785996 free trail 240 bytes choice point 196 bytes SLG subgoal space O bytes O in use O free SLG unific space 65536 bytes O in use 65536 free SLG completion 65536 bytes O in use 65536 free SLG trie space O bytes O in use O free call ret trie O bytes trie hash tables O bytes Maximum stack use global 224 local 1384 trail 240 cp 492 Maximum stack use SLG completion 0 Max level 0 O Trail unwinds O levels 0 570 sec cputime 5 088 sec elapsetime shows how the emulator output looks if it is invoked with the s option without it the Maximum use line is not shown Information about the allocation size is provided since the sizes can be changed through emulator options see Section 3 5 shell SystemCall Calls the operating system with the atom SystemCall as argument It succeeds if SystemCall is executed successfully
134. inciples of Declarative Programming 10th International Symposium PLILP 98 Held Jointly with the 6th International Conference ALP 98 number 1490 in LNCS pages 21 35 Pisa Italy Sept 1998 Springer B Demoen and K Sagonas Memory Management for Prolog with Tabling In Proceedings of ISMM 98 ACM SIGPLAN International Symposium on Memory Management pages 97 106 Vancouver B C Canada Oct 1998 ACM Press B Demoen and K Sagonas CHAT the Copy Hybrid Approach to Tabling In G Gupta editor Practical Aspects of Declarative Languages First International Workshop number 1551 in LNCS pages 106 121 San Antonio Texas Jan 1999 Springer S Dietrich Extension Tables for Recursive Query Evaluation PhD thesis SUNY at Stony Brook 1987 J Freire T Swift and D Warren Beyond depth first Improving tabled logic programs through alternative scheduling strategies Journal of Functional and Logic Programming 1998 J Freire T Swift and D Warren A formal framework for scheduling in SLG In International Workshop on Tabling in Parsing and Deduction 1998 ISO working group JTC1 SC22 Prolog international standard Technical report International Standards Organization 1995 D Kemp and R Topor Completeness of a top down query evaluation procedure for stratified databases In Logic Programming Proc of the Fifth International Conference and Symposium pages 178 194 1988 M Kifer G Lausen and J Wu Logical
135. ion we informally describe this translation which resembles the DCG rules of other Prologs in most particulars Each grammar rule is translated into a Prolog clause as it is consulted or compiled This DCG term expansion is as follows A DCG rule such as p X gt q X will be translated expanded into the Prolog rule p X Li Lo q x Li Lo If there is more than one non terminal on the right hand side as in LA term like foo is just a syntactic sugar for the term foo Readers familiar with Quintus Prolog may notice the difference in the treatment of the various kinds of not For example in Quintus Prolog a not 1 that is not enclosed within curly brackets is interpreted as a non terminal grammar symbol CHAPTER 8 DEFINITE CLAUSE GRAMMARS 126 p X Y gt q x gt r x Y gt s Y the corresponding input and output arguments are identified translating into p X Y Li Lo q X Li L1 r X Y Li L2 s Y L2 Lo Terminals are translated using the built in predicate C 3 See section 8 3 for its description For instance p X gt go to q X stop is translated into p X 0 C S0 go S1 C S1 to S2 q X 2 3 C S3 stop S Extra conditions expressed as explicit procedure calls naturally translate into themselves For example positive_number X gt N integer N N gt OP fraction F formmumber N F X tra
136. is a real number conversion from a real to its character list representation is not implemented yet atom_codes Atom CharCodeList The standard predicate atom_codes 2 performs the conversion between an atom and its character list representation If Atom is supplied and is an atom CharList is unified with a list of ASCII codes representing the name of that atom In that case CharList is exactly the list of ASCII character codes that appear in the printed representation of Atom If on the other hand Atom is a variable then CharList must be a proper list of ASCII character codes In that case Atom is instantiated to an atom containing exactly those characters even if the characters look like the printed representation of a number If both of the arguments of atom_codes 2 are uninstantiated or CharList is not a proper list of ASCII characters atom_codes 2 aborts and an error message will be sent to the standard error stream Examples atom_codes Foo L 70 111 111 E atom_codes L 91 93 E atom_codes X 102 111 111 foo ps atom_codes X X CHAPTER 6 STANDARD PREDICATES 83 atom_codes X Foo X Foo atom_codes X 52 51 49 X 431 atom_codes X 52 51 49 integer X no atom_codes X 52 Y 49 Instantiation error in argument 2 of atom_codes 2 Aborting atom_codes 431 L Type error in argument 1 of
137. ity The above examples could have been written equally well as Head Tail foo bar Tail For convenience a further notational variant is allowed for lists of integers that correspond to ASCII character codes Lists written in this notation are called strings For example I am a HiLog string represents exactly the same list as 73 32 97 109 32 97 32 72 105 76 111 103 32 115 116 114 105 110 103 CHAPTER 4 SYNTAX 41 4 2 From HiLog to Prolog From the discussion about the syntax of HiLog terms it is clear that the HiLog syntax allows the incorporation of some higher order constructs in a declarative way within logic programs As we will show in this section HiLog does so while retaining a clean first order declarative semantics The semantics of HiLog is first order because every HiLog term and formula is automatically encoded converted in predicate calculus in the way explained below Before we briefly explain the encoding of HiLog terms let us note that the HiLog syntax is a simple but notationally very convenient encoding for Prolog terms of some special form In the same way that in Prolog is just an external shorthand for the term 1 2 in the presence of an infix operator declaration for see section 4 3 so X a b is just an external shorthand for the Prolog compound term apply X a b Also in the presence of a hilog declaration see section 3 8 4 for h the HiLog term whose external
138. ive quote characters evaluate as a single quote character Finally to facilitate proper argument delimitation certain characters can be stacked when they occur in a macro argument so that the argument separator or macro end sequence are not parsed if the argument body is not balanced This allows nesting macro calls without using quotes If an improperly balanced argument is needed quote characters should be added in front of some stacked characters to make it balanced The macro construction sequences described above can be different for meta macros and for user macros this is e g the case in cpp mode Note that since meta macros can only have up to two arguments the delimitation rules for the second argument are somewhat sloppier and unquoted argument separator sequences are allowed in the second argument of a meta macro Unless one of the standard operating modes is selected the above syntax sequences can be specified either on the command line using the M and U options respectively for meta macros and user macros or inside an input file via the mode meta and mode user meta macro calls In both cases the mode description consists of 9 parameters for user macro specifications namely the macro start sequence the short macro end sequence the argument start sequence the argument separator the long macro end sequence the string listing characters to stack the string listing characters to unstack the argument reference sequence
139. known terms appearing in a particular current module It succeeds iff Predicate_Indicator is a predicate indicator for any term that appears in the database Note that this includes terms both in system and in user defined modules even terms that may be not yet loaded in the system The behaviour of current_functor 1 may be contrasted with that of current_predicate 1 which reports only those predicates which have been loaded in the system both Prolog and foreign predicates or are dynamic predicates There are no error conditions associated with this predicate if its argument is not a predicate indicator the predicate simply fails Predicate current_functor 1 comes in two flavours depending on the form of its argument Predicate_Indicator 1 If Predicate_Indicator is of the form Module Functor Arity then the execution of current_functor 1 will backtrack through all the current modules of the system user defined system defined and global modules 2 If however Predicate_Indicator is uninstantiated or has the form Functor Arity then predicate current_functor 1 backtracks only through the terms appearing in the global modules of the system in other words searches only modules usermod and global This flavour is only for convenience since this is the common use of predicate current_functor 1 Note that all the following are equivalent current_functor Functor Arity current_functor Predicate current_functor user
140. l Term basics Reads a term that is in canonical format from the current input stream and returns it in CHAPTER 6 STANDARD PREDICATES 68 Term On end of file it returns the atom end_of file If it encounters an error it prints an error message on stderr and returns the atom read_canonical_error This is significantly faster than read 1 but requires the input to be in canonical form write_canonical Stream Term write_canonical 2 has the same behavior as write_canonical 1 but the output stream is explicitly designated using the first argument print Term This predicate is intended to provide a handle for user defined pretty printing Currently it is defined as write 1 6 2 Special I O fmt_read Format types T1 T2 args A1 A2 RetCode This predicate implements C style formatted input It reads the current input according to the Format string Format has the same syntax as the input format in C The term types lists the types of the arguments they must match the types specified in Format Here 1 means string 2 means integer and 3 means float The term args specifies the variables for the input RetCode specifies the return code 0 ok 1 end of file read_line Line Status Reads the next line from the current input and puts it in Line If the line is larger than the available buffer then Status is 0 If the line was read in full up to and including the newline character then Status is 1
141. laration HiLog symbols The HiLog symbols are always atoms but if the argument of hilog_symbol though instantiated is not an atom the predicate simply fails So one can enumerate all the HiLog symbols by using the following query CHAPTER 6 STANDARD PREDICATES 104 hilog symbol X current_op Precedence Type Name This predicate is used to examine the set of operators currently in force It succeeds when the atom Name is currently an operator of type Type and precedence Precedence None of the arguments of current_op 3 need to be instantiated at the time of the call but if they are they must be of the following types Precedence it must be an integer in the range from 1 to 1200 Type it must be one of the atoms xfx xfy yfx fx fy hx hy xf yf Name it must be either an atom or a list of atoms Exceptions not yet implemented domain_error Precedence is not between 1 1200 or Type is not one of the listed atoms type_error Name is not an atom hilog_ op Precedence Type Name This predicate has exactly the same behaviour as current_op 3 with the only difference that Type can only have the values hx and hy 6 10 Modification of the Database XSB provides an array of features for modifying the dynamic database Using assert 1 clauses can be asserted using first argument indexing in a manner that is now standard to Prolog imple mentations While this is the default behavior for XSB other behavior can be specifie
142. lashes can cause the system to behave strangely since these modules will probably have different semantics from that expected by the XSB system code The list of module names in XSB s system library directories can be found in appendix C Apart from the user libraries XSB now has a simple packaging system A package is an appli cation consisting of one or more files that are organized in a subdirectory of one of the XSB system or user libraries The system directory XSB_DIR packages has several examples of such packages Packages are convenient as a means of organizing large XSB applications and for simplifying user interaction with such applications User level packaging is implemented through the predicate bootstrap_userpackage LibraryDir PackageDir PackageName which must be imported from the packaging module To illustrate suppose you wanted to create a package foobar inside your own library my_lib Here is a sequence of steps you can follow 1 Make sure that my_1lib is on the library search path by putting an appropriate assert statement in your xsbrc P 2 Make subdirectory my_1ib foobar and organize all the package files there Designate one file say foo P as the entry point e the application file that must be loaded first 3 Create the interface program my_1ib foobar P with the following content bootstrap_userpackage my_lib foobar foobar foo The interface program and the pack
143. latter can be done by responding with b at the prompt which recursively invokes an XSB sub session At this point you can enter the debugger control commands and type end_of_file This returns XSB back to the debugger prompt but with new settings 1 debug_ctl prompt off Set non interactive mode globally This means that trace will be printed from start to end and the user will never be prompted during the trace 2 debug_ctl prompt on Make tracing spying interactive 3 debug_ctl profile on Turns profiling on This means that each time a call execution reaches the Fail or Exit port CPU time spent in that call will be printed The actual call can be identified by locating a Call prompt that has the same number as the cpu time message 4 debug_ctl profile off Turns profiling off 5 debug_ctl redirect File Redirects debugging output to a file This also includes program output errors and warnings Note that usually you cannot see the contents of File until it is closed e until another redirect operation is performed usually debug ctl redirect tty see next 6 debug_ctl redirect tty Attaches the previously redirected debugging error pro gram output and warning streams back to the user terminal CHAPTER 7 DEBUGGING 122 7 debug ctl show PortList Allows the user to specify at which ports should trace messages be printed PortList must be a list of port names i e a sublist of Call Exit
144. ldren each one of which is the root of the tree of terms t1 t2 tk Sometimes it is convenient to write certain functors as operators Binary functors that is functors that are applied to two arguments may be declared as infiz operators and unary functors that is functors that are applied to one argument may be declared as either prefix or postfix operators Thus it is possible to write the following X Y P Q X lt Y X P More about operators in HiLog can be found in section 4 3 4 1 6 Lists As in Prolog lists form an important class of data structures in HiLog They are essentially the same as the lists of Lisp a list is either the atom representing the empty list or else a compound term with functor and two arguments which are the head and tail of the list respectively where the tail of a list is also a list Thus a list of the first three natural numbers is the structure CHAPTER 4 SYNTAX 40 IN 1 x ZN 2 a ZN 3 O which could be written using the standard syntax as 1 2 3 but which is normally written in a special list notation as 1 2 3 Two examples of this list notation as used when the tail of a list is a variable are Head Tai1 foo bar Tail which represent the structures IN IN Head Tail foo IN bar Tail respectively Note that the usual list notation H T does not add any new power to the language it is simply a notational convenience and improves readabil
145. les atom HiLog no atom 10 no atom HiLog yes atom X a b no atom h yes atom yes atom integer X Succeeds if X is currently instantiated to an integer otherwise it fails real 7X Succeeds if X is currently instantiated to a floating point number otherwise it fails float X Same as real 1 Succeeds if X is currently instantiated to a floating point number otherwise it fails This predicate is included for compatibility with earlier versions of SBProlog number X Succeeds if X is currently instantiated to either an integer or a floating point number real otherwise it fails CHAPTER 6 STANDARD PREDICATES 72 atomic X Succeeds if X is currently instantiated to an atom or a number otherwise it fails Examples atomic 10 yes atomic p yes atomic h yes atomic h X no atomic foo no atomic foo yes atomic X no atomic X Y no compound X Succeeds if X is currently instantiated to a compound term with arity greater that zero i e to a nonvariable term that is not atomic otherwise it fails Examples compound 1 no compound foo 1 2 3 yes compound foo bar yes compound foo yes compound foo no compound X a b yes compound a b yes structure X Same as compound 1 Its existence is on
146. les it usually means that the predicate is either a function symbol or an unloaded predicate symbol including constants amame The predicate is dynamic loaded The predicate including internal predicates is a Prolog predicate loaded into the module in question this is always the case for predicates in global modules unloaded The predicate is yet unloaded into the module in question foreign The predicate is a foreign predicate This implies that the predicate is already loaded in the system because currently there is no way for XSB to know that a predicate is a foreign predicate until it is loaded in the system exported The predicate symbol is exported by the module in question in other words the predicate symbol is visible to any other module in the system local The predicate symbol is local to the module A imported_from Mod The predicate symbol is imported into the module in Ii e spied The predicate symbol has been declared spied eee either conditionally or unconditionally built_in The predicate symbol has the same Functor and Arity as one of XSB s builtin standard predicates CHAPTER 6 STANDARD PREDICATES 100 Finally since dynamic is usually declared as an operator with precedence greater than 999 writing the following predicate_property X dynamic will cause a syntax error The way to achieve the desired result is to parenthesize the operator like in predicate_prope
147. les only those predicates of a module which appear to static analysis to contain an infinite loop or which are called directly through tnot 1 It is perhaps more illuminating to CHAPTER 3 SYSTEM DESCRIPTION 32 demonstrate these conditions through an example rather than explaining them For instance in the program auto_table pla s f a s X p f a r X q X W r Y m X tnot f X mode api ap1 HIT L HIL1 api T L L1 mode ap ap F F ap HIT L HIL1 ap T L L1 mem H HIT mem H _IT mem H T The compiler prints out the messages Compiling predicate s 1 as a tabled predicate Compiling predicate r 1 as a tabled predicate Compiling predicate m 1 as a tabled predicate Compiling predicate mem 2 as a tabled predicate Terminating conditions were detected for ap1 3 and ap 3 but not for any of the other predi cates auto_table gives an approximation of tabled programs which we hope will be useful for most programs The minimal set of tabled predicates needed to insure termination for a given program is undecidible It should be noted that the presence of meta predicates such as cal1 1 makes any static analysis useless so that the auto_table directive should not be used in such cases Predicates can be explicitly declared as tabled as well through the table 1 When table 1 is used the directive takes the form table F A where F is the f
148. ling configure option Detailed explanations can be found in 18 Type of Memory Management Routines for managing execution stacks for tabled evaluations can be quite complex due to interdependencies of tabled subgoals Indeed memory man agement algorithms can be based on common elements are shared among computation states or are copied The default configuration of XSB shares these elements while the option enable chat copies these elements While sharing and copying have minor performance differences the main reason to try the enable chat configuration is to use a heap garbage collector that has been written for it See 35 14 15 16 for in depth discussion of the engine memory management 2 1 1 Possible Installation Problems Lack of Space for Optimized Compilation of C Code When making the optimized version of the emulator the temporary space available to the C compiler for intermediate files is sometimes not sufficient For example on one of our SPARCstations that had very little tmp space the 04 option could not be used for the compilation of files enuloop c and tries c without changing the default tmp directory and increasing the swap space Depending on your C compiler the amount and nature of tmp and swap space of your machine you may or may not encounter problems If you are using the SUN C compiler and have disk space in one of your directories say dir add the following option to the entries of any files that cannot b
149. ll backtrack into such a scope To complete our example of retracting the intersection of dynamic predicates p X q X retract_nr p X retract_nr q X fail reclaim_space p _ reclaim_space q _ would do the trick Notice that the reclaim_space calls must be made after execution has completely failed out of choice points for q X and p X Predicate reclaim_space 1 is not standard but must be imported from module assert As with retract_nr the use of this predicate is discouraged it is provided for partial compatibility with SB Prolog index PredSpec IndexSpec In general XSB supports hash based indexing on alternate arguments or combinations of arguments along with trie based indexing The availability of various kinds of indexing depends on whether code is static e g compiled or dynamic e g asserted or loaded with load_dyn 1 The executable directive index 2 does not re index an already existing predicate but takes effect only if the program store contains no clauses for PredSpec Index directives can be given to the compiler as part of source code or executed during program execution analogously to op 3 CHAPTER 6 STANDARD PREDICATES 107 e Hash based Indexing Static Predicates In this case IndexSpec must be a non negative integer which indi cates the argument on which an index is to be constructed If IndexSpec is 0 then no index is kept possibly an efficient strategy for predicates with only one or
150. ll to a tabled predicate Example 5 2 2 An example of using cuts in a tabled predicate is a tabled meta interpreter table demo 1 demo true demo A B demo A demo B demo C cal1 C More elaborate tabled meta interpreters can be extremely useful for instance to implement various extensions of definite or normal programs In Version 2 1 of XSB a cut over tables occurs only when the user makes a call to a tabled predicate from the interpreter level but does not generate all solutions In such a case the user will see the warning Removing incomplete tables appear Any complete tables will not be removed They can be abolished by using one of XSB s predicates for abolishing tables Potential Pitfalls in Tabling While the judicious use of tabling can make some programs faster its indiscriminate use can make other programs slower Naively tabling append 3 is one case append L L append HIT L HIT1 append T L T1 can in the worst case copy N sublists of the first and third arguments into the table transforming a linear algorithm into a quadratic one Exercise 5 2 5 If you need convincing that tabling can sometimes slow a query down type the query genlist 1000 L prolog_append L a Out and then type the query CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 51 genlist 1000 L table_append L a Out append 3 is a particularly bad predicate to table Type the que
151. lt compile size This default compile is set to be 20 000 bytes in cmp1ib config P but can be reset by the user If the source file is larger than the default compile size the file will be loaded using load_dyn 1 and otherwise it will be compiled load_dyn 1 can also be called separately see the section Asserting Dynamic Code for details While load_dyn gives reasonibly good execution times compilation can always be done by using compile 1 2 explicitly Currently Version 2 1 a foreign language module is compiled when at least one of files c or H has been changed from the time the corresponding object files have been created Whether the file is compiled or dynamically loaded the byte code for the file is loaded into XSB s database The default action upon loading is to delete any previous byte code for predicates defined in the file If this is not the desired behavior the user may add to the file a declaration multifile lt Predicate_List gt where Predicate_List is a list of predicates in functor arity form The effect of this declaration is to delete only those clauses of predicate arity that were defined in the file itself After loading the module all exported predicates of that module are imported into the cur rent environment the current working module usermod For non modules see Section 3 3 all predicates are imported into the current working module CHAPTER 3 SYSTEM DESCRIPTION 24 Finally any queries
152. ly for compatibility with SB Prolog version 3 1 is_list X Succeeds if X is a proper list In other words if it is either the atom or H T where H is any Prolog or HiLog term and T is a proper list otherwise it fails Examples CHAPTER 6 STANDARD PREDICATES 73 is_list p a b c h a b yes is_list _ _ yes is_list a b X no is_list alb no is_charlist X Succeeds if X is a Prolog string i e a list of characters Examples is_charlist abc yes is_charlist abc no is_charlist X Size Like above but also returns the length of that string in the second argument which must be a variable is_most_general_term X Succeeds if X is compound term with all distinct variables as arguments or if X is an atom It fails if X is a cons node is_most_general_term f _ _ _ _ yes is_most_general_term abc yes is_most_general_term f X Y Z X no is_most_general_term f X Y Z a no is_most_general_term _ _ no callable X Succeeds if X is currently instantiated to a term that standard predicate call 1 could take as an argument and not give an instantiation or type error Note that it only checks for errors of predicate call 1 In other words it succeeds if X is an atom or a compound term otherwise it fails Predicate callable 1 has no associated error conditions Examples callable p yes callable p 1
153. m to evaluate stable models may have to fall back on generating possible assumption sets in pathological cases For a ground program if it is ensured that residual clauses are produced for all atoms using the residual program may bring a performance gain since the search space of algorithms to compute stable models will be correspondingly reduced In fact by using XSB in conjunction with a Stable Model generator Smodels 30 an efficient system has been devised for model checking of concurrent systems that is 10 20 times faster than competing systems 28 5 4 Tabled Aggregation The following shortest path predicate is a modification of the path 2 predicate of Section 5 2 table path 3 path X Y C path X Z C1 edge Z Y C2 C is C1 C2 path X Y C edge X Y C Exercise 5 4 1 path 3 has a simple declarative meaning it computes the path between two vertices of a graph along with the cost of the path Since path 3 is tabled would you expect it to terminate Try the query path 1 5 X over the graph provided in the file table_examples P If we could use tabling to compute the path with least cost or the shortest path the program would not only omit extraneous information but it would also terminate Recall that for simple horn programs variant based tabling ensures termination by only returning a given answer A once and failing on subsequent derivations of A If this strategy could be extended so that the engine only returne
154. macro call or of a comment By default when a newline or whitespace character forms the end of a macro or a comment it is parsed as part of the macro call or comment and therefore removed from output Use the n option to keep the last character in the input stream if it was whitespace or a newline U argil arg9 User defined mode The nine following command line arguments are taken to be respectively the macro start sequence the macro end sequence for a call without arguments the argument start sequence the argument separator the argument end sequence the list of characters to stack for argument balancing the list of characters to unstack the string to be used for referring to an argument by number and finally the quote character if there is none an empty string should be provided These settings apply both to user macros and to meta macros unless the M option is used to define other settings for meta macros See the section on syntax specification for more details M argl arg7 User defined mode specifications for meta macros This option can only be used together with M The seven following command line arguments are taken to be respectively the macro start sequence the macro end sequence for a call without arguments the argument start sequence the argument separator the argument end sequence the list of characters to stack for argument balancing and the list of characters to unstack See below for more details defa
155. ment may not be sufficient Indexing may be improved in such cases by the use of unification factoring Unification Factoring is a program transformation that factors out common parts of clause heads allowing differing parts to be used for indexing as illustrated by the following example p f X Y p X Y _ p a X q X p b X r X p f a X q X p f b X r X The transformation thus effectively allows p 2 to be indexed on atoms a 0 and 6 0 Unification Factoring is transparent to the user predicates created by the transformation are internal to the system and do not appear during tracing The following compiler directives control the use of unification factoring ti F A Specifies that predicate F A should be compiled with unification factoring enabled ti off F A Specifies that predicate F A should be compiled with unification factoring dis abled ti_all Specifies that all predicates defined in the file should be compiled with unification factoring enabled ti_off_all Specifies that all predicates defined in the file should be compiled with unification factoring disabled By default higher order predicates more precisely predicates named apply with arity greater than 1 are compiled with unification factoring enabled It can be disabled using the ti_off directive For all other predicates unification factoring must be enabled explicitly via the ti or ti_all directive If both
156. minate with a call to predicate abort 0 so an exception encountered at some break level other than the top level will return the interpreter at the top level The user should be aware of the fact that abort 0 does not close any files which may have been opened If the program under execution is doing file manipulation using see 1 and tell 1 then strange behavior may occur after the program is aborted and restarted unless the user manually closes the files Aborting closes all incomplete tables those which may not have a complete set of answers Closed tables are unaffected even if the tables were created during the aborted computation break Causes the current execution to be suspended at the beginning of the next call The in terpreter then enters break level 1 and is ready to accept input as if it were at top level If another call to break 0 is encountered it moves up to break level 2 and so on While execution is done at break level n gt 0 the prompt changes to n To close a break level and resume the suspended execution the user can type the the atom end_of_file or the end of file character applicable on the system usually CTRL d on UNIX systems Predicate break O then succeeds note in the following example that the calls to CHAPTER 6 STANDARD PREDICATES 109 break 0 do not succeed and the execution of the interrupted program is resumed Alterna tively the suspended execution can be abandoned by calling the standard p
157. mod Predicate current _functor global Predicate So to backtrack through all of the functors of positive arity function and predicate symbols that appear in the global modules of the system regardless of whether they are system or a user defined use current_functor Functor Arity Arity gt 0 current_functor Name Term_Indicator Succeeds iff Term_Indicator is the most general term corresponding to one of the currently known terms having Name as their functor appearing in a current module Both system and user defined modules are checked Or procedurally current_functor 2 unifies Name with the name of a functor known to the database and Term_Indicator with the most general CHAPTER 6 STANDARD PREDICATES 96 term corresponding to that functor The flavours of this predicate are analogous to the ones of current _functor 1 according to whether Term_Indicator has one of the following two forms 1 Module Term 2 Term for global modules If Term_Indicator is uninstantiated then this predicate succeeds only for global modules As in current _functor 1 even unloaded predicates are reported if they have been imported and are are known to the database For example if a predicate foo 2 and and a function symbol foo 1 are defined into module blah the following query will return current functor foo blah Term Term foo0 _638788 638792 Term foo 638788 no If a module is specified current
158. mode stack and restores the operating mode at the end of the included file The included file may override this behavior by starting with a mode restore call and ending with a mode push call Additionally when the m command line option is specified gpp will automatically switch to the cpp compatibility mode upon including a file whose name ends with either c or h e exec command This causes gpp to execute the specified command line and include its standard output in the current output Note that this meta macro is disabled unless the z command line flag was specified for security reasons If use of ezec is not allowed a warning message is printed and the output is left blank Note that the specified command line is evaluated before being executed thus allowing the use of macros in the command line However the output of the command is included verbatim and not evaluated If you need the output to be evaluated you must use defeval see above to cause a double evaluation APPENDIX A GPP GENERIC PREPROCESSOR 144 e eval expr The eval meta macro attempts to evaluate expr first by expanding macros normal gpp eval uation and then by performing arithmetic evaluation The syntax and operator precedence for arithmetic expressions are the same as in C the only missing operators are lt lt gt gt and assignment operators If unable to assign a numerical value to the result the returned text is simply the result
159. n which XSB was built the release number the various directories that XSB uses to find its libraries etc CHAPTER 6 STANDARD PREDICATES 102 To find all features and their values ask the following query xsb_configuration FeatureName Value fail Here is how xsb_configuration might look like xsb_configuration architecture i686 pc linux gnu configuration is usualy the same as architecture but it can also 4h contain special tags it e g 1686 pc linux gnu dbg for a verion 4 built with debugging enabled xsb_configuration configuration i686 pc linux gnu dbg xsb_configuration host_os linux gnu xsb_configuration os_version 2 34 xsb_configuration os_type linux gnu xsb_configuration host_vendor pc xsb_configuration host_cpu i686 xsb_configuration compiler gcc xsb_configuration compiler_flags ansi pedantic Wall g xsb_configuration loader_flags lm 1dl Wl export dynamic xsb_configuration compile_mode debug The following is XSB release information xsb_configuration major_version 1 xsb_configuration minor_version 9 xsb_configuration beta_version 3 xsb_configuration version 1 9 b3 xsb_configuration codename Code Breaker xsb_configuration release_date date 1998 10 17 4h XSB query evaluation directive xsb_configuration scheduling_strategy batched Support for other languages xs
160. n files for different architectures reside in different directories and there is no danger of an architecture conflict Moreover you can keep using the same bin xsb script regardless of the architecture It will detect your configuration and will use the right files for the right architecture CHAPTER 2 GETTING STARTED WITH XSB 7 Choice of the C Compiler and Other options The configure script will attempt to use gcc if it is available Otherwise it will revert to cc or acc Some versions of gcc are bro ken in which case you would have to give configure an additional directive with cc If you must use some special comiler use with cc your own compiler You can also disable optimization to change the default enable debug and there are many other options Type configure help to see them all Also see the file XSB_DIR INSTALL for more details Other options are of interest to advanced users who wish to experiment with XSB or to use XSB for large scale projects In general however users need not concern themselves with these options Type of Scheduling Strategy The ordering of operations within a tabled evaluation can dras tically affect its performance XSB provides two scheduling strategies Batched Evaluation and Local Evaluation Batched Evaluation is the default scheduling strategy for XSB and evaluates queries to reduce the time to the first answer of a query Local Evaluation can be chosen via the enable local schedu
161. n with delay In EPIA 1999 Available at http www cs sunysb edu tswift T Swift Tabling for non monotonic programming Annals of Mathematics and Artificial Intelligence 1999 To Appear Available at http cs sunysb edu tswift H Tamaki and T Sato OLDT resolution with tabulation In Third International Conference on Logic Programming pages 84 98 1986 A van Gelder K Ross and J Schlipf Unfounded sets and well founded semantics for general logic programs JACM 38 3 620 650 1991 BIBLIOGRAPHY 163 45 L Vieille Recursive query processing The power of logic Theoretical Computer Science 69 1 53 1989 46 A Walker Backchain iteration Towards a practical inference method that is simple enough to be proved terminating sound and complete J Automated Reasoning 11 1 1 23 1993 Originally formulated in New York University TR 34 1981 47 D S Warren Programming the PTQ grammar in XSB In R Ramakrishnan editor Appli cations of Logic Databases pages 217 234 Kluwer Academic Publishers 1995 48 J Xu The PSB Prolog User Manual 1990 Index 1 0 93 124 125 128 trace 0 122 3 85 A 1 53 xsb_xpp_include_dir 26 2717 0 69 xsb_configuration 2 101 2 69 t not 1 70 gt 2 70 2 79 2 69 2 80 1 69 C 3 128 abolish 1 106 abolish all_tables 0 117 abolish_table_call 1 117 abolish_table_pred 1 117 abort 0 108 arg 3 77 arg0 3 78 a
162. name 2 81 n1 0 65 nl 1 65 nodebug 0 121 nonvar 1 71 nospy 1 121 not 1 69 number 1 71 number_chars 2 84 number_codes 2 83 number_digits 2 84 once 1 93 optimize 26 165 otherwise 0 68 package_configuration 2 18 phrase 2 127 phrase 3 127 predicate_property 2 98 print 1 68 prompt 2 109 proper_hilog 1 74 put 1 65 read 1 66 read 2 66 read_canonical 1 67 read _line 2 68 real 1 71 reclaim_space 1 106 repeat 2 70 retract 1 105 retract_nr 1 106 retractall 1 105 see 1 63 seeing 1 64 seen 0 64 set_global_compiler_options 1 25 setof 3 84 shell 1 110 shell 2 111 spec_dump 27 spec_off 27 spec_repr 27 spy 1 120 statistics 0 109 structure 1 72 suppl_table 27 32 tab 1 65 table 1 108 table_once 1 93 table_state 2 118 tbagof 3 86 tell 1 64 telling 1 64 term_expansion 2 128 tfindall 3 85 ti_dump 27 ti_long names 27 tnot 1 53 69 INDEX told 0 64 true 0 68 tsetof 3 86 unfold_off 27 unload_package 1 18 var 1 70 write 1 66 write 2 66 write_canonical 1 67 write_canonical 2 68 write_prolog 1 67 write_prolog 2 67 writeln 1 66 writeln 2 67 writeq 1 67 writeg 2 67 xpp_on 26 xsb_configuration 2 103 xsb_flag 2 103 abort trace facility 120 aggregate predicates prolog 84 tabling 86 byte code files compiler 24 cmplib 24 comparison of terms 90 Compiler 24 directives 30 inlines 35 invoking 24 options
163. nate in XSB since ancestor 2 is compiled as a tabled predicate Prolog systems however would go into an infinite loop The user can declare that SLG resolution is to be used for a predicate by using table declarations as here Alternately an auto_table compiler directive can be used to direct the system to invoke a simple static analysis to decide what predicates to table see Section 3 8 4 This power to solve recursive queries has proven very useful in a number of areas including deductive databases language processing 24 25 program analysis 12 9 5 model checking 32 and diagnosis 34 For efficiency we have implemented SLG at the abstract machine level so that tabled predicates will be executed with the speed of compiled Prolog We finally note that for definite programs SLG resolution is similar to other tabling methods such as OLDT resolution 43 see Chapter 5 for details Example 1 0 1 The use of tabling also makes possible the evaluation of programs with non stratified negation through its implementation of the well founded semantics 44 When logic pro gramming rules have negation paradores become possible As an example consider one of Russell s paradoxes the barber in a town shaves every person who does not shave himself written as a logic program table shaves 2 shaves barber Person person Person tnot shaves Person Person person barber person mayor Logically speaking the meaning of
164. nfiguration site_config_dir SiteConfigDir xsb_configuration site_config_libdir SiteConfigLibdir Where user s arch specific libraries are found by default xsb_configuration user_config_libdir UserConfigLibdir xsb_flag FlagName Value Succeeds iff the current value of the XSB flag Flag_Name is Value So one can enumerate all the flag names which the system currently understands together with their current values by using the following query xsb_flag FlagName Value fail The flag names currently supported are Flag Name debugging on iff debug mode is on off otherwise on iff trace mode is on off otherwise goa the goal passed to XSB on command line with the e switch true if nothing is passed dcg_style the DCG style currently used xsb or standard standard is used in Quintus SICSTUS etc See Section 8 4 for more details garbage_collection none sliding or copying depending on the garbage col lection strategy that is currently being employed see also Sec tion 3 5 Note that xsb_flag is used only for dynamic XSB settings i e settings that might change be tween sessions or within the same session For static configuration information the predicate xsb_configuration 2 is used hilog_symbol Symbol Succeeds iff Symbol has been declared as a HiLog symbol or procedurally unifies Symbol with one of the currently known because of a prior dec
165. ng the single argument to appear earlier In that case the joint index would never be used e Trie based Indexing The executable declaration index Predspec trie causes clauses for Predspec to be asserted using tries see 33 which is available through the XSB web page The name trie indexing is something of a misnomer since the trie itself both indexes the term and represents it In XSB the above trie index is formed using a left to right traversal of the unit clauses These indexes can be very effective if discriminating information lies deep within a term and if there is sharing of left prefixes of a term can reduce the space needed to represent terms Furthermore asserting a unit clause as a trie is much faster than asserting it using default WAM code Despite these advantages representing terms as tries leads to semantic differences from asserted code of which the user should be aware First the order of clauses within a trie is arbitrary using asserta 1 or assertz for a predicate currently using trie indexing will give the same behavior as using assert Also the current version of XSB only allows trie indexing for unit clauses Trie based indexing is available only for dynamic predicates dynamic PredSpec is an executable predicate which converts a predicate specified as Predicate Arity to a dynamic predicate If Predicate is not previously defined it will be initialized to empty so that calls to it quietly fail instead
166. nslates to positive_number X Li Lo Li N L1 integer N N gt 0 L1 L2 fraction F L2 L3 form_number N F N L3 Lo Similarly a cut is translated literally Push back lists a proper list of terminals on the left hand side of a DCG rule translate into a sequence of C 3 goals with the first and third arguments reversed For example it_is X is not gt aint becomes it_is X Li Lo Li aint L1 C Lo is L2 C L2 not Li CHAPTER 8 DEFINITE CLAUSE GRAMMARS 127 Disjunction has a fairly obvious translation For example the DCG clause expr E gt expr X term Y E is X y term E translates to the Prolog rule expr E Li Lo expr X Li L1 2C L1 43 L2 h 0 43 term Y L2 L3 E is X Y L3 Lo term E Li Lo 8 3 Definite Clause Grammar predicates The library predicates of XSB that support DCGs are the following phrase Phrase List This predicate is true iff the list List can be parsed as a phrase i e sequence of terminals of type Phrase Phrase can be any term which would be accepted as a nonterminal of the grammar or in general it can be any grammar rule body and must be instantiated to a nonvariable term at the time of the call otherwise an error message is sent to the standard error stream and the predicate fails This predicate is the usual way to commence execution of grammar rules If List is bound
167. nvention does not apply to the subdirectories of the examples directory which illustrate advanced features of XSB CHAPTER 2 GETTING STARTED WITH XSB glob loc space 786432 bytes 432 in use global 240 bytes local 192 bytes trail cp space 786432 bytes 468 in use trail 132 bytes choice point 336 bytes SLG subgoal space O bytes O in use SLG unific space 65536 bytes O in use SLG completion 65536 bytes O in use SLG trie space O bytes O in use call ret trie O bytes trie hash tables O subgoals currently in tables O subgoal check insert attempts inserted O answer check insert attempts inserted 11 786000 free 785964 free O free 65536 free 65536 free O free O bytes O subgoals in the tables O answers Time 0 610 sec cputime 18 048 sec elapsetime yes halt I had enough End XSB cputime 1 19 secs elapsetime 270 25 secs my_favourite_prompt gt 2 6 Exiting XSB in the tables If you want to exit XSB issue the command halt or simply type CTRL d at the XSB prompt To exit XSB while it is executing queries strike CTRL c a number of times Chapter 3 System Description Throughout this chapter we use XSB_DIR to refer to the directory in which XSB was installed 3 1 Entering and Exiting XSB After the system has been installed the emulator s executable code appears in the file XSB_DIR bin xsb or if after being built XSB is later installed at a central location SHARED_X
168. odule unless it is declared otherwise via an export or import declaration Symbols that are local to a given module are not visible to other modules e Every other symbol essentially function symbols in a module is assumed to be global its module prefix is usermod unless declared otherwise e Ifa symbol is imported from another module via an explicit import declaration the module prefix of the symbol is the module it is imported from any other symbol takes the module where the symbol occurs as its module prefix e The XSB interpreter is entered with usermod as its working module e Symbols that are either defined in non modules loaded into the system or that are dynamically created by the use of standard predicates such as read 1 functor 3 2 etc are contained in usermod The following facts about the module system of XSB may not be immediately obvious e If users want to use a symbol from another module they must explicitly import it otherwise the two symbols are different even if they are of the same functor arity form e A module can only export predicate symbols that are defined in that module As a conse quence a module cannot export predicate symbols that are imported from other modules This happens because an import declaration is just a request for permission to use a symbol from a module where its definition and an export declaration appear e The implicit module for a particular symbol appearing in a mod
169. of stratified as not using recursion through negation is this program stratified Would the program still be stratified if the order of the literals in the body of clauses for lrd_p lrd_q or lrd_r were changed The rules for p q and r are involved in a positive loop and no answers are ever produced Each of these atoms can be failed thereby proving s Exercise 5 3 2 thus illustrates an instance of how tabling differs from Prolog in executing stratified programs since Prolog would not fail finitely for this program Completely Evaluated Subgoals Knowing when a subgoal is completely evaluated can be useful when programming with tabling Simply put a subgoal S is completely evaluated if an evaluation can produce no more answers for S The computational strategy of XSB makes great use of complete evaluation so that understanding this concept and its implications can be of great help to a programmer Consider a simple approach to incorporating negation into tabling Each time a negative goal is called a separate table is opened for the negative call This evaluation of the call is carried on to termination If the evaluation terminates its answers if any are used to determine the success of failure of the calling goal This general mechanism underlies early formulations for tabling stratified programs 21 40 Of course this method may not be efficient Every time a new negative goal is called a new table must be started and run to termina
170. of issuing Undefined predicate error messages If the CHAPTER 6 STANDARD PREDICATES 108 predicate is previously defined and dynamic dynamic 1 is a noop If previously defined as compiled Predicate will be converted to dynamic which means that clauses can be added although the compiled portion cannot be manipulated Note that dynamic 1 can be used like a compiler directive since it will be passed through to be executed when the module is loaded Note however that the semantics is different from that of the standard 20 when the file contains clauses defining the so specified predicate table PredSpec is an executable predicate where PredSpec is a predicate specification for a dynamic predi cate This is also a compiler directive when PredSpec specifies a compiled predicate See the section of this manual on compiler directives This predicate declares a dynamic predicate to be tabled It simply saves information to be used at the time of assert and so it must be called before any clauses are asserted into the specified predicate in order for the predicate to be tabled 6 11 Execution State abort Abandons the current execution and returns to the top level This predicate is normally used in one of the following two cases e when some error condition or exception has occurred and carrying on the computation is of no further use e when using the debugger see Chapter 7 Currently all exception handling routines ter
171. of macro expansion without any arithmetic evaluation The only ex ceptions to this rule are the and operators which if one of the sides does not evaluate to a number perform string comparison instead ignoring trailing and leading spaces Inside arithmetic expressions the defined special user macro is also available it takes only one argument which is not evaluated and returns 1 if it is the name of a user macro and 0 otherwise e if expr This meta macro invokes the arithmetic evaluator in the same manner as eval and compares the result of evaluation with the string 0 in order to begin a conditional block In particular note that the logical value of expr is always true when it cannot be evaluated to a number e mode keyword This meta macro controls gpp s operating mode See below for a list of mode commands The key to gpp s flexibility is the mode meta macro Its first argument is always one of a list of available keywords see below its second argument is always a sequence of words separated by whitespace Apart from possibly the first of them each of these words is always a delimiter or syntax specifier and should be provided as a C string delimited by double quotes The various special matching sequences listed in the section on syntax specification are available Any mode command is parsed in a mode where is understood to be a C style string so it is safe to put any character insi
172. og write_prolog 2 has the same behavior as write_prolog 1 but the output stream is explic itly designated using the first argument writeq Term Acts as write Term but atoms and functors are quoted whenever necessary to make the result acceptable as input to read 1 writeq 1 treats treats terms of the form VAR N the same way as write 1 writing A if N 0 etc writeq 1 always succeeds without producing an error writeq Stream Term writeq 2 has the same behavior as writeg 1 but the output stream is explicitly designated using the first argument write_canonical Term This predicate is provided so that the HiLog term Term if written to a file can be read back using read 1 regardless of special characters appearing in Term or prevailing operator declarations Like write_prolog 1 write_canonical 1 writes all proper HiLog terms to the current output stream using the standard Prolog syntax see Section 4 1 on the standard syntax of HiLog terms write_canonical 1 also quotes atoms and functors as writeq 1 does to make them acceptable as input of read 1 Operator declarations are not taken into consideration and compound terms are therefore always written in the form predicate name arg1 argn Unlike writeq 1 write_canonical 1 does not treat terms of the form VAR N spe cially It writes square bracket lists using 2 and that is foo bar is written as foo bar read_canonica
173. og_Clause dcg Succeeds iff the DCG rule DCG_Rule translates to the Prolog clause Prolog Clause At the time of call DCG_Rule must be bound to a term whose principal functor is gt 2 or else the predicate fails dcg 2 must be explicitly imported from the module dcg 8 4 Two differences with other Prologs The DCG expansion provided by XSB is in certain cases different from the ones provided by some other Prolog systems e g Quintus Prolog SICStus Prolog and C Prolog The most important of these differences are 1 XSB expands a DCG clause in such a way that when a 0 is the last goal of the DCG clause the expanded DCG clause is always steadfast That is the DCG clause a gt py E pes gets expanded to the clause a A B b A C C B c A B and not to the clause CHAPTER 8 DEFINITE CLAUSE GRAMMARS 129 a A B b A B cCA B as in Quintus SICStus and C Prolog The latter expansion is not just optimized but it can have a different unintended meaning if a 2 is called with its second argument bound However to obtain the standard expansion provided by the other Prolog systems the user can simply execute set_dcg style standard To switch back to the XSB style DCG s call set_dcg_style xsb This can be done anywhere in the program or interactively By default XSB starts with the XSB style DCG s To change that start XSB as follows xsb e set_dcg _style standard Problems o
174. og_arg 1 apply foo bar A A bar CHAPTER 6 STANDARD PREDICATES 79 Note the difference between the last two examples The difference is due to the fact that apply 1 is a Prolog term while apply 2 is a proper HiLog term Term Functor ArgList Succeeds when Term is any Prolog or HiLog term Functor is its Prolog functor and ArgList is the list of its arguments The use of 2 pronounced univ although convenient can nearly always be avoided Whenever efficiency is critical it is advisable to use the predicates functor 3 and arg 3 since 2 is implemented by calls to these predicates The behaviour of 2 is as follows e If initially Term is uninstantiated then the list in the second argument of 2 must be instantiated either to a proper list list of determinate length whose head is an atom or to a list of length 1 whose head is a number e If the arguments of 2 are both uninstantiated or if either of them is not what is expected 2 aborts producing an appropriate error message Examples X 15 1 X _595692 L _595692 1 p a b c L L p a b c h a b c L L apply h a b c map p a b L L apply map p a b T foo T foo T BIX T 3 X 1 T apply X a b T apply X a b T 1 2 Error Wrong type s in argument 2 of 2 Aborting T atb 2 Error Wrong type s
175. ollect all the solutions into a list which may then be processed in any way desired So XSB provides several builtin all solutions predicates which collect solutions into lists Sometimes however one wants simply to perform some aggregate operation over the set of solutions for example to find the maximum or minimum of the set of solutions XSB uses tabling and HiLog to provide a general and powerful aggregation facility through the use of two new builtins setof X Goal Set This predicate may be read as Set is the set of all instances of X such that Goal is prov able If Goal is not provable setof 3 fails The term Goal specifies a goal or goals as in CHAPTER 6 STANDARD PREDICATES 85 call Goal Set is a set of terms represented as a list of those terms without duplicates in the standard order for terms see Section 6 7 If there are uninstantiated variables in Goal which do not also appear in X then a call to this evaluable predicate may backtrack gener ating alternative values for Set corresponding to different instantiations of the free variables of Goal Variables occurring in Goal will not be treated as free if they are explicitly bound within Goal by an existential quantifier An existential quantification can be specified as Y 7G meaning there exists a Y such that G is true where Y is some Prolog term usually a variable Exceptions Same as predicate ca11 1 see Section 6 8 bagof X Goal Bag This pr
176. olume of such trace information can very become large very quickly so this method of tracing is not recommended in general XSB debugger also provides means for the low level control of what must be traced Normally various low level predicates are masked out from the trace since these predicates do not make sense to the application programmer However if tracing below the application level is needed you can retract some of the facts specified in the file syslib debugger_data P and in some cases assert into them All these predicates are documented in the header of that file Here we only mention the four predicates that an XSB developer is more likely to need To get more trace you should retract from the first three predicates and assert into the last one e hide_this_show Pred Arity specifies calls predicate name and arity that the debugger should not show at the prompt However the evaluation of this hidden call is traced e hide_this_hide Pred Arity specifies calls to hide Trace remains off while evaluating those predicates Once trace is off there is no way to resume it until the hidden predicate exits or fails e show_this_hide Pred Arity calls to show at the prompt However trace is switched off right after that CHAPTER 7 DEBUGGING 123 e trace_standard predicate Pred Arity Normally trace doesn t go inside standard predi cates i e those specified in syslib std_xsb P If you need to trace some of those you mus
177. ons an XSB programmer can intermix the use of tabled negation tnot 1 with Prolog s negation A 1 or equivalently fail_if 1 or not 1 These restrictions are discussed in detail below for now we focus on differences in behavior or these two predicates in stratified programs Recall that A S calls S and if S has a solution Prolog executes a cut over the subtree created by S and fails tnot 1 on the other hand does not execute a cut so that all subgoals in the computation path begun by the negative call will be completely evaluated The major reason for not executing the cut is to insure that XSB evaluates ground queries to Datalog programs with negation with polynomial data complexity As seen in Section 5 2 this property cannot be preserved if negation cuts over tables There are other small differences between tnot 1 and 1illustrated in the following exer cise Exercise 5 3 3 In general making a call to non ground negative subgoal in Prolog may be unsound cf 29 but the following program illustrates a case in which non ground negation is sound ngr_p ngr_p _ ngr_p a Its tabled analog is table ngr_tp 1 ngr_tp tnot ngr_tp _ ngr_tp a Version 2 1 of XSB will flounder on the call to ngr_tp but not on the call to ngr_p 0 The description of tnot 1 in Section 6 4 describes other small differences between 1and tnot 1 as implemented in XSB Before leaving th
178. or Pis not a callable term P Exactly the same as fail_if 1 Its existence is only for compatibility with other Prolog systems not P If the goal P has a solution fails otherwise it succeeds It is defined by not P call P fail not _ Argument P must be ground for sound negation although no runtime checks are made by the system Note that in contrast to the other two kinds of negation as failure A 1 and fail_if 1 predicate not 1 is not compiled by the compiler but the above definition is used Exceptions The same as ca11 1 see section 6 8 tnot P Tabling The semantics of tnot 1 allows for correct execution of programs with according to the well founded semantics P must be a tabled predicate For a detailed description of the actions of CHAPTER 6 STANDARD PREDICATES 70 tabled negation for in XSB Version 2 1 see 35 37 Chapter 5 contains further discussion of the functionality of tnot 1 Exceptions instantiation_error P is not ground floundering occurs type_error Pis not a callable term table_error P is not a call to a tabled predicate gt t not P Tabling Same as tnot 1 but does not check for floundering This predicate is not standard but should be explicitly imported from module tables Since it is not safe its use is discouraged P gt Q R Analogous to if P then Q else R i e defined as if by P gt Q R P Q P gt Q R R P gt Q When occ
179. or space reclamation One alternative is to use finda11 3 to collect the intersection first before retracting Another is to use the predicates retract_nr 1 and reclaim_space 1 described below Asserting clauses as WAM code might be considerably slow for some applications To remedy this XSB provides an alternative to assert 1 which implements assert s functionality using the trie based tabling data structures 33 Though trie based dynamic code can be created and usually executed significantly faster than using assert 1 users of the following predicates should be aware that trie based assert can be used only for unit clauses where a relation is viewed as a set and where the order of the facts is not important XSB does not at this time fully support dynamic predicates defined within compiled code The only way to generate dynamic code is by explicitly asserting it or by using the standard predicate load_dyn 1 to read clauses from a file and assert them see the section Asserting Dynamic Code in Volume 2 There is a dynamic 1 predicate see page 107 that declares a predicate within the system so that if the predicate is called when no clauses are presently defining it the call will quietly fail instead of issuing an Undefined predicate error message assert Clause adds a dynamic clause Clause to the database Clause must be of one of the forms Head or Head Body Note that because of the precedence of 2 using the second
180. or tabled aggregation XSB can be configured to use local scheduling via the configuration option enable local scheduling and remaking XSB This will not affect the default version of XSB which will also remain available Chapter 6 Standard Predicates Standard predicates are always available to the Prolog interpreter and do not need to be imported or loaded explicitly as do other Prolog predicates Our standard predicates are listed below Stan dard predicates whose semantics depend on HiLog terms or on SLG evaluation are marked as HiLog or Tabling It is possible for the user to add standard predicates not provided in the standard release See the section on Customizing XSB 6 1 Input and Output Presently input and output can only be done with respect to the current input and output streams These can be set reset or checked using the file handling predicates described below The default input and output streams are internally denoted by userin and userout the user accesses them both via the name user and they refer to the user s terminal 6 1 1 File Handling see F Makes file F the current input stream e If there is an open input stream associated with the file that has F as its file name and that stream was opened previously by see 1 then it is made the current input stream e Otherwise the specified file is opened for input and made the current input stream If the file does not exist see 1 fails
181. out from these terms during the specialisation process As a result some head unification get_ or unify_ and some argument register put_ WAM instructions of the original program become unneces sary These instructions are eliminated from both the specialised calls as well as from the specialised versions of the predicates Though these properties are sufficient to get the idea behind specialisation the actual specialisation performed by the XSB compiler can be better understood by the following example The example shows the specialisation of a predicate that checks if a list of HiLog terms is ordered ordered ordered X ordered X Y Z ordered Ul X lt Y _ ordered Y Z ordered X ordered X Y Z i i 2 2 X lt Y ordered Y Z index _ ordered _ ordered X _ ordered X Y Z X lt Y _ ordered Y Z The transformation driven by the partially instantiated call ordered Y Z effectively allows predicate ordered 2 to be completely deterministic when used with a proper list as its argument and to not use any unnecessary heap space for its execution We note that appropriate index directives are automatically generated by the XSB compiler for all specialised versions of predicates The default specialisation of partially instantiated calls is without any folding of the clauses that the calls select Using the spec_repr compiler option see Section 3 8 2 specialisation with
182. ows upward from the bottom of the region and the choice point stack grows downward from the top of the region Because this region is expanded automatically from Version 1 6 0 onward this option should rarely need to be used Default initial size 768 Kbytes m size Allocates size Kbytes of space to the local global stack area The global stack grows upward from the bottom of the region and the local stack grows downward from the top of the region Default 768 Kbytes CHAPTER 3 SYSTEM DESCRIPTION 21 o size Allocates size Kbytes of space to thecompletion stack area Because this region is expanded automatically from Version 1 6 0 onward this option should rarely need to be used Default initial size 64 Kbytes u size Allocates size Kbytes of space to the unification and table copy stack Default 64 Kbytes This option should rarely need to be used D Tells XSB to use a top level command loop driver specified here instead of the standard XSB interpreter This is most useful when XSB is used as a server r Turns off automatic stack expansion g gc_type Chooses the garbage collection strategy that is employed choice of the strategy is between none meaning perform no garbage collection or garbage collection based on sliding or on copying Since garbage collection is only available when the emulator is based on a CHAT model see also the installation options this option only makes sense in this context it is ineffective
183. pare 0p T1 T2 Succeeds if the result of comparing terms T1 and T2 is Op where the possible values for Op are if T1 is identical to T2 lt if T1 is before T2 in the standard order gt if T1 is after T2 in the standard order Thus compare T1 T2 is equivalent to Ti T2 Predicate compare 3 has no associated error conditions sort Li L2 The elements of the list L1 are sorted into the standard order and any identical i e elements are merged yielding the list L2 The time to perform the sorting is O nlogn where n is the length of list L1 Examples sort 3 14 X a X a 2 a X a L L X 3 14 2 a a X no Exceptions instantiation_error Argument 1 of sort 2 is a variable or is not a proper list keysort L1 L2 The list L1 must consist of elements of the form Key Value These elements are sorted into order according to the value of Key yielding the list L2 The elements of list L1 are scanned from left to right Unlike sort 2 in keysort 2 no merging of multiple occurring elements takes place The time to perform the sorting is O nlogn where n is the length of list L1 Note that the elements of L1 are sorted only according to the value of Key not according to the value of Value The sorting of elements in L1 is not guaranteed to be stable Examples keysort 3 a 1 b 2 c 1 a 3 al L L 1 b 1 a 2 c 3 a 3 a no Exceptions instantiation_error Argument 1 of keyso
184. pecial T O 2 2 n ee okie fore 4 eames fae oe See EEE es 68 6 3 CONVENDIENCE visi RE Se RA Bo lee Ree od 68 6 4 Negation and Contra 69 CONTENTS iii 6 5 Meta Logical 2 4 ete e E A Oe a ee a a 70 6 6 All Solutions and Aggregate Predicates 2 0 0 0 0 eee ee es 84 6 6 1 Tabling Aggregate Predicates 2 o aa ee 86 6 0 GOMparisOny 2 5 a a ea ee ee es da ae a ee ee ge ek 90 6 8 Meta Predicates S23 4 4 ae goa hee ee ES ae a ay ees 93 6 9 Information about the State of the Program o 93 6 10 Modification of the Database e 104 6 11 Execution State a dere ee E E e ee a a ee ee ge ae 108 6 12 Table 0 ese nese e ne fen a ae Se ee Ae Eta ae RO A oe A 112 7 Debugging 119 GA Hieh Level SV racine icc nt a eee NE Ae fa Bl Seca ROE ee OE a KX 119 T2 Low Level Tracing i sock Cad Oe wee ee Pa EE ae oe ee aS 122 8 Definite Clause Grammars 124 8 1 General Description woraus een Soe a A ook Re ee 2 es ie 124 8 2 Translation of Definite Clause Grammar rules 125 8 3 Definite Clause Grammar predicates 2 2 ee 127 8 4 Two differences with other Prologs o o e ee 128 8 5 Interaction of Definite Clause Grammars and Tabling 129 9 Restrictions and Current Known Bugs 131 9 1 Gurrent Restrictions so aara ees pee dae Be Ph a Se Se a 131 92 Known Bugs ria eat ara eh ed ae gl Boe A ed RG ed 132 A GPP Generic Pr
185. precise A tabled computation can be viewed as a directed graph in which there is a link from one non completed tabled predicate P1 to a non completed tabled predicate P2 if P2 or tnot P2 is called by P1 Of course this graph constantly changes through an evaluation as resolution proceeds subgoals are completed and so on Any directed graph can be uniquely partitioned into a set of maximal strongly connected components or SCCs and these sets correspond to sets of mutually recursive predicates The SCCs then are reminiscent of the LRD stratifiedstratification discussed in Section 5 3 1 except that both positive and negative links are counted as dependencies From this view to optimally compute tabled aggregation non optimal answers from a given subgoal S must be returned within the SCC of S but not outside the SCC This action is performed by Local Scheduling It is illustrative to compare local scheduling to Batched Scheduling the default scheduling of XSB Batched scheduling returns answers as they are derived and resembles Prolog s tuple at a time scheduling Local scheduling was shown to be quite efficient in terms of time and space in 18 and is the fastest scheduling strategy that we know of for computing a sequence of answers The same paper also introduced Local Scheduling which computes all answers for each SCC and return only the best answer or answers out of the SCC when the SCC is completely evaluated exactly the thing f
186. predicate term_expansion 2 the user can specify any desired transformation to be done as clauses are read by the reader of XSB s parser Extra conditions in the form of explicit Prolog literals or control constructs such as if then elses gt 2 or cuts 0 may be included in the Body of the DCG rule and they work exactly as one would expect An overview of the syntax of DCGs supported by XSB is as follows 1 A non terminal symbol may be any HiLog term other than a variable or a number A variable which appears in the body of a rule is equivalent to the appearance of a call to the built in predicate phrase 3 as it is described below 2 A terminal symbol may be any HiLog term In order to distinguish terminals from nonter minals a sequence of one or more terminal symbols a P y 0 is written within a grammar rule as a Prolog list 6 y 0 with the empty sequence written as the empty list The list of terminals may contain variables but it has to be a proper list or else an error message is sent to the standard error stream and the expansion of the grammar rule that 124 CHAPTER 8 DEFINITE CLAUSE GRAMMARS 125 contains this list will fail If the terminal symbols are ASCII character codes they can be written as elsewhere as strings 3 Extra conditions expressed in the form of Prolog predicate calls can be included in the body right hand side of a grammar rule by enclosing such conditions in curly bracket
187. put endif APPENDIX A GPP GENERIC PREPROCESSOR 149 To finish here are some examples involving mode switching The following example is self explanatory starting in default mode mode push define f x x x mode standard TeX f blah mode string mode comment x f urf blah define F00 bar and some more mode pop f F00 A good example where a user defined mode becomes useful is the gpp source of this document available with gpp s source code distribution Another interesting application is selectively forcing evaluation of macros in C strings when in cpp mode For example consider the following input define blah x and he said x blah foo Obviously one would want the parameter z to be expanded inside the string There are several ways around this problem mode push mode nostring X define blah x and he said x mode pop mode quote define blah x and he said x mode string QQQ define blah x and he said x The first method is very natural but has the inconvenient of being lengthy and neutralizing string semantics so that having an unevaluated instance of x in the string or an occurrence of would be impossible without resorting to further contorsions The second method is slightly more efficient because the local presence of a quote character makes it easier to control what is evaluated and wha
188. r a parenthesis character The set of additional matched characters in comparison with Yo is customizable using the mode charset par command The default setting is to have the characters in as parentheses Moreover all of these matching subsets except w and W can be negated by inserting a i e by writing 2 instead of Az APPENDIX A GPP GENERIC PREPROCESSOR 141 Note an important distinctive feature of start sequences when the first character of a macro or comment string start sequence is or one of the above special sequences it is not taken to be part of the sequence itself but is used instead as a context check for example a start sequence beginning with n matches only at the beginning of a line but the matching newline character is not taken to be part of the sequence Similarly a start sequence beginning with matches only if some whitespace is present but the matching whitespace is not considered to be part of the start sequence and is therefore sent to output If a context check is performed at the very beginning of a file or more generally of any body to be evaluated the result is the same as matching with a newline character this makes it possible for a cpp mode file to start with a meta macro call A 5 Evaluation Rules Input is read sequentially and interpreted according to the rules of the current mode All input text is first matched against the specified comment
189. r instantiated by this call CHAPTER 6 STANDARD PREDICATES 106 abolish PredSpec Removes the definition of the specified predicate PredSpec is of the form Pred Arity Everything about the abolished predicate is completely forgotten by the system including the dynamic property There is also an abolish 2 which takes Pred and Arity as its two arguments clause Head Body Returns through backtracking all dynamic clauses in the database whose head matches Head and Body matches Body For facts the Body is true retract_nr Clause Performs just as retract 1 does except that it does not reclaim the space used by the retracted clause This is provided to allow programmers to modify dynamic clauses while executing them a practice that is discouraged For example to retract an intersection as described above one could do p X q X retract_nr p X retract_nr q X fail In order to reclaim space after using retract_nr 1 see reclaim_space 1 below Predi cate retract_nr 1 is not a standard predicate and must be imported from module assert retract_nr 1 is provided for partial compatibility with the retract 1 predicate of SB Prolog reclaim_space Head Runs through the dynamic code for the predicate indicated by Head and reclaims space for any clauses that have been deleted from that predicate by retract_nr 1 This cannot safely be used when execution is still within some invocation of the specified predicate or wi
190. re outdated i e Older than the last modification time of the source files mi_foreign This option is used only when mode analysis is performed on XSB system modules This option is needed when analyzing standard and machine in syslib sysmod Mainly used by developers when compiling system modules If specified standard pred icates listed in Appendix B are automatically available for use only if they are primitive predicates see the file syslib machine P for a current listing of such predicates When compiling in this mode non primitive standard predicates must be explicitly imported from the appropriate system module verbo Compiles the files modules specified in verbose mode printing out information about the progress of the compilation of each predicate profile This option is usually used when modifying the XSB compiler When specified the compiler prints out information about the time spent in each phase of the compilation process asm_dump compile_off Generates a textual representation of the SLG WAM assembly code and writes it into the file module A where module is the name of the module file being compiled WARNING This option was created for compiler debugging and is not intended for general use There might be cases where compiling a module with these options may cause generation of an incorrect A and 0 file In such cases the user can see the SLG WAM instructions that are generated for a module by compiling
191. rectory The default is usr include if no I option is specified Multiple I options may be specified to look in several directories e D name val Define the user macro name as equal to val This is strictly equivalent to using the define meta macro but makes it possible to define macros from the command line If val makes references to arguments or other macros it should conform to the syntax of the mode specified on the command line Note that macro argument naming is not allowed on the command line e 2 Set text mode to Unix mode LF terminator Any CR character in the input is systematically discarded This is the default under Unix systems e zZ Set text mode to DOS mode CR LF terminator In this mode all CR characters are removed from the input and all output LF characters are converted to CR LF This is the default if gpp is compiled with the WIN_NT option e x Enable the use of the exec meta macro Since ezec includes the output of an arbitrary shell command line it may cause a potential security threat and is thus disabled unless this option is specified e m Enable automatic mode switching to the cpp compatibility mode if the name of an included file ends in h or c This makes it possible to include C header files with only minor modifications APPENDIX A GPP GENERIC PREPROCESSOR 136 n Prevent newline or whitespace characters from being removed from the input when they occur as the end of a
192. redicate abort 0 which causes a return to the top level An example of break 0 s use is the following break Break level 1 1 break Break level 2 2 end_of_file End break level 2 yes 1 Entering a break closes all incomplete tables those which may not have a complete set of answers Closed tables are unaffected even if the tables were created during the computation for which the break was entered halt Exits the XSB session regardless of the break level On exiting the system cpu and elapsed time information is displayed prompt NewPrompt 01dPrompt Sets the prompt of the top level interpreter to NewPrompt and returns the old prompt in OldPrompt An example of prompt 2 s use is the following prompt Yes master gt P P no Yes master gt fail no Yes master gt cputime CPU_Time Returns the CPU_Time at the time of the call in seconds The difference between results of successive calls to this predicate can measure the time spent in specific predicates statistics Prints on the current output stream e Information about allocation of memory see Section 3 5 containing the global stack heap and local environment stack trail and choice point stack CHAPTER 6 STANDARD PREDICATES 110 SLG subgoal space tablestack SLG unification stack SLG completion stack the space occupied by subgoal and answer tables
193. replacement of the selected clauses with the representative of these clauses is performed Using this compiler option predicate ordered 2 above would be specialised as follows ordered ordered XIY _ ordered X Y index _ ordered 2 2 _fordered X _ ordered X YIZ X lt Y _ ordered Y Z We note that in the presense of cuts or side effects the code replacement operation is not always sound i e there are cases when the original and the residual program are not computationally equivalent with respect to the answer substitution semantics The compiler checks for sufficient but not necessary conditions that guarantee computational equivalence and if these conditions are not met specialisation is not performed for the violating calls The XSB compiler prints out messages whenever it specialises calls to some predicate For example while compiling a file containing predicate ordered 1 above the compiler would print out the following message CHAPTER 3 SYSTEM DESCRIPTION 30 Specialising partially instantiated calls to ordered 1 The user may examine the result of the specialisation transformation by using the spec_dump compiler option see Section 3 8 2 Finally we have to mention that for technical reasons beyond the scope of this document specialisation cannot be transparent to the user predicates created by the transformation do appear during tracing 3 8 4 Compiler Directives The following
194. rt 2 is a variable or is not a proper list type_error The elements of L1 are not of the form Key Value CHAPTER 6 STANDARD PREDICATES 93 6 8 Meta Predicates call X If X is a nonvariable term in the program text then it is executed exactly as if X appeared in the program text instead of cal1 X e g pla call a x r s x is equivalent to e Pads GO r Y A However if X is a variable in the program text then if at runtime X is instantiated to a term which would be acceptable as the body of a clause the goal ca11 X is executed as if that term appeared textually in place of the ca11 X except that any cut occurring in X will remove only those choice points in X If X is not instantiated as described above an error message is printed and cal1 1 fails Exceptions instantiation_error Argument 1 of ca11 1 is not instantiated type error Argument 1 of cal1 1 is not a callable term X where X is a variable executes exactly the same as ca11 X However the explicit use of call 1 is considered better programming practice The use of a top level variable subgoal elicits a warning from the compiler once X once 1 is defined as once X call X once 1 should be used with care in tabled programs The compiler can not determine whether a tabled predicate is called in the scope of once 1 and such a call may lead to runtime errors If a tabled predicate may occur in the scope of once
195. rty X dynamic module_property Module Property The standard predicate module_property 2 can be used to find the properties of any current module Succeeds iff Module is the name of a current module having Property as one of its properties Or procedurally Property is unified with the currently known properties of the module having Module as its name Currently the following properties are associated with modules implicitly Property Explanation unloaded The module including system modules though it is current is yet unloaded in the system loaded The module including system modules is loaded in the Eee system this is always the case for global modules listing Lists in the current output stream the clauses for all dynamic predicates found in module usermod Note that listing O does not list any compiled predicates unless they have the dynamic property see predicate_property 2 A predicate gets the dynamic property when it is explicitly declared as dynamic or automatically acquires it when some clauses for that predicate are asserted in the database In cases where a predicate was compiled but converted to dynamic by asserting additional clauses for that predicate listing O will just display an indication that there exist compiled clauses for that predicate and only the dynamically created clauses of the predicate will be listed For example CHAPTER 6 STANDARD PREDICATES 101 user Compiling user
196. ry table_append L a Out i e with no genlist 2 and backtrack through a few answers Will table_append 3 ever succeed for this predicate Why not Suppose DCG predicates Section 8 are defined to be tabled How is this similar to tabling append Another issue to be aware of when using tabling in XSB is tracing XSB s tracer is a standard 4 port tracer that interacts with the engine at each call exit redo and failure of a predicate see Chapter 7 When tabled predicates are traced these events may occur in unexpected ways as the following example shows Exercise 5 2 6 Consider a tabled evaluation when the query a 0 X is given to the following program table mut_ret_a 2 mut_ret_b 2 mut_ret_a X Y mut_ret_d X Y mut_ret_a X Y mut_ret_b X Z mut_ret_c Z Y mut_ret_b X Y mut_ret_c X Y mut_ret_b X Y mut_ret_a X Z mut_ret_d Z Y mut_ret_c 2 2 mut_ret_c 3 3 mut_ret_d 0 1 mut_ret_d 1 2 mut_ret_d 2 3 mut_ret_a 0 1 can be derived immediately from the first clause of mut_ret_a 2 All other answers to the query depend on answers to the subgoal mut_ret_b 0 X which arises in the evaluation of the second clause of muat_ret_a 2 Each answer to mut_ret_b 0 X in turn depends on an answer to mut_ret_a 0 X so that the evaluation switches back and forth between deriving answers for mut_ret_a 0 X and mut_ret_b 0 X Try tracing this evaluation using creep and skip Do you find the behavior intuiti
197. ry xsb 2 If you decide to move XSB to some other place make sure that the entire directory tree is moved XSB executable looks for the files it needs relatively to its current position in the file system You can compile XSB under Microsoft Visual C compiler to create a console supported top loop or a DLL by following these steps 1 cd build 2 Type makexsb_wind CFG option DLL yes ORACLE yes SITE_LIBS libraries e The items in square brackets are optional e The options for CFG are release or debug The latter is used when you want to compile XSB with debugging enabled e The other parameters to makexsb_wind are optional The DLL parameter tells Visual C to compile XSB as a DLL The ORACLE parameter compiles XSB with support for Oracle DBMS If ORACLE is specified you must also specify the necessary Oracle libraries using the parameter SITE_LIBS CHAPTER 2 GETTING STARTED WITH XSB 9 3 The above command will compile XSB as requested and will put the XSB executable in XSB_DIR config x86 pc windows bin xsb exe If you requested to compile XSB as a DLL then the DLL will be placed in XSB_DIR config x86 pc windows bin xsb d11l Note the XSB executable and the DLL can coexist in the same source tree structure However if you first compiled XSB as an executable and then want to compile it as a DLL or vice versa then you must run makexsb_wind clean in between 2 3 Invoking XSB Under Unix XSB
198. s f and gt gt For example one can write positive_integer N gt N integer N N gt 0 4 The left hand side of a DCG rule must consist of a single non terminal possibly followed by a sequence of terminals which must be written as a unique Prolog list Thus in XSB unlike SB Prolog version 3 1 push back lists are supported 5 The right hand side of a DCG rule may contain alternatives written using the usual Prolog s disjunction operator or using the usual BNF disjunction operator 6 The Prolog control primitives if then else gt 2 nots not 1 fail_if 1 A 1 or tnot 1 and cut 0 may also be included in the right hand side of a DCG rule These symbols need not be enclosed in curly brackets All other Prolog s control primitives such as repeat O must be enclosed explicitly within curly brackets if they are not meant to be interpreted as non terminal grammar symbols 8 2 Translation of Definite Clause Grammar rules The procedural interpretation of a DCG rule is that it takes an input list of symbols or character codes analyzes some initial portion of that list and produces the remaining portion possibly en larged if pushback lists are used as output for further analysis As an abbreviation the arguments required for the input and output lists are not written explicitly in a grammar rule but are added when the rule is translated into an ordinary Prolog clause In this sect
199. s record Key Term Ref recorda Key Term Ref recorded Key Term Ref recordz Key Term Ref rename QldName NewName repeat retract Term retractall Predicate see File APPENDIX B STANDARD PREDICATES AND FUNCTIONS 156 seeing File seen set_global_compiler_options 0OptionsList setof Elem Goal Set skip Char sort Input Output spy Predicate List statistics statistics Number structure Term tab Count table_prop Pred Type Value table_prop Pred Type OldValue NewValue table_state Call State tbagof Elem Goal List tell File telling File tfindall Elem Goal List tilde_expand filename File_name Expanded Filename tnot Goal told trace true tsetof Elem Goal List ttywrite Term Same as write but always writes to the standard output ttywritenl Terms Terms can be a comma list of terms The predicate calls ttywrite 1 to print all the terms in the list and then prints a new line symbol at the end shell Command shell Command Result var Term write Term write Stream Term writeln Term write_prolog Term write_prolog Stream Term writeq Term write_canonical Term xsb_flag Flag Value Query bod gt lt l o Ii lt APPENDIX B STANDARD PREDICATES AND FUNCTIONS 157 Term List Term List Goal B 2 List of Standard Functions X Y X Y X Y X Y X Y integer division X mod Y ALY bitwis
200. s sent to the designated output stream get0 N N is the ASCII code of the next character read from the current input stream If the current input stream reaches its end of file a 1 is returned Compatibility Note Unlike other Prologs such as C Prolog the input stream is not closed on encountering the end of file character get N N is the ASCII code of the next non blank printable character from the current input stream It has the same behaviour as get0 1 when an end of file character is encountered Compatibility Note Unlike other Prologs such as C Prolog the input stream is not closed on encountering the end of file character put N Puts the ASCII character code N to the current output stream Exceptions instantiation error N is not instantiated at the time of the call type error N is not an integer at the time of the call tab N Puts N spaces to the current output stream Exceptions instantiation_error N is not instantiated at the time of the call type _error N is not an integer at the time of the call CHAPTER 6 STANDARD PREDICATES 66 6 1 3 Term I O read Term A HiLog term is read from the current or designated input stream and unified with Term according to the operator declarations in force See Section 4 1 for the definition and syntax of HiLog terms The term must be delimited by a full stop i e a followed by a carriage return space or tab Predicate read 1 does not return until a valid
201. s where this can be done only for predicate symbols Usually the following three files are associated with a particular module e A single source file whose name is the module name plus the suffix P e An optional header file whose name is the module name plus the suffix H e An object byte code file whose name consists of the module name plus the suffix 0 The header file is normally used to contain declarations and directives while the source file usually contains the actual definitions of the predicates defined in that module The module hierarchy of XSB is therefore flat nested modules are not possible In order for a file to be a module it should contain one or more export declarations which specify that a set of symbols appearing in that module is visible and therefore can be used by any other module A module can also contain local declarations which specify that a set of symbols are visible by this module only and therefore cannot be accessed by any other module Any file either module or not may also contain import declarations which allow symbols defined in and exported by other modules to be used in the current module We note that only exported symbols can be imported for example importing a local symbol will cause an environment conflict error Export local and import declarations can appear anywhere in the source or header files and have the following forms 7 export syMi Sym
202. set of datalog atoms Currently this must be done manually as explained in 47 available in the tech reports directory Chapter 9 Restrictions and Current Known Bugs In this chapter we indicate some features and bugs of XSB that may affect the users at some point in their interaction with the system If at some point in your interaction with the system you suspect that you have run across a bug not mentioned below please report it to xsb contact cs sunysb edu Please try to find the smallest program that illustrates the bug and mail it to this address together with a script that shows the problem We will do our best to fix it or to assist you to bypass it 9 1 Current Restrictions e The maximum arity for predicate and function symbols is 255 e The maximum length of atoms that can be stored in an XSB object code file is in principle 2 1 but in practice it is 278 1 i e in 32 bit platforms it is bounded by the size of the maximum integer see below e In the current version you should never try to rename a byte code file generated for a module though you can move it around in your file system Since the module name is stored in the file renaming it causes the system to load it into wrong places However byte code files for non modules can be renamed at will e XSB allows up to 1 Gigabyte of address space for 32 bit SUNs and 512 Megabytes of address space for other 32 bit platforms For SUNs the address space for int
203. specification i e a unary HiLog predicate Op must be a Hilog operation i e a 3 ary HiLog predicate that defines an associative operator The predicate must define a binary function in which the first two arguments determine the third Id must be the identity of the operator bagReduce returns with Arg bound to the reduce of the elements of the bag determined by SetPred under the operation Op l e Arg becomes the result of applying the operator to all the elements in the bag that unify with SetPred See the bagSum operator below to see an example of bagReduce s use filterReduce 4 acts as bagReduce 4 with two differences First it does not depend on HiLog so that filterReduce 4 will be more robust especially when XSB s module system is used In addition filterReduce 4 aggregates solutions to Pred using a variance rather than unification An example of the use of filterReduce 4 is given in Chapter 5 bagP0 SetPred Arg 0Order HiLog Tabling CHAPTER 6 STANDARD PREDICATES 88 filterP0 SetPred Arg 0rder Tabling SetPred must be a HiLog set specification i e a unary HiLog predicate Order must be a binary Hilog relation that defines a partial order bagPO returns nondeterministically with Arg bound to the maximal elements under Order of the bag SetPred bagP0 3 can be used with Order being subsumption to reduce a set of answers and keep only the most general answers See the bagMax operator below to see an example of bagPO
204. ssert 1 105 asserta 1 105 assertz 1 105 atom 1 71 atom_chars 2 83 atom_codes 2 82 atomic 1 72 auto_table 27 31 bagAvg 2 90 bagCount 2 90 bagMax 2 88 bagMin 2 89 bagP0 3 88 bagReduce 4 87 bagSum 2 89 164 bootstrap_userpackage 3 17 break 0 108 call 1 93 callable 1 73 cd 1 111 clause 2 106 close 1 64 65 compound 1 72 consult 1 2 22 copy term 2 80 cputime 1 109 current_atom 1 95 current_functor 1 95 current_functor 2 95 current_input 1 94 current_module 1 94 current_module 2 94 current_op 3 104 current_output 1 94 current_predicate 1 96 current_predicate 2 97 dcg 2 128 debug 0 121 debug ct1 2 121 debugging O 121 delete_returns 2 117 display 1 67 dynamic 1 107 edit 1 111 expand_term 2 127 fail 0O 68 fail_if 1 69 file_exists 1 65 filterP0 2 88 filterP0 3 88 filterP0 4 60 filterReduce 4 60 87 INDEX float 1 71 fmt_read 4 68 fmt_write 2 68 functor 3 74 get 1 65 get0 1 65 get_call 3 113 get_calls 3 112 get_calls_for_table 2 114 get_residual 2 116 get_returns 2 114 get_returns 3 115 get_returns_for_call 2 115 halt 0 109 hilog_arg 3 78 hilog functor 3 76 hilog op 3 104 hilog symbol 1 103 import 1 18 index 2 106 integer 1 71 is_charlist 1 73 is_charlist 2 73 is_list 1 72 is_most_general_term 1 73 listing 0 100 listing 1 101 1s 0 111 mi_warn 28 modeinfer 27 module_property 2 100 multifile 2 23
205. start anywhere while in cpp mode they must be at the beginning of a line the default mode has no knowledge of comments and strings but has a quote character while cpp mode has extensive comment string specifications but no quote character Moreover the arguments to meta macros need to be correctly parenthesized in default mode while no such checking is performed in cpp mode This makes it easier to nest meta macro calls in default mode than in cpp mode For example consider the following HTML mode input which tests for the availability of the erec command lt ifeq lt exec echo blah gt blah gt exec allowed lt else gt exec not allowed lt endif gt There is no cpp mode equivalent while in default mode it can be easily translated as ifeq exec echo blah blah exec allowed else exec not allowed endif In order to nest meta macro calls in cpp mode it is necessary to modify the mode description either by changing the meta macro call syntax or more elegantly by defining a silent string and using the fact that the context at the beginning of an evaluated string is a newline character mode string QQQ ifeq exec echo blah blah exec allowed APPENDIX A GPP GENERIC PREPROCESSOR 148 ttelse exec not allowed endif Note however that comments strings cannot be nested inside would go undetected so one needs to be careful about what to include inside s
206. stly 18 cites a case in which the shorter path program which should be less than cubic in the number of vertices in a graph has exponential complexity because of the non optimal solutions that are returned Fortunately this has an easy solution Exercise 5 4 3 The actual shortest_path program has the following definition filterReduce Call Op Id Res filterReduce1 Call Op Id Res fail filterReduce Call Op Id Res filterReducel Call 0p Id Res shortest_path X Y C filterReduce sp X Y min infinity C sp X Y C shortest_path X Z C1 edge Z Y C2 C is C1 C2 sp X Y C edge X Y C min X Y Y number X min X Y X number Y min One Two Min One gt Two gt Min Two Min One Once again try the query shortest_path 1 5 C By simply failing out of filterReduce1 4 and then rereading the maximal value from the table an efficient shortest_path algorithm is derived whose complexity is roughly cubic in the number or vertices of the graph This solution is not general for all predicates but does work for deriving the shortest path A more general solution is provided in Section 5 4 1 filterReduce 4 is an extremely useful predicate It can write database aggregation functions such as min max count sum and average However it can also be used to implement paraconsis tent and quantitative reasoning through Generalized Annotated Programs 23 as detailed in the section on GAPs in Volume 2
207. string start sequences of the current mode except those which are disabled by the i modifier unless the body being evaluated is the con tents of a comment string whose modifier enables macro evaluation The most recently defined comment string specifications are checked for first Important note comments may not appear between the name of a macro and its arguments doing so results in undefined behavior Anything that is not a comment string is then matched against a possible meta macro call and if that fails too against a possible user macro call All remaining text undergoes substitution of argument reference sequences by the relevant argument text empty unless the body being evaluated is the definition of a user macro and removal of the quote character if there is one Note that meta macro arguments are passed to the meta macro prior to any evaluation al though the meta macro may choose to evaluate them see meta macro descriptions below In the case of the mode meta macro gpp temporarily adds a comment string specification to enable recognition of C strings and prevent any evaluation inside them so no interference of the characters being put in the C string arguments to mode with the current syntax is to be feared On the other hand the arguments to a user macro are systematically evaluated and then passed as context parameters to the macro definition body which gets evaluated with that environment The only ex
208. t assert into this predicate In principle by retracting all facts from the first three predicates and asserting enough facts into the last one it is possible to achieve the behavior that approximates the T option However unlike T debugging can be done interactively This does not obviate T however First it is easier to use T than to issue multiple asserts and retracts Second T can be used when the error occurs early on before the moment when XSB shows its first prompt Chapter 8 Definite Clause Grammars 8 1 General Description Definite clause grammars DCGs are an extension of context free grammars that have proven useful for describing natural and formal languages and that may be conveniently expressed and executed in Prolog A Definite Clause Grammar rule in Prolog is executable because it is just a notational variant of a Prolog term that has the following general form Head gt Body with the declarative interpretation that a possible form for Head is Body The procedural inter pretation of a grammar rule is that it takes an input list of symbols or character codes analyses some initial portion of that list and produces the remaining portion possibly enlarged as output for further analysis The arguments required for the input and output lists are not written explicitly in the DCG rule but are added when the rule is translated expanded into an ordinary Prolog clause during parsing By defining the hook
209. t a proper list Aborting Exceptions instantiation_error Argument 2 of 2 is not a proper list copy term Term Copy Makes a Copy of Term in which all variables have been replaced by brand new variables which occur nowhere else It can be very handy when writing meta interpreters for logic based CHAPTER 6 STANDARD PREDICATES 81 languages The version of copy term 2 provided is space efficient in the sense that it never copies ground terms Predicate copy_term 2 has no associated errors or exceptions Examples copy_term X Y X _598948 _598904 lt i copy_term f a X Y X _598892 Y f a _599112 name Constant CharList The standard predicate name 2 performs the conversion between a constant and its character list representation Tf Constant is supplied and is any atom or number CharList is unified with a list of ASCII codes representing the name of the constant In that case CharList is exactly the list of ASCII character codes that appear in the printed representation of Constant If on the other hand Constant is a variable then CharList must be a proper list of ASCII character codes In that case name 2 will convert a list of ASCII characters that can represent a number to a number rather than to a character string As a consequence of this there are some atoms for example 18 which cannot be constructed by using name 2 If conversion to an atom is preferred in these cases
210. t associative Thus it is required that both of the arguments of the operator must be subexpressions of lower precedence than the operator itself that is the principal functor of each subexpression must be of lower precedence unless the subexpression is written in parentheses which automatically gives it zero precedence Operators of type xfy are right associative only the first left hand subexpression must be of lower precedence the right hand subexpression can be of the same precedence as the main operator Left associative operators type yfx are the other way around An atom named Name can be declared as an operator of type Type and precedence Precedence by the command CHAPTER 4 SYNTAX 44 op Precedence Type Name The same command can be used to redefine one of the predefined XSB operators see appendix B 3 However it is not allowed to alter the definition of the comma operator An operator declaration can be cancelled by redeclaring the Name with the same Type but Precedence 0 As a notational convenience the argument Name can also be a list of names of operators of the same type and precedence It is possible to have more than one operator of the same name so long as they are of different kinds infix prefix or postfix An operator of any kind may be redefined by a new declaration of the same kind Declarations for all these built in operators can be found in appendix B 3 For example the buil
211. t in operators and are as if they had been declared by the command op 500 yfx 1 so that 1 2 3 is valid syntax and denotes the compound term 1 2 3 or pictorially IN 3 Z N 1 2 In XSB the list functor 2 is one of the standard operators that can be thought as declared by the command op 661 xfy So in XSB 1 2 0 represents the structure IN ae CHAPTER 4 SYNTAX 45 Contrasting this picture with the picture above for 1 2 3 shows the difference between yfx operators where the tree grows to the left and xfy operators where it grows to the right The tree cannot grow at all for xfx type operators It is simply illegal to combine xfx operators having equal precedences in this way If these precedence and associativity rules seem rather complex remember that you can always use parentheses when in any doubt In XSB at the time when this is written the possible types for prefix operators are fx fy hx hy and the possible types for postfix operators are xf yf We end our discussion about operators by just mentioning that prefix operators of type hx and hy are proper HiLog operators The discussion of proper HiLog operators and their properties is deferred for the manual of a future version Chapter 5 Using Tabling in XSB A Tutorial Introduction XSB has two ways of evaluating predicates The default is to use Prolog style evaluation but by usin
212. t isn t but has the drawback that it is sometimes impossible to find a reasonable quote character without having to either significantly alter the source file or enclose it inside a mode push pop construct For example any occurrence of in the string would have to be quoted APPENDIX A GPP GENERIC PREPROCESSOR 150 The last method demonstrates the efficiency of evaluated strings in the context of selective evaluation since comments strings cannot be nested any occurrence of or inside the gets output as plain text as expected inside a string and only macro evaluation is enabled Also note that there is much more freedom in the choice of a string delimiter than in the choice of a quote character A 8 Advanced Examples Here are some examples of advanced constructions using gpp They tend to be pretty awkward and should be considered as evidence of gpp s limitations The first example is a recursive macro The main problem is that since gpp evaluates everything a recursive macro must be very careful about the way in which recursion is terminated in order to avoid undefined behavior most of the time gpp will simply crash In particular relying on a if else endif construct to end recursion is not possible and results in an infinite loop because gpp scans user macro calls even in the unevaluated branch of the conditional block A safe way to proceed is for example as follows we give the example in Te
213. t to clutter the picture As an example a program which uses more heap and local stack than the default configuration of XSB might be run by invoking XSB with the command xsb m 2000 CHAPTER 3 SYSTEM DESCRIPTION 22 3 6 Memory Management All execution stacks are automatically expenaded in Version 2 1 including the local stack heap region the trail choice point region and the completion stack region Each of these regions begin with an initial value set by the user or the default stated in Section 3 5 and double their size until it is not possible to do so with available system memory At that point XSB tries to find the maximal amount of space that will still fit in system memory Garbage collection is automatically performed for retracted clauses In addition heap garbage collection is automatically included when the enable chat configuration option is used The program area the area into which the code is loaded is also dynamically expanded as needed and the area occupied by dynamic code created using assert 1 or the standard predicate load_dyn 1 is reclaimed when that code is retracted Version 1 8 improves memory management for retracted dynamic code Version 2 1 provides memory management for table space as well Space for tables is dynami cally allocated as needed and reclaimed through use of the predicate abolish_all_tables O see Section 6 12 3 7 Compiling and Consulting In XSB both compiled and interpreted cod
214. termines the location of the libraries it needs based on the full path name by which it was invoked The smart script bin xsb also uses its full path name to determine the location of the various scripts that it needs in order to figure out the configuration of your machine Therefore there are certain limitations on how XSB can be invoked Here are some legal ways to invoke XSB 1 invoking the smart script bin xsb or the XSB executable using their absolute or relative path name 2 using an alias for bin xsb or the executable 3 creating a new shell script that invokes either bin xsb or the XSB executable using their full path names Here are some ways that are guaranteed to not work in some or all cases 1 creating a hard link to either bin xsb or the executable and using it to invoke XSB Symbolic links should be ok 2 changing the relative position of either bin xsb or the XSB executable with respect to the rest of the XSB directory tree Type of Machine The configureation script automatically detects your machine and OS type and builds XSB accordingly Moreover you can build XSB for different architectures while using the same tree and the same installation directory provided of course that these ma chines are sharing this directory say using NFS or Samba All you will have to do is to login to a different machine with a different architecture or OS type and repeat the above sequence of comands The configuratio
215. tes e g allowing non ground negation in tnot 1 5 2 Tabling in Definite Programs Definite programs also called Horn Clause Programs are those programs without negation In XSB this means without the 1 fail_if 1 not 1 or tnot 1 operators Consider the Prolog program path X Y path X Z edge Z Y path X Y edge X Y together with the query path 1 Y This program has a simple declarative meaning there is a path from X to Y if there is a path from X to some node Z and there is a path from Z to Y or if there is a direct path from X to Y Prolog however enters into an infinite loop when computing an answer to this query The inability of Prolog to answer such queries which arise frequently comprises one of its major limitations as an implementation of logic A number of approaches have been developed to address this problem by reusing partial answers to the query path 1 Y 17 43 3 45 46 The ideas behind these algorithms can be described in the following manner First the implementation keeps track of all calls to tabled predicates or tabled subgoals such as path 1 Y in the above example Whenever a new tabled subgoal S is called a check is first made to see whether S is in the table If so S is resolved against answers in the table if not S is entered into the table and the subgoal is resolved against program clauses as in Prolog Answers are handled in the same way When an answer to a tabled subgoal S is
216. the bold literals the three atoms are all undefined since they are neither proved true nor fail However if the evaluation could only look at the literal in italics simpl_r it would discover that simpl_r is involved in a positive loop and since there is only one clause for simpl_r the evaluation could conclude that the atom was false This is exactly what XSB does delays the evaluation of tnot simpl s in the clause for simpl_r and looks ahead to the next literal in the body of that clause This action of looking ahead of a negative literal is called delaying A delayed literal is moved into the delay list of a current path of computation Whenever an answer is derived the delay list of the current path of computation is copied into the table If the delay list is empty the answer is unconditional otherwise it is conditional Of course for definite programs any answers will be unconditional we therefore omited delay lists when discussing such programs In the above program delaying occurs for the negative literals in clause for simp1_p X simp1_s and simpl r In the first two cases conditional answers can be derived while in the third simp1_r will fail as mentioned above Delayed literals eventually become evaluated through simplification Consider an answer of the form simp1_p X tnot simpl_s where the is used to represent the end of the delay list If after the answer is copied into the table simpl_s turns out to be false
217. the cor responding HiLog term creating new uninstantiated variables for its arguments As happens with functor 3 all constants can be their own principal function symbols Examples hilog functor f a b c F A F f A 3 hilog_functor X a b c F A X _595836 F _595836 A 3 CHAPTER 6 STANDARD PREDICATES TT hilog_functor map P a b F A _595828 map _595828 2 gt yu l hilog_functor T p 2 T p _595708 _595712 hilog_functor T h 2 apply h _595712 _595716 H i hilog_functor T X 3 T apply _595592 _595736 _595740 595744 _595592 p lt I hilog_functor T p f a 2 T apply p f a _595792 _595796 hilog_functor T h p a L1 L2 1 T apply apply apply h p a _595984 _595776 _596128 L1 _595984 L2 _595776 hilog_functor T a b 3 T apply a b _595820 _595824 _595828 arg Index Term Argument Unifies Argument with the Index argument of Term where the index is taken to start at 1 Initially Index must be instantiated to any integer and Term to any non variable Prolog or HiLog term The arguments of the Term are numbered from 1 upwards An atomic term has 0 arguments If the initial conditions are not satisfied or J is out of range the call quietly fails Examples arg 2 p a b A arg 2 h a b A Aza arg 0 foo A arg 2 a b c A A
218. ther class of operating system C functions can be made callable from XSB either directly within a process or using a socket library XSB can access external data in a variety of ways through an Oracle interface through an ODBC interface or through a variety of mechanisms to read data from flat files These interfaces are all described in Volume 2 of this CHAPTER 1 INTRODUCTION 4 manual Another feature of XSB is its support for extensions of normal logic programs through pre processing libraries Currently supported are Extended logic programs under the well founded semantics F Logic and Annotated Logic Programs These libraries are described in Volume 2 of this manual Source code is provided for the whole of XSB including the engine interfaces and supporting functions written in C along with the compiler top level interpreter and libraries written in Prolog It should be mentioned that we adopt some standard notational conventions such as the name arity convention for describing predicates and functors to denote input arguments to denote output arguments for arguments that may be either input or output and for arguments that are both input and output can be changed by the procedure See Section 3 8 4 for more details Also the manual uses UNIX syntax for files and directories except when it specifically addresses other operating systems such as Windows Finally we note that XSB is under continuous development and
219. tion We would like to use information already derived from the computation to answer a new query if at all possible just as with definite programs XSB addresses this problem by keeping track of the state of each subgoal in the table A call can have a state of complete incomplete or not_yet_called Calls that do have table entries may be either complete or incomplete A subgoal in a table is marked complete only after it is determined to be completely evaluated otherwise the subgoal is incomplete If a tabled subgoal is not present in the table it is termed not_yet_called XSB contains predicates that allow a user to examine the state of a given table Section 6 12 Using these concepts we can overview how tabled negation is evaluated for stratified programs If a literal tnot S is called where S is a tabled subgoal the evaluation checks the state of S If S is complete the engine simply determines whether the table contains an answer for S Otherwise the engine suspends the computation path leading to tnot S until S is completed and calls S if necessary Whenever a suspended subgoal tnot S is completed with no answers the engine resumes the evaluation at the point where it had been suspended We note that because of this behavior tracing programs that heavily use negation may produce behavior unexpected by the user CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 54 tnot 1 vs A 1 Subject to some semantic restricti
220. two clauses Dynamic Predicates For a dynamic predicate to which no clauses have yet been asserted IndexSpec is either an IndexElt or a list of IndexElts Each IndexElt specifies an argument or group of arguments on which to build an index Syntacti cally an IndexE1t in its turn is a non negative integer or a sequence of up to three non negative integers separated by e g 1 2 3 For example index p 3 2 1 indicates that clauses asserted to the predicate p 3 should be indexed on both the second and the first argument Subsequent calls to p 3 will first check to see if the second argument is nonvariable and if so use that index If the second argument is variable it will check to see if the first argument is nonvariable and if so use that index As another example one could specify index p 5 1 2 1 4 After clauses are asserted to it a call to p 5 would first check to see if both the first and second arguments are nonvariable and if so use an index based on both those values Otherwise it would see if the second argument is nonvariable and if so use an index based on it Otherwise it would see if the fourth argument is nonvariable and if so use an index based on it As a last resort it would use no index but backtrack through all the clauses in the predicate Notice that it may well make sense to include an argument that appears in a joint specification later alone as 1 in this example but it never makes sense forci
221. ty of the compound term that results from the encoding of t is one more than the arity of t Note that the encoding of HiLog terms described above implies that even though the HiLog terms p a b h a b externally appear to have the same form in the presence of a hilog declaration for h but not for p they are completely different This is because these terms are shorthands for the terms whose internal representation is p a b apply h a b respectively Furthermore only h a b is unifiable with the HiLog term whose external represen tation is X a b We end this short discussion on the encoding of HiLog terms with a small example that illus trates the way the encoding described above is being done Assuming that the following declarations of parameter symbols have taken place hilog h hilog hilog before the compound terms of page 38 were read by XSB the encoding of these terms in predicate calculus using the described transformation is as follows foo bar prolog a X apply hilog X apply 123 john 500 apply X kostis sofia apply X Y Z apply Y W f a apply b c d apply map double apply apply h apply map P A B C 4 3 Operators From a theoretical point of view operators in Prolog are simply a notational convenience and add absolutely nothing to the power of the language For example in most Prologs is an infix operator so CHAPTER 4 SYNTAX 43 2 1 is an alt
222. u can write portable program to run under XSB and and other prologs that support C style preprocessing and use conditional compilation to account for the differences ifdef XSB_PROLOG XSB specific stuff else other Prolog s stuff endif common stuff However as mentioned earlier XSB lets the user filter programs except the programs that belong to XSB distribution through any preprocessor the user wants To this end one only needs to assert the appropriate command into the predicate xpp_program which should be imported from module parse The command should not include the file name XSB appends the name of the file to be compiled to the command supplied by the user For instance executing assert xpp_program usr bin m4 E G CHAPTER 3 SYSTEM DESCRIPTION 27 before calling the compiler will have the effect that the next XSB program passed to the compiler will be first preprocessed by the M4 macro package Note that the XSB compiler automatically clears out the xpp program predicate so there is no need to tidy up each time But this also means that if you need to compile several programs with a non standard preprocessor then you must specify that non standard preprocessor each time the program is compiled auto_table When specified as a compiler option the effect is as described in Section 3 8 4 Briefly a static analysis is made to determine which predicates may loop under Prolog s SLD evalua tion These predicates
223. uch a silent evaluated string Remember that macros without arguments are actually understood to be aliases when they are called with arguments as illustrated by the following example default or cpp mode define DUP x x x define FOO and I said DUP F00 blah The usefulness of the defeval meta macro is shown by the following example in HTML mode lt define APPLY lt defeval TEMP lt 1 1 gt gt lt TEMP 2 gt gt lt define lt foo x gt lt x gt and lt x gt gt lt APPLY foo BLAH gt The reason why defeval is needed is that since everything is evaluated in a single pass the input that will result in the desired macro call needs to be generated by a first evaluation of the arguments passed to APPLY before being evaluated a second time To translate this example in default mode one needs to resort to parenthesizing in order to nest the defeval call inside the definition of APPLY but need to do so without outputting the parentheses The easiest solution is define BALANCE x x define APPLY f v BALANCE defeval TEMP f TEMP v define foo x x and x APPLY foo BLAH As explained above the simplest version in cpp mode relies on defining a silent evaluated string to play the role of the BALANCE macro The following example default or cpp mode demonstrates arithmetic evaluation define x 4 The answer is eval x x 2 16 x 1998 x Hif defined x amp amp 3 x 5 gt 17 This should be out
224. ugh other facts such as shaves barber mayor may be true Formally the meaning of normal programs may be given using the well founded semantics and it is this semantics that XSB adopts for negation The Intuition behind Stratified Programs Before considering the full well founded seman tics we discuss how XSB can be used to evaluate programs with stratified negation Intuitively a program uses stratified negation whenever there is no recursion through negation Indeed most programmers most of the time use stratified negation Exercise 5 3 1 The program win X move X Y tnot move Y is stratified when the move 2 relation is a binary tree This can be seen by loading the file XSB_DIR examples tree along with table_examples P and typing the query win 1 win 1 calls win 2 through negation win 2 calls win 4 through negation and so on but no subgoal ever calls itself recursively through negation The previous example of win 1 over a binary tree is a simple instance of a stratified program but it does not even require tabling A more complex example is presented below Exercise 5 3 2 Consider the query 1lrd_s to the following program CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 53 lrd_p lrd_q tnot lrd_r tnot lrd_s lrd_q lrd_r tnot lrd_p lrd_r lrd_p tnot lrd_q lrd_s tnot lrd_p tnot 1lrd_q tnot lrd_r Should lrd_s be true or false Try it in XSB Using the intuitive definition
225. uld be set on or off No prefix indicates the option should be set on 3 8 5 Inline Predicates Inline predicates represent primitive operations in the WAM Calls to inline predicates are com piled into a sequence of WAM instructions in line i e without actually making a call to the predicate Thus for example relational predicates like gt 2 gt 2 etc compile to essentially a subtraction followed by a conditional branch Inline predicates are expanded specially by the compiler and thus cannot be redefined by the user without changing the compiler The user does not need to import these predicates from anywhere There are available no matter what options are specified during compiling Table 3 1 lists the inline predicates of XSB Version 2 1 Those predicates that start with _ are internal predicates that are also expanded in line during compilation 1 3 2 2 lt 2 2 1 lt 2 I gt 9 2 2 gt 2 2 2 gt 2 is 2 00 lt gt 2 Q lt 2 20 gt 2 27 gt 2 2 2 fail O true 0 var 1 nonvar 1 halt 0 21 gt 0 gt cutto 1 _ savecp 1 builtin 1 Table 3 1 The Inline Predicates of XSB We warn the user to be very cautious when defining predicates whose functor starts with since the names of these predicates may interfere with some of XSB s internal predicates The situation may be particularly severe for predicates like _ builtin 1 that are treated specially by the XSB compiler C
226. ule must be uniquely deter mined As a consequence a symbol of a specific functor arity cannot be declared as both exported and local or both exported and imported from another module or declared to be imported from more than one module etc These types of environment conflicts are detected at compile time and abort the compilation e It is an error to import a symbol from a module that does not export it This error is not detected at compile time but at run time when a call to that symbol is made If the symbol is defined in but not exported from the module that defines it an environment conflict error will take place If the symbol is not defined in that module an undefined predicate function error will be be reported to the user e In the current implementation at any time only one symbol of a specific functor arity form can appear in a module As an immediate consequence of this fact only one functor arity symbol can be loaded into the current working module usermod An attempt to load a module that redefines that symbol results in a warning to the user and the newly loaded symbol overrides the definition of the previously loaded one CHAPTER 3 SYSTEM DESCRIPTION 16 3 4 The Dynamic Loader and its Search Path The dynamic or automatic loader comprises one of XSB s differences from other Prolog systems In XSB the loading of user modules Prolog libraries including the XSB compiler itself is delayed until predicates in them ar
227. ult mode The default mode is a vaguely cpp like mode but it does not handle comments and presents various incompatibilities with cpp Typical meta macros and user macros look like this define x y macro arg This mode is equivalent to U nnu nun Gu Mas nyu meu nyu ng UN NU M u4 n non n n n u wy C cpp compatibility mode This is the mode where gpp s behavior is the closest to that of cpp Unlike in the default mode meta macro expansion occurs only at the beginning of lines and C comments and strings are understood This mode is equivalent to n U nn nn gu now nyu wou mya u un gt M n w n non n n n nu nn c x x c n c m n nwo 8 A myn LASN s mou 43 my u APPENDIX A GPP GENERIC PREPROCESSOR 137 e T TeX like mode In this mode typical meta macros and user macros look like this define x y macrofarg No comments are understood This mode is equivalent to U UN NU wi ufu zq nyu ufu HYN ugu gu e H HTML like mode In this mode typical meta macros and user macros look like this lt define x y gt lt macro arg gt No comments are understood This mode is equivalent to U heyy Wu NB n Wu Well Wu es INN e P Prolog compatible cpp like mode This mode differs from the cpp compatibility mode by its handling of comments and is equivalent to n U nn nn N mow n now qu ng unn gt M n w n non n n n nu nn
228. uments of Term are all distinct e If Arity is 0 then Functor must be either an atom or a number and it is unified with Term e If Arity is anything else then functor 3 aborts Exceptions CHAPTER 6 STANDARD PREDICATES domain_error Functor is instantiated to a compound term instantiation error Both Term and either Functor or Arity are uninstantiated Examples functor p f a b t F A F p A 3 functor T foo 3 T foo _595708 _595712 595716 functor T 1 3 A T 1 3 A 0 functor foo F 0 F foo functor foo F A functor O A A 0 functor 2 3 4 F A functor atb F A F A 2 functor f a b c F A F f A 3 functor X a b c F A F apply A 4 functor map P a b F A apply A 3 j I functor T foo a 1 Error Wrong type in argument 2 of functor 3 Aborting functor T F 3 Error Uninstantiated argument 2 of functor 3 Aborting functor T foo A CHAPTER 6 STANDARD PREDICATES 76 Error Uninstantiated argument 3 of functor 3 Aborting hilog functor Term F Arity HiLog The XSB standard predicate hilog_functor 3 succeeds e when Term is a Prolog term and the principal function symbol functor of Term is F and the arity number of arguments of Term is Arity or e when Term is a HiLog term having name F and the number of arguments F is
229. unctor of the predicate to be tabled and A its arity Another use of tabling is to filter out redundant solutions for efficiency rather than termination In this case suppose that the directive edb 1 were used to indicate that certain predicates were CHAPTER 3 SYSTEM DESCRIPTION 33 likely to have a large number of clauses Then the action of the declaration suppl_table in the program edb r1 2 edb r2 2 edb r3 2 suppl_table join X Z r1 X X1 r2 X1 X2 r3 X2 Z would be to table join 2 The suppl_table directive is the XSB analogue to the deductive database optimization supplementary magic templates 4 suppl1 table O is shorthand for suppl_table 2 which tables all predicates containing clauses with two or more edb facts or tabled predicates By specifying suppl_table 3 for instance only predicates containing clauses with three or more edb facts or tabled predicates would be tabled This flexibility can prove useful for certain data intensive applications Indexing Directives The XSB compiler usually generates an index on the principal functor of the first argument of a predicate Indexing on the appropriate argument of a predicate may significantly speed up its execution time In many cases the first argument of a predicate may not be the most appropriate argument for indexing and changing the order of arguments may seem unnatural In these cases the user may generate an index on any other argument by me
230. ure versions of XSB Known Bugs The current version of XSB does not fully support dynamic code In fact the declartion dynamic essentially instructs XSB to fail on that code if it is undefined Currently the C foreign language interface does not work when XSB is also compiled with the Oracle interface on Solaris Variables that appear in compiled arithmetic comparison predicates should only be bound to numbers and not evaluable arithmetic expressions That is the variables are not evalu ated to obtain an arithmetic value but the XSB compiler assumes that they are evaluated For example executing compiled code for the following program will cause an Arithmetic exception error p X X 1 p cos 0 This behaviour is only exhibited in compiled code The reader cannot read an infix operator immediately followed by a left parenthesis In such a case you get a syntax error To avoid the syntax error just leave a blank between the infix operator and the left parenthesis For example instead of writing X a b write X a b The reader cannot properly read an operator defined as both a prefix and an infix operator For instance the declaration op 1200 xf lt op 1200 xfx lt CHAPTER 9 RESTRICTIONS AND CURRENT KNOWN BUGS 133 will lead to a syntax error e When the code of a predicate is reloaded many times if the old code is still in use at the time of loading unexpecte
231. urring other than as one of the alternatives of a disjunction is equivalent to P gt Q fail repeat Generates an infinite sequence of choice points in other words it provides a very convenient way of executing a loop It is defined by the clauses repeat repeat repeat 6 5 Meta Logical To facilitate manipulation of terms as objects in themselves XSB provides a number meta logical predicates These predicates include the standard meta logical predicates of Prolog along with their usual semantics In addition are provided predicates which provide special operations on HiLog terms For a full discussion of Prolog and HiLog terms see Section 4 1 var X Succeeds if X is currently uninstantiated i e is still a variable otherwise it fails Term X is uninstantiated if it has not been bound to anything except possibly another uninstantiated variable Note in particular that the HiLog term X Y Z is considered to be instantiated There is no distinction between a Prolog and a HiLog variable Examples CHAPTER 6 STANDARD PREDICATES 71 var X yes var X no var X Y Z no var X yes var X Y no nonvar X Succeeds if X is currently instantiated to a non variable term otherwise it fails This has exactly the opposite behaviour of var 1 atom X Succeeds only if the X is currently instantiated to an atom that is to a Prolog or HiLog non numeric constant Examp
232. various properties associated with Term_Indicator e If Property is bound to a valid predicate property then predicate_property 2 succes sively unifies Term_Indicator with the skeletal specifications of all known to the system predicates having the specified Property e If Term_Indicator is a variable then it is unified successively through backtracking with the most general term for a predicate whose known properties are unified with Property e If Term_Indicator is a skeletal specification not a known to the system or Property is not a valid predicate property the call simply fails CHAPTER 6 STANDARD PREDICATES 99 For example all the loaded predicate skeletal specifications in module usermod may be enumerated using predicate_property Pred loaded Also the following query finds all predicate skeletal specifications that are exported by module blah predicate_property blah Pred exported Currently the following properties are associated with predicates either implicitly or by dec laration where double lines show property categories and a predicate can have at most one property of each category Property Explanation unclassified The predicate symbol is not yet classified according to this category This property has various meanings Usually for exported predicate symbols in system or user defined modules it means that the predicate is yet unloaded because it has not been used In global modu
233. ve or not Table Directives and Declarations Often it is tedious to decide which predicates must be tabled To address this XSB can automatically table predicates in files The declaration auto_table chooses predicates to table to assist in termination while supp1 table chooses predicates to table to optimize data oriented queries Both are explained in Section 3 8 4 Exercise 5 2 7 The reader may have noted that the command table was referred to as a directive while auto_table and suppl_table were both referred to as declarations The difference is that CHAPTER 5 USING TABLING IN XSB A TUTORIAL INTRODUCTION 52 the user can execute a directive at the command line but not a compiler declaration For instance restart XSB and at the XSB prompt type the directive table dyn_path 2 and load_dyn dyn_examples Try the queries to path 2 of the previous examples Note that it is important to dynamically load dyn_examples P otherwise the code in the file will be compiled without knowledge of the tabling declaration 5 3 Stratified Normal Programs Normal programs extend definite programs to include default negation which posits a fact as false if all attempts to prove it fail As shown in Example 1 0 1 which presented one of Russell s paradoxes as a logic program the addition of default negation allows logic programs to express contradictions As a result some assertions such as shaves barber barber may be undefined altho
234. when the emulator is SLG WAM based s Maintains information on the size of program stacks for the predicate statistics 0 This option may be expected to slow execution by around 10 Default off T Generates a trace at entry to each called predicate both system and user defined This option is available mainly for people who want to modify and or extend XSB and it is not the normal way to trace XSB programs For the latter the builtin predicates trace 0 or debug 0 should be used see Chapter 7 Note This option is not available when the system is being used at the non tracing mode see Section 7 t Traces through code at SLG WAM instruction level This option is for internal debugging and is not fully supported It is also not available when the system is being used at the non debug mode see Section 7 e goal Pass goal to XSB at starup This goal is evaluated right before the first prompt is issued For instance xsb e write Hello nl will print a heart warming message when XSB starts up nobanner Start XSb without showing the startup banner Useful in batch scripts and for inter process communication when XSB is launched as a subprocess quietload Do not tell when a new module gets loaded Again is useful in non interactive activities and for interprocess communication noprompt Do not show te XSB prompt This is useful only in batch mode and in interprocess communication when you do not want the promp
235. xpp_program 26 167
236. xtensive pattern matching libraries which are especially useful for Web applications e Preprosessors and Interpreters so that XSB can be used to evaluate programs that are based on advanced formalisms such as extended logic progams according to the Well Founded Semantics 1 Generalized Annotated Programs 23 and F Logic 22 e Source code availability for portability and extensibility Though XSB can be used as a Prolog system we avoid referring to XSB as such because of the availability of SLG resolution and the handling of HiLog terms These facilities while seemingly simple significantly extend its capabilities beyond those of a typical Prolog system We feel that these capabilities justify viewing XSB as a new paradigm for Logic Programming To understand the implications of SLG resolution 8 recall that Prolog is based on a depth first search through trees that are built using program clause resolution SLD As such Prolog is susceptible to getting lost in an infinite branch of a search tree where it may loop infinitely SLG evaluation available in XSB can correctly evaluate many such logic programs To take the simplest of examples any query to the program Many of the Prolog components of XSB are based on PSB Prolog 48 which itself is based on version 2 0 of SB Prolog 13 CHAPTER 1 INTRODUCTION 2 table ancestor 2 ancestor X Y ancestor X Z parent Z Y ancestor X Y parent X Y will termi
237. y x y concat F00 BAR ifeq concat foo bar foo bar This is output else This is not output endif In HTML mode and without argument naming one gets similarly lt define FOO This is gt lt define BAR a message gt lt define concat 1 2 gt lt concat lt FO0 gt lt BAR gt gt lt ifeq lt concat foo bar gt foo bar gt This is output lt else gt This is not output lt endif gt The following example in standard mode illustrates the use of the quote character define FOO This is a multiline definition define BLAH x My argument is x BLAH urf BLAH urf APPENDIX A GPP GENERIC PREPROCESSOR 147 Note that the multiline definition is also valid in cpp and Prolog modes despite the absence of quote character because followed by a newline is then interpreted as a comment and discarded In cpp mode C strings and comments are understood as such as illustrated by the following example define BLAH foo BLAH BLAH BLAH It s a xstring The main difference between Prolog mode and cpp mode is the handling of strings and comments in Prolog a string may not begin immediately after a digit and a comment may not begin immediately after an operator character Furthermore comments are not removed from the output unless they occur in a command The differences between cpp mode and default mode are deeper in default mode commands may

The XSB System Version 2.1 Volume 1: Programmer`s Manual

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