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C-Prolog User`s Manual

1. N 1 product product If X is instantiated to a variable then Y must be instantiated either to a list of determinate length whose head is an atom or to a list of length 1 whose head is a number name X L If X is an atom or a number then L is a list of the ASCII codes of the characters comprising the name of X E g name product 112 114 111 100 117 99 116 i e name product product name 1976 49 57 55 54 name hello 104 101 108 108 111 name If X is instantiated to a variable L must be instantiated to a list of ASCII character codes E g name X 104 101 108 108 111 C Prolog User s Manual 27 X hello name X hello X hello call X If X is instantiated to a term which would be acceptable as body of a clause the goal call X is executed exactly as if that term appeared textually in place of the call 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 call fails where X is a variable Exactly the same as call X 5 6 Sets When there are many solutions to a problem and when all those solutions are required to be col lected together this can be achieved by repeatedly backtracking and gradually building up a list of the solutions The following evaluable predicates are provided to automate this process setof X P S Read this as S is t
2. Calls the evaluable predicate break thus putting you at interpreter top level with the execution so far sitting underneath you When you end the break with the END OF INPUT character you will be reprompted at the port at which you broke The new execution is completely separate from the suspended one the invocation numbers will start again from 1 during the break Debug mode is not switched off as you call the break but if you do switch it off then it will be re switched on when you finish the break and go back to the old execution However any changes to the leashing or to spy points will remain in effect n Nodebug Turns off debug mode Notice that this is the correct way to switch debugging off at a trace point You cannot use the b option because it always restores debug mode upon return 4 7 Reconsulting during Debugging It is possible and sometimes useful to reconsult a file whilst in the middle of a program execution However this can lead to unexpected behaviour under the following circumstances a procedure has been successfully executed it is subsequently re defined by a reconsult and is later re activated by back tracking When the backtracking occurs all the new clauses for the procedure appear to the interpreter to be potential alternative solutions even though identical clauses may already have been used Thus large amounts of unwanted activity takes place on backtracking The problem does not arise if you d
3. square root sin X sine cos X cosine tan X tangent asin X arc sine acos X arc cosine atan X C Prolog User s Manual arc tangent floor X the largest integer not greater than X XY X to the power Y XAY integer bitwise conjunction XIY integer bitwise disjunction X lt lt Y integer bitwise left shift of X by Y places X gt gt Y integer bitwise right shift of X by Y places X integer bitwise negation cputime CPU time since C Prolog was started in seconds heapused Heap space in use in bytes xX a list of just one element evaluates to X if X is an integer 24 Since a quoted string is just a list of integers this allows a quoted character to be used in place of its ASCII code e g A behaves within arithmetic expressions as the integer 65 The arithmetic evaluable predicates are as follows where X and Y stand for arithmetic expressions and Z for some term Note that this means that is only evaluates one of its arguments as an arithmetic expression the right hand side one whereas all the comparison predicates evaluate both their arguments Zis X Arithmetic expression X is evaluated and the result is unified with Z Fails if X is not an arithmetic expression X The values of X and Y are equal X Y The values of X and Y are not equal X lt Y The value of X is less than the value of Y X gt Y The value of X is greater than the value of Y X lt Y The value of X
4. This means that you will never see messages concerning these predicates during debugging C Prolog User s Manual 18 4 6 Options Available during Debugging This section describes the particular options that are available when the system prompts you after printing out a debugging message All the options are one letter mnemonics They are read from the terminal with any blanks being completely ignored up to the next end of line The creep option needs only the new line The only option which you really have to remember is h This provides help in the form of the fol lowing list of available options END OF LINE creep a abort c creep f fail l leap b break s skip h help r retry r lt n gt retry goal lt n gt q quasi skip n nodebug g goal stack read clauses from terminal e exit Prolog The first three options are the basic control decisions c END OF LINE Creep Causes the interpreter to single step to the very next port and print a message Then if the port is leashed the user is prompted for further interaction Otherwise it continues creeping If leashing is off creep is the same as leap see below except that a complete trace is printed on the terminal l Leap causes the interpreter to resume running your program only stopping when a spy point is reached or when the program terminates Leaping can thus be used to follow the execution at a higher level than exhaustive tracing All you need to do is to
5. Y Terms X and Y are not strictly identical X lt Y Term X precedes term Y X lt Y Term X precedes or is identical Y X gt Y Term X follows term Y X gt Y Term X follows or is identical to term Y FIR Perform the re consult s specified by FIR
6. applied before system defined ones C Prolog User s Manual 34 Appendix I Summary of Evaluable Predicates abolish F N abort arg N T A assert C assert C R asserta C asserta C R assertz C assertz C R atom 7 atomic T bagof X P B break call P clause P Q clause P Q R close F compare C X Y consult F current_atom A current_functor A 7 current_predicate A P db_reference 7 debug debugging display T erase R erased R expanded_exprs O N expand_term T7 X exists F fail fileerrors functor T F N get C get0 C halt instance R T integer T Yis X keysort L S leash M listing listing P name A L nl nodebug nofileerrors nonvar T nospy P number 7 op P TA primitive T print T Abolish the procedure named F arity N Abort execution of the current directive The Mth argument of term Tis A Assert clause C Assert clause C ref R Assert C as first clause Assert C as first clause ref R Assert C as last clause Assert C as last clause ref R Term Tis an atom Term T is an atom or integer The bag of Xs such that P is provable is B Break at the next procedure call Execute the procedure call P There is a clause head P body Q There is an clause head P body Q ref R Close file F C is the result of comparing terms X and Y Extend the program with clauses from file F One of the currently defined atoms is A A current functor is named A m
7. as in member X d e f With backtracking this will successively return each element of the list Now suppose that the first clause had been written instead member X X L In this case the above call would extract only the first element of the list d On backtracking the cut would immediately fail the whole procedure A procedure of the form x p q xX r is similar to x if p then q else r in an Algol like language A cut discards all the alternatives since the parent goal even when the cut appears within a disjunc tion This means that the normal method for eliminating a disjunction by defining an extra predicate cannot be applied to a disjunction containing a cut 4 Debugging Facilities This section describes the debugging facilities that are available in C Prolog The purpose of these facilities is to provide information concerning the control flow of your program The main features of the debugging package are as follows The Procedure box model of Prolog execution which provides a simple way of visualising control flow especially during backtracking Control flow is viewed at the procedure level rather than at the level of individual clauses The ability to exhaustively trace your program or to selectively set spy points Spy points allow you to nominate interesting procedures at which the program is to pause so that you can interact The wide choice of control and information option
8. L L2 The elements of the list LZ are sorted into the standard order and any identical i e elements are merged yielding the list L2 The time taken to do this is at worst order N log N where N is the length of L keysort L L2 The list LZ must consist of items of the form Key Value These items are sorted into order accord ing to the value of Key yielding the list L2 No merging takes place The time taken to do this is at worst order N log N where N is the length of L1 5 8 Modification of the Program The predicates defined in this section allow modification of the program as it is actually running Clauses can be added to the program asserted or removed from the program retracted Some of the predicates make use of an implementation defined identifier or database reference which uniquely identifies every clause in the interpreted program This identifier makes it possible to access clauses directly instead of requiring a search through the program every time However these facilities are intended for more complex use of the database and are not required and undoubtedly should be avoided by novice users assert C The current instance of C is interpreted as a clause and is added to the program with new private variables replacing any uninstantiated variables The position of the new clause within the pro cedure concerned is implementation defined C must be instantiated to a non variable asser
9. existing clauses in the database get thrown away Reconsulting is useful for telling Prolog about corrections in your program Clauses may also be typed in directly at the terminal To enter clauses at the terminal you must give the directive user The interpreter is now in a state where it expects input of clauses or directives To get back to the top level of the interpreter type the END OF INPUT character Typing clauses directly into C Prolog is only recommended if the clauses will not be needed permanently and are few in number For significant bits of program you should use an editor to produce a file containing the text of the program 1 5 Directives Questions and Commands When Prolog is at top level signified by an initial prompt of with continuation lines prompted with that is indented out from the left margin it reads in terms and treats them as direc tives to the interpreter to try and satisfy some goals These directives are called questions Remember that Prolog terms must terminate with a period and that therefore Prolog will not execute anything for you until you have typed the period and then END OF LINE at the end of the directive Suppose list membership has been defined by member X X _ member X _ L member X L Note the use of anonymous variables written If the goal s specified in a question can be satisfied and if th
10. halt Typing the END OF INPUT charater at top level also causes C Prolog to terminate 2 Prolog Syntax 2 1 Terms The data objects of the language are called terms A term is either a constant a variable or a com pound term The constants include numbers such as 0 999 5 23 0 23E 5 Constants also include atoms such as a void Algol 68 The symbol for an atom can be any sequence of characters written in single quotes if there is possibility of confusion with other symbols such as variables or numbers As in other programming languages con stants are definite elementary objects Variables are distinguished by an initial capital letter or by the initial character for example X Value A Al 3 RESULT If a variable is only referred to once it does not need to be named and may be written as an anonymous variable indicated by the underline character A variable should be thought of as standing for some definite but unidentified object A variable is not simply a writeable storage location as in most programming languages rather it is a local name for some data object cf the variable of pure LISP and constant declarations in Pascal The structured data objects of the language are the compound terms A compound term comprises a functor called the principal functor of the term and a sequence of one or more terms called arguments A functor is characterised by its name which is an atom and i
11. is less than or equal to the value of Y X gt Y The value of X is greater than or equal to the value of Y expanded_exprs Old New C Prolog User s Manual 25 Unifies Old to the current value of the expression expansion flag and sets the value of the flag to New The possible values of the flag are off the default for not expanding arithmetic expressions into procedure calls and on to do the expansion 5 3 Convenience P Q P and Q P Q P or Q true Always succeeds X Y Defined as if by the clause Z Z that is X and Y are unified 5 4 Extra Control Cut discard all choice points made since the parent goal started execution P If the goal P has a solution fail otherwise succeed It is defined as if by P P fail _ P gt Q R Analogous to af 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 occurring other than as one of the alternatives of a disjunction is equivalent to P gt Q fail repeat Generates an infinite sequence of backtracking choices It behaves as if defined by the clauses repeat repeat repeat fail Always fails 5 5 Meta Logical var X Tests whether X is currently instantiated to a variable nonvar X Tests whether X is currently instantiated to a non variable term C Prolog User s Manual 26 atom X Checks that X is currently instantiated to an atom i e a non variable term of arity 0 other
12. tell H executed H then becomes the current output stream Subsequent calls to see F or to tell H make F or H the current input or output stream respectively Any input or out put is always to the current stream When no input or output stream has been specified the standard ersatz file user denoting the user s terminal is utilised for both When the terminal is waiting for input on a new line a prompt will be displayed as follows interpreter waiting for command interpreter wating for command continuation line consult user wating for continuation line default for waiting for other user input cm 66 99 29 When the current input or output stream is closed the user s terminal becomes the current input or out put stream The only file that can be simultaneously open for input and output is the ersatz file user A file is referred to by its name written as an atom e g myfile F 123 data_Ist tom greeks All I O errors normally cause an abort except for the effect of the evaluable predicate nofileerrors decribed below End of file is signalled on the user s terminal by typing the END OF INPUT character Any more input requests for a file whose end has been reached causes an error failure 5 1 1 Reading in Programs consult F Instructs the interpreter to read in the program which is in file F When a directive is read it is immediately executed When a clause is r
13. than a number or database reference number X Checks that X is currently instantiated to a number integer X Checks that X is currently instantiated to an integer atomic X Checks that X is currently instantiated to an atom number or database reference primitive X Checks that X is currently instantiated to a number or database reference db_reference X Checks that X is currently instantiated to a database reference functor 7 F N The principal functor of term T has name F and arity N where F is either an atom or provided N is 0 a number Initially either T must be instantiated to a non variable or F and N must be instan tiated to respectively either an atom and a non negative integer or an integer and 0 If these conditions are not satisfied an error message is given In the case where T is initially instantiated to a variable the result of the call is to instantiate T to the most general term having the principal functor indicated arg T X Initially Z must be instantiated to a positive integer and T to a compound term The result of the call is to unify X with the th argument of term T The arguments are numbered from 1 upwards If the initial conditions are not satisfied or J is out of range the call merely fails X Y Y is a list whose head is the atom corresponding to the principal functor of X and whose tail is the argument list of that functor in X E g product 0 N N 1 product 0 N N 1 N 1
14. the directive file which will instruct the interpreter to consult the program The file specification file must be a Prolog atom It may be any file name note that if this file name contains characters which are not normally allowed in an atom then it is necessary to surround the whole file specification with single quotes since quoted atoms can include any character for example people greeks The specified file is then read in Clauses in the file are stored in the database ready to be executed while C Prolog User s Manual 3 any directives are obeyed as they are encountered When the end of the file is found the interpreter displays on the terminal the time spent in reading in and the number of bytes occupied by the program In general this directive can be any list of filenames such as myprogram extras testbits In this case all three files would be consulted If a filename is preceded by a minus sign as in testbits moreideas then that file is reconsulted The difference between consulting and reconsulting is important and works as follows if a file is consulted then all the clauses in the file are simply added to C Prolog s database If you consult the same file twice then you will get two copies of all the clauses However if a file is reconsulted then the clauses for all the procedures in the file will replace any existing clauses for those procedures that is any such previously
15. this predicate of any arity then nothing will be done If you really want to place a spy point on acurrently non existent procedure then you must use the full form lt atom gt lt arity gt you will get a warning message in this case If you set some spy points when debug mode is off then it will be automatically switched on nospy X This reverses the effect of spy X all the procedures given by X will have previously set spy points removed from them The options available when you arrive at a spy point are described below 4 5 Format of Debugging Messages We will now look at the exact format of the message output by the system at a port All trace mes sages are output to the terminal regardless of where the current output is directed This allows you to trace programs while they are performing file I O The basic format is as follows 23 6 Call foothello there 123 The indicates that this is a spy point If this port is not for a procedure with a spy point set then there will be two spaces there instead If this port is the requested return from a Skip then the second character becomes gt This gives four possible combinations ck This is a spy point ck gt This is a spy point and you also did a Skip last time you were in this box This is not a spy point but you did a Skip last time you were in this box rar This is not a spy point The number in parentheses is the un
16. to rematch the original goal with an alternative clause and then try to satisfy any subgoals in the body of this new clause Textually we follow the code back wards up the way we came looking for new ways of succeeding possibly dropping down on to another clause and following that if necessary Note that this is somewhat less informative than the Redo port described in Programming in Prolog because it does not show the path in the program from a failed goal back to the first goal where alternative clauses exist but only this goal Fail This arrow represents a failure of the initial goal which might occur if no clause is matched or if subgoals are never satisfied or if any solution produced is always rejected by later processing Con trol now passes out of the Fail port of the descendant box and the system continues to backtrack Textually we move back to the code which called this procedure and keep moving backwards up the code looking for choice points In terms of this model the information we get about the procedure box is only the control flow through these four ports This means that at this level we are not concerned with which clause matches and how any subgoals are satisfied but rather we only wish to know the initial goal and the final outcome However it can be seen that whenever we are trying to satisfy subgoals what we are actually doing is passing through the ports of their respective boxes If we were to follow this then we
17. AX machines under the UNIX and VAX VMS operating systems on the Sun workstation under 4 1 2 UNIX and has been ported with minor changes to other MC68000 based workstations and to the Three Rivers PERQ Prolog is a simple but powerful programming language originally developed at the University of Marseilles as a practical tool for programming in logic From a user s point of view the major attraction of the language is ease of programming Clear readable concise programs can be written quickly with few errors Prolog is especially suitable for high level symbolic programming tasks and has been applied in many areas of Artificial Intelligence research The system consists of a Prolog interpreter and a wide range of builtin system defined procedures Its design was based on the Edinburgh DECsystem 10 Prolog system and the system is closely compati ble with DECsystem 10 Prolog and thus is also reasonably close to PDP 11 UNIX and RT 11 Prolog This manual is not intended as an introduction to the Prolog language and how to use it For this pur pose you should study Programming in Prolog W Clocksin amp C Mellish Springer Verlag 1981 This manual assumes that you are familiar with the principles of the Prolog language its purpose being to explain how to use C Prolog and to describe all the evaluable predicates provided by C Prolog 1 2 Using C Prolog Overview C Prolog offers the user an interactive programming environment wi
18. C Prolog User s Manual Version 1 5 October 24 1996 Edited by Fernando Pereira SRI International Menlo Park California from material by David Warren SRI International Menlo Park California David Bowen Lawrence Byrd Dept of Artificial Intelligence University of Edinburgh Luis Pereira Dept de Informatica Universidade Nova de Lisboa Abstract This is a revised edition of the user s manual for C Prolog a Prolog interpreter written in C for 32 bit machines C Prolog is based on an earlier Prolog interpreter written in IMP for the EMAS operating system by Luis Damas who borrowed many aspects of the design from the DECsystem 10 20 Prolog system developed by David Warren Fernando Pereira Lawrence Byrd and Luis Pereira This manual is based on the EMAS Prolog manual which in turn was based on the DECsystem 10 20 Prolog manual Formerly at EACAAD Dept of Architecture University of Edinburgh C Prolog User s Manual 1 1 Using C Prolog 1 1 Preface This manual describes C Prolog a Prolog interpreter written in C C Prolog was developed at EdCAAD Dept of Architecture University of Edinburgh and is based on a previous interpreter written in IMP for the EMAS operating system by Luis Damas of the Dept of Computer Science University of Edin burgh C Prolog was designed for machines with a large uniform address space and assumes a pointer cell 32 bits wide At the time of writing it has been tested on V
19. CII order complex terms ordered first by arity then by the name of principal functor then by the arguments in left to right order For example here is a list of terms in the standard order X 9 1 fie foe fum X Y fie 0 2 fie 1 1 These are the basic predicates for comparison of arbitrary terms X Tests if the terms currently instantiating X and Y are literally identical in particular variables in equivalent positions in the two terms must be identical For example the question X Y fails answers no because X and Y are distinct uninstantiated variables However the question X Y X Y succeeds because the first goal unifies the two variables see page 42 X Y Tests if the terms currently instantiating X and Y are not literally identical Tl lt T2 Term TZ is before term T2 in the standard order Tl gt T2 Term 7 is after term 72 in the standard order TI lt T2 Term T is not after term 72 in the standard order TI gt T2 Term TZ is not before term 72 in the standard order Some further predicates involving comparison of terms are compare Op T 72 The result of comparing terms T7 and T2 is Op where the possible values for Op are C Prolog User s Manual 29 if Tl is identical to T2 lt if TI is before 72 in the standard order gt if T is after T2 in the standard order Thus compare 7 72 is equivalent to TI T2 sort
20. Call port of the current goal or if given a numeric argument n to the Call port of the invocation numbered n IF the invocation being retried has been deleted by the cut control primitive the most recent active invocation before it will be retried If n is out of range the current invocation is retried This option is extremely useful to go back to an earlier state of computation if some point of interest was overshot in a debugging session Note however that side effects such as database modification are not undone C Prolog User s Manual 19 g Goal Stack Shows the goal that called the current one and the one that called it and so on that is the stack of pending goals Goals are printed one per line from the most recent to the least recent Each goal is labeled by its recursion level its invocation number i and the ordinal position p of the clause currently used to solve the goal a Abort Causes an abort of the current execution All the execution states built so far are destroyed and you are put right back at the top level of the interpreter This is the same as the evaluable predi cate abort Read clauses from terminal starts reading clauses typed by the user as if doing a consult user _ An END OF INPUT returns to the debugger e Exit from Prolog causes an irreversible exit from the Prolog system This is the same as the evaluable predicate halt h Help Displays the table of options given above b Break
21. Here then is an informal definition of the procedural semantics To execute a goal the system searches forwards from the beginning of the program for the first clause whose head matches or unifies with the goal The unification process finds the most gen eral common instance of the two terms which is unique if it exists If a match is found the matching clause instance is then activated by executing in turn from left to right each of the goals if any in its body If at any time the system fails to find a match for a goal it back tracks that is it rejects the most recently activated clause undoing any substitutions made by the match with the head of the clause Next it reconsiders the original goal which activat ed the rejected clause and tries to find a subsequent clause which also matches the goal For example if we execute the goal in the query C Prolog User s Manual 13 concatenate X Y a b we find that it matches the head of the first clause for concatenate with X instantiated to a X1 The new variable X1 is constrained by the new goal produced which is the recursive procedure call concatenate X1 Y b Again this goal matches the first clause instantiating X1 to b X2 and yielding the new goal concatenate X2 Y Now this goal will only match the second clause instantiating both X2 and Y to Since there are no further goals to be executed we have a solution X a b Y representing a true inst
22. T the last record under key K ref R rename F G Rename file F to G repeat Succeed repeatedly retract C Erase the first clause of form C save F Save the current state of Prolog in file F see F Make file F the current input stream seeing F The current input stream is named F seen Close the current input stream setof X P B The set of Xs such that P is provable is B sh Start a recursive shell skip C Skip input characters until after character C sort L S The list L sorted into order yields S spy P Set spy points on the procedure s P statistics Display execution statistics system S Execute command S tab V Output N spaces tell F Make file F the current output stream telling F The current output stream is named F told Close the current output stream trace Switch on debugging and start tracing true Succeed var T Term T is a variable write 7 Write the term T writeq 7 Write the term 7 quoting names if necessary LC The following Prolog text uses lower case NOLC The following Prolog text uses upper case only Cut any choices taken in the current procedure P Goal P is not provable X lt Y As numbers X is less than Y X lt Y As numbers X is less than or equal to Y X gt Y As numbers X is greater than Y X gt Y As numbers X is greater than or equal to Y X Y Terms X and Y are equal i e unified T L The functor and args of term T comprise the list L Y Terms X and Y are strictly identical
23. ables Note that variables in different clauses are completely independent even if they have the same name the lexical scope of a variable is limited to a single clause Each distinct variable in a clause should be interpreted as standing for an arbitrary entity or value To illustrate this here are some examples of clauses containing variables with possible declara tive and procedural readings employed X employs Y X Any X is employed if any Y employs X To find whether a person X is employed find whether any Y employs X derivative X X 1 For any X the derivative of X with respect to X is 1 The goal of finding a derivative for the expression X with respect to X itself is satisfied by the result 1 ungulate X aquatic X Ts it true for any X that X is an ungulate and X is aquatic Find an X which is both an ungulate and aquatic In a program the procedure for a particular predicate is the sequence of clauses in the program whose head goals have that predicate as principal functor For example the procedure for a 3 ary predi cate concatenate might consist of the two clauses concatenate X L1 L2 X L3 concatenate L1 L2 L3 concatenate L L where concatenate L1 L2 L3 means the list L1 concatenated with the list L2 is the list L3 In Prolog several predicates may have the same name but different arities Therefore when it is i
24. acter Input Output nl A new line is started on the current output stream get0 N N is the ASCII code of the next character from the current input stream If the current input stream reaches its end of file the ASCII character code for control Z is returned and the stream closed 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 on end of file skip Skips to just past the next ASCII character code N from the current input stream N may be an integer expression Skipping past the end of file causes an error put V ASCII character code N is output to the current output stream N may be an integer expression tab N spaces are output to the current output stream N may be an integer expression 5 2 Arithmetic Arithmetic is performed by evaluable predicates which take as arguments arithmetic expressions and evaluate them An arithmetic expression is a term built from evaluable functors numbers and variables At the time of evaluation each variable in an arithmetic expression must be bound to a number or to an arithmetic expression The result of evaluation will always be converted back to an integer if possible Each evaluable functor stands for an arithmetic operation The adjective integer in the descrip tions below means that the operation only makes sense for integers and will fail for floating point numbers Be
25. al Equipment Corporation 3UNIX is a Trademark of Bell Laboratories C Prolog User s Manual 2 We assume that there are three keys or key combinations that achieve the effects of terminating an input line marking end of input and interrupting execution of a program Because these depend on operat ing system and individual tastes they are denoted in this manual by END OF LINE END OF INPUT and INTER RUPT respectively they are carriage return control Z and control C on the computer this is being written on Since Prolog makes syntactic use of the difference between upper and lower case it is important that you have your terminal set up so that it accepts lower case in the normal way This means for a start that you must be using an upper and lower case terminal and not for example an upper case only teletype It is possible to use Prolog using upper case only see Section 2 4 but it is unnecessarily painful We shall assume both upper and lower case throughout this manual If you type the cprolog command Prolog will output a banner and prompt you for directives as follows C Prolog version 1 5 te There will be a pause between the first line and the prompt while the system loads itself It is possible to type ahead during this period if you get impatient If you give an argument to the cprolog command C Prolog will interpret it as the name of a file containing a saved state created earlier and will restore that s
26. ance of the original goal concatenate a b a b If this solution is rejected backtracking will generate the further solutions X a Y b X Y a b in that order by rematching against the second clause for concatenate goals already solved once using the first clause 3 3 Occurs Check Prolog s unification does not have an occurs check i e when unifying a variable against a term the system does not check whether the variable occurs in the term When the variable occurs in the term unification should fail but the absence of the check means that the unification succeeds producing a cir cular term Trying to print a circular term or trying to unify two circular terms will cause an infinite loop and possibly make Prolog run out of stack space The absence of the occur check is not a bug or design oversight but a conscious design deci sion The reason for this decision is that unification with the occur check is at best linear on the sum of the sizes of the terms being unified whereas unification without the occur check is linear on the size of the smallest of the terms being unified In any practical programming language basic operations are sup posed to take constant time Unification against a variable should be thought of as the basic operation of Prolog but this can take constant time only if the occur check is omitted Thus the absence of a occur check is essential to make Prolog into a practical programming
27. ary Thus the brackets are necessary in X 2 2 3 Syntax Errors Syntax errors are detected when reading Each clause directive or in general any term read in by the evaluable predicate read that fails to comply with syntax requirements is displayed on the termi nal as soon as it is read A mark indicates the point in the string of symbols where the parser has failed to continue its analysis For example typing member X X L gives evntax error member X X wh ape kK C Prolog User s Manual 10 L Syntax errors do not disrupt the re consulting of a file in any way except that the clause or command with the syntax error will be ignored All the other clauses in the file will have been read in properly If the syntax error occurs at top level then you should just retype the question Given that Prolog has a very simple syntax it is usually quite straightforward to see what the problems is look for missing brackets in particular The book Programming in Prolog gives further examples 2 4 Using a Terminal without Lower Case The syntax of Prolog assumes that a full ASCII character set is available With this full character set or LC convention variables are normally distinguished by an initial capital letter while atoms and other functors must start with a lower case letter unless enclosed in single quotes When lower case is not available the no lower case or NOLC convention has to be adopted Wi
28. at top level but if you explicitly called save from within your program then the execution of your program will continue Saved states can only be restored when C Prolog is initially run from command level Version 1 5 provides no way of restoring a saved state from inside C Prolog Note that when a new version of C Prolog is installed saved states created with the old version may become unusable You are thus advised to rely on source files for your programs and not on some gigantic saved state 1 8 Program Execution And Interruption Execution of a program is started by giving the interpreter a directive which contains a call to one of the program s procedures Only when execution of one directive is complete does the interpreter become ready for another directive However one may interrupt the normal execution of a directive by hitting the INTER RUPT key on your terminal In response to the prompt Action h for help you can type either a t d or c followed by END OF LINE The a response will force Prolog to abort back to top level the t option will switch on tracing the d response will switch on debug ging and continue the execution and the c response will just continue the execution 1 9 Nested Executions Break and Abort C Prolog provides a way to suspend the execution of your program and to enter a new incarnation of the top level
29. aved state Saved states will be explained fully later cprolog prog Restore prog C Prolog uses six major internal data areas the heap the global stack the local stack the trail the atom area and the auxiliary stack Although the system is initially configured with reasonable allocations for each of those areas a particular program might exceed one of the allocations causing an error message Ideally the system should adjust the available storage among areas automatically but this facility is not implemented yet Instead the user may specify when starting Prolog the allocations in K bytes for some or all of the areas This is done by giving command line switches between the program name and the optional file argument For example cprolog h 1000 g 1000 1 500 bigprogram specifies heap and global stack allocations of 1000 K and a local stack allocation of 500 K The full set of switches is hN heap allocation is N K bytes gN global stack allocation is N K bytes IN local stack allocation is N K bytes tN trail allocation is N K bytes aN atom area allocation is N K bytes xN auxiliary stack allocation is N K bytes 1 4 Reading in Programs A program is made up of a sequence of clauses possibly interspersed with directives to the interpreter The clauses of a procedure do not have to be immediately consecutive but remember that their relative order may be important To input a program from a file file give
30. cause arithmetic expressions are compound terms they use up storage that is only recovered on backtracking The evaluable predicate expanded _exprs can be used to avoid this overhead by preexpand ing arithmetic expressions into calls to arithmetic evaluation predicates However this makes program read in slower and clauses bigger In general an arithmetic operation that combines integers and floating point numbers will return a floating point number However if the result is integral it is converted back to integer representation and the same applies to numbers read in by the reader Thus the goal p 2 0 will succeed with the clause p 2 and the result of the query X is 2 1 5 is X 5 C Prolog User s Manual 23 Numbers may be integers 33 0 9999 or floating point numbers 1 333 2 6E 7 0 555E 2 Note that the decimal cannot be confused with the end of clause because the latter must be fol lowed by blank space space tab or END OF LINE However if an operator is declared as infix it will only be possible to apply it to two numbers if they are separated by blank space from the operator The evaluable functors are as follows where X and Y are arithmetic expressions X Y addition X Y subtraction X Y multiplication X Y division XIIY integer division X mod Y X integer modulo Y X unary minus exp X exponential function log x natural logarithm log10 X base 10 logarithm sqrt X
31. cessible through the predicates that search through the database but will still be accessible through its database reference if this is available in the execution state after the call to erase Only when all instances of the item s database reference have been forgotten through database erasures and or backtracking will the item be actually removed from the database erased R Suceeds if R is a database reference to a database item that has been erased otherwise fails instance Ref Term A most general instance of the recorded term whose database reference is Ref is unified with Term Ref must be instantiated to a database reference Note that instance will even pick database items that have been erased 5 11 Debugging debug Debug mode is switched on Information will now be retained for debugging purposes and execu tions will require more space nodebug Debug mode is switched off Information is no longer retained for debugging and spy points are removed trace Debug mode is switched on and the interpreter starts tracing from the next call to a user goal If trace was given in a command on its own the goal s traced will be those of the next command Since this is a once off decision a call to trace is necessary whenever tracing is required right from the start of an execution otherwise tracing will only happen at spy points spy Spec Spy points will be placed on all the procedures given by Spec All control flow throug
32. cute some goals then you should use a command That is to execute a directive 66 99 in a file it must be preceded by otherwise it will be treated as a clause 1 6 Saving A Program Once a program has been read the interpreter will have available all the information necessary for its execution This information is called a program state The state of a program may be saved on a file for future execution To save a program into a file file perform the command TE The extra parentheses with the immediately next to them are currently essential due to a problem with prefix operators like and lists They are not required for commands that do not contain lists This restriction will be eventually removed C Prolog User s Manual 5 save file Save can be called at top level from within a break level q v or from anywhere within a program 1 7 Restoring A Saved Program Once a program has been saved into a file file C Prolog can be restored to this saved state by invok ing it as follows cprolog file After execution of this command the interpreter will be in exactly the same state as existed immediately prior to the call to save except for open files which are automatically closed by save That is to say exe cution will start at the goal immediately following the call to save just as if save had returned successfully If you saved the state at top level then you will be back
33. ead it is put after any clauses already read by the inter preter for that procedure reconsult F Like consult except that any procedure defined in the reconsulted file erases any clauses for that procedure already present in the interpreter reconsult makes it possible to amend a program without having to restart from scratch and consult all the files which make up the program File Files This is a shorthand way of consulting or reconsulting a list of files A file name may optionally be preceded by the operator to indicate that the file should be reconsulted rather than con sulted Thus C Prolog User s Manual 21 2 file1 file2 file3 is merely a shorthand for consult file1 reconsult file2 consult file3 5 1 2 File Handling see F File F becomes the current input stream seeing F F is unified with the name of the current input file seen Closes the current input stream tell F File F becomes the current output stream telling F F is unified with the name of the current output file told Closes the current output stream close F File F currently open for input or output is closed fileerrors Undoes the effect of nofileerrors nofileerrors After a call to this predicate the I O error conditions incorrect file name can t see file can t tell file and end of file cause a call to fail instead of the default action whic
34. ere are no variables as in this example member b a b c then the system answers yes and execution of the question terminates If variables are included in the question then the final value of each variable is displayed except for anonymous variables Thus the question member X a b c would be answered by X a At this point the interpreter is waiting for you to indicate whether that solution is sufficient or whether you want it to backtrack to see if there are any more solutions Simply typing END OF LINE terminates the ques tion while typing followed by END OF LINE causes the system to backtrack looking for alternative solutions If no further solutions can be found it outputs no 6699 The outcome of some questions is shown below where a number preceded by is a system C Prolog User s Manual 4 generated name for a variable member X tom dick harry X tom X dick X harry no member X a b f Y c member X f b Z d Y b X f b c Z c Just END OF LINE typed here yes member X f _ g X f _ 1728 yes Ec When C Prolog reads terms from a file or from the terminal following a call to user it treats them all as program clauses In order to get the interpreter to execute directives from a file they must be preceded by for questions or for commands Commands are like questions except that they do not cause answers to be printed o
35. g term T A current predicate is named A m g goal P T is a database reference Switch on debugging Output debugging status information Display term T on the terminal Erase the clause or record ref R The object with ref R has been erased Expression expansion if N on Term T is a shorthand which expands to term X The file F exists Backtrack immediately Enable reporting of file errors The top functor of term T has name F arity N The next non blank character input is C The next character input is C Halt Prolog exit to the monitor A m g instance of the record ref R is T Term T is an integer Y is the value of arithmetic expression X The list L sorted by key yields S Set leashing mode to M List the current program List the procedure s P The name of atom or number A is string L Output a new line Switch off debugging Disable reporting of file errors Term Tis a non variable Remove spy points from the procedure s P Term Tis a number Make atom A an operator of type T precedence P T is a number or a database reference Portray or else write the term T C Prolog User s Manual 35 prompt A B Change the prompt from A to B put C The next character output is C read T Read term T reconsult F Update the program with procedures from file F recorda K 7 R Make term T the first record under key K ref R recorded K T7 R Term T is recorded under key K ref R recordz K 7 R Make term
36. ge A logic program consists simply of a sequence of statements called clauses which are analogous to sentences of natural language A clause comprises a head and a body The head either consists of a sin gle literal or is empty The body consists of a sequence of zero or more literal that is it too may be empty The body literals are called goals or procedure calls A clause with empty head is a negative clause or query and represents a command to the interpreter to start proving the goals in its body The form of a query is 5After all it could not be read C Prolog User s Manual 11 P Q R but as described in Section 1 5 the can be omitted when typing a command to the interpreter From here on when we use the term clause we mean a positive clause that is one with nonempty head If the body of a clause is empty the clause is called a unit clause and is written in the form P where P is the head literal We interpret this declaratively as P is true and procedurally as P can be satisfied If the body of a clause is nonempty the clause is called a nonunit clause and is written in the form P Q R S where P is the head and Q R and S are the goals which make up the body We can read such a clause either declaratively as P is true if Q and R and S are true or procedurally as To satisfy goal P satisfy goals Q R and S Clauses in general contain vari
37. h is to type an error message and then call abort exists F Succeeds if the file F exists rename F N If File F is renamed to N If N is the file is deleted If F was a currently open stream it is closed first 5 1 3 Input and Output of Terms read X The next term delimited by a full stop i e a followed by a carriage return or a space is read from the current input stream and unified with X The syntax of the term must accord with current operator declarations If a call read X causes the end of the current input stream to be reached X is unified with the atom end of file Further calls to read for the same stream will then cause an error failure write X The term X is written to the current output stream according to operator declarations in force display X The term X is displayed on the terminal in standard parenthesised prefix notation writeq Term C Prolog User s Manual 22 Similar to write Term but the names of atoms and functors are quoted where necessary to make the result acceptable as input to read print Term Print Term onto the current output This predicate provides a handle for user defined pretty printing If Term is a variable then it is written using write Term If Term is non variable then a call is made to the user defined procedure portray Term If this succeeds then it is assumed that Term has been output Otherwise print is equivalent to write 5 1 4 Char
38. h the ports of these procedures will henceforth be traced If debug mode was previously off then it will be switched on Spec can either be a predicate specification of the form Name Arity or Name or a list of such specifications When the Name is given without the Arity this refers to all predicates of that name which currently have definitions If there are none then nothing will be done Spy points can be placed on particular undefined procedures only by using the full form Name Arity nospy Spec Spy points are removed from all the procedures given by Spec as for spy leash Mode Sets the leashing mode to Mode where Mode can be one of the following full prompt on Call Exit BackTo and Fail tight prompt on Call BackTo and Fail half prompt on Call and BackTo loose prompt on Call off never prompt N N is an integer If the binary notation of N is 2 cebf the digits c e b and f correspond to the Call Exit BackTo and Fail respectively and are 1 0 if the corresponding port is leashed un leashed The initial leashing mode is half C Prolog User s Manual 32 debugging Outputs information concerning the status of the debugging package This will show whether debug mode is on and if it is what spy points have been set what mode of leashing is in force 5 12 Environmental NOLC Establishes the no lower case convention described in Section 2 4 LO Establishes the full character set convention descr
39. he set of all instances of X such that P is provable where that set is non empty The term P specifies a goal or goals as in call P S is a set of terms represented as a list of those terms without duplicates in the standard order for terms see Section 5 3 If there are no instances of X such that P is satisfied then the predicate fails The variables appearing in the term X should not appear anywhere else in the clause except within the term P Obviously the set to be enumerated should be finite and should be enumerable by Prolog in finite time It is possible for the provable instances to contain variables but in this case the list S will only provide an imperfect representation of what is in reality an infinite set If there are uninstantiated variables in P which do not also appear in X then a call to this evaluable predicate may backtrack generating alternative values for S corresponding to different instantiations of the free variables of P It is to cater for such usage that the set S is constrained to be non empty For example the call setof X X likes Y S might produce two alternative solutions via backtracking Y beer S dick harry tom Y cider S bill jan tom The call setof Y S setof X X likes Y S SS would then produce SS beer dick harry tom cider bill jan tom Variables occurring in P will not be treated as free if they are explicitly bound within P by an existe
40. hed to Old the atom bound to New specifies the new prompt In particular the goal prompt X X matches the current prompt to X without changing it Note that this only affects the prompt issued for reads in the user s program it will not change the propmts used by the system at top level etc system String Calls the operating system with string String as argument For example system Is will produce a directory listing on UNIX sh Suspends C Prolog and enters a recursive command interpreter On UNIX the shell used will be that specified in the environment variable SHELL statistics C Prolog User s Manual 33 Shows the current allocations and amounts used for each of the six working areas of C Prolog and also the runtime since C Prolog started For example statistics atom space 64K 15596 bytes used aux stack 8K 0 bytes used trail 64K 48 bytes used heap 256K 30664 bytes used global stack 256K 0 bytes used local stack 256K 300 bytes used Runtime 1 42 sec RS 5 13 Preprocessing expand term 7 72 Each top level term T read when consulting a file is rewritten into T2 before being asserted or exe cuted The default transformations provided by this predicate are the ones for grammar rules and for inline expansion of arithmetic expressions The user may define further transformations as clauses for the predicate term_expansion 2 which has similar arguments User defined transformations are
41. ibed in Section 2 4 It is the default setting op priority type name Treat name name as an operator of the stated type and priority refer to Section 2 2 name may also be a list of names in which case all are to be treated as operators of the stated type and priority break Causes the current execution to be suspended at the next procedure call Then the message Break level 1 is displayed The interpreter is then ready to accept input as though it was at top level If another call of break is encountered it moves up to level 2 and so on To close the break and resume the execution which was suspended type the END OF INPUT character Execution will be resumed at the procedure call where it had been suspended Alternatively the suspended execution can be aborted by calling the evaluable predicate abort Refer to Section 1 9 abort Aborts the current execution taking you back to top level Refer to Section 1 9 save F The system saves the current state of the system into file F Refer to Section 1 6 save F When Saves the current state of the system into file F When is unified with 0 or 1 depending on whether the system is returning from the save goal in the original Prolog session or after the saved state in F has been restored by invoking Prlog with file F as argument prompt Old New The sequence of characters prompt which indicates that the system is waiting for user input is represented as an atom and matc
42. inite clauses tells us which goals can be considered true according to a given program and is defined recursively as fol lows A goal is true if it is the head of some clause instance and each of the goals if any in the body of that clause instance is true where an instance of a clause or term is obtained by substitut ing for each of zero or more of its variables a new term for all occurrences of the variable For example if a program contains the preceding procedure for concatenate then the declarative semantics tells us that concatenate a b a b is true because this goal is the head of a certain instance of the first clause for concatenate namely concatenate a b a b concatenate b b and we know that the only goal in the body of this clause instance is true since it is an instance of the unit clause which is the second clause for concatenate The declarative semantics makes no reference to the sequencing of goals within the body of a clause nor to the sequencing of clauses within a program This sequencing information is however very relevant for the procedural semantics which Prolog gives to definite clauses The procedural semantics defines exactly how the Prolog system will execute a goal and the sequencing information is the means by which the Prolog programmer directs the system to execute his program in a sensible way The effect of executing a goal is to enumerate one by one its true instances
43. ique invocation identifier This is continuously incrementing regardless of whether or not you are actually seeing the invocations provided that debug mode is on This number can be used to cross correlate the trace messages for the various ports since it is unique for every invocation It will also give an indication of the number of procedure calls made since the start of the execution The invocation counter starts again for every fresh execution of a command and it is also reset when retries see later are performed The number following this is the current depth that is the number of direct ancestors this goal has The next word specifies the particular port Call Exit BackTo or Fail The goal is then printed so that you can inspect its current instantiation state The final is the prompt indicating that you should type in one of the option codes allowed see next section If this particular port is unleashed then you will obviously not get this prompt since you have specified that you do not wish to interact at this point Notice that not all procedure calls are traced there are a few basic procedures which have been made invisible since it is more convenient not to see them These include all primitive I O evalu able predicates for example get put read write all basic control structures that is gt and all debugging control evaluable predicates for instance debug spy leash trace
44. is case the directive should be a command as shown above Another common method of organisation is to have one file just containing commands to declare all the necessary operators This file is then always con sulted first Note that a comma written literally as a punctuation character can be used as though it were an infix operator of precedence 1000 and type xfy XY X Y represent the same compound term But note that a comma written as a quoted atom is not a standard operator Note also that the arguments of a compound term written in standard syntax must be expres sions of precedence below 1000 Thus it is necessary to bracket the expression P Q in assert P Q The following syntax restrictions serve to remove potential ambiguity associated with prefix operators In a term written in standard syntax the principal functor and its following must not be separated by any blankspace Thus point X Y Z is invalid syntax If the argument of a prefix operator starts with a this must be separated from the opera tor by at least one space or other non printable character Thus pq r where is the prefix operator is invalid syntax and must be written as p3q r If a prefix operator is written without an argument as an ordinary atom the atom is treated as an expression of the same precedence as the prefix operator and must therefore be bracketed where necess
45. is clearly impractical to creep through the entire program Spy points make it possible to stop the program whenever it gets to a particular procedure which is of interest Once there one can set further spy points in order to catch the control flow a bit further on or one can start creeping Setting a spy point on a procedure indicates that you wish to see all control flow through the vari ous ports of its invocation boxes When control passes through any port of a procedure with a spy point set on it a message is output and the user is asked to interact Note that the current mode of leashing does not affect spy points user interaction is requested on every port Spy points are set and removed by the following evaluable predicates which are also standard opera tors C Prolog User s Manual 17 spy X Sets spy points on all the procedures given by X X is either a single predicate specification or a list of such specifications A predicate specification is either of the form lt atom gt lt arity gt which means the procedure with the name lt atom gt and an arity of lt arity gt for example member 2 foo 0 hello 27 or it is of the form lt atom gt which means all the procedures with the name lt atom gt that currently have clauses in the data base e g member foo hello This latter form may refer to multiple procedures which have the same name but different arities If you use the form lt atom gt but there are no clauses for
46. is exemplified by XIL a b L representing X L a b L respectively Note that this list syntax is only syntactic sugar for terms of the form _ and does not provide any additional facilities that were not available in Prolog For convenience a further notational variant is allowed for lists of integers which correspond to ASCII character codes Lists written in this notation are called strings For example Prolog represents exactly the same list as 80 114 111 108 111 103 2 2 Operators Operators in Prolog are simply a notational convenience For example the expression 2 1 could also be written 2 1 It should be noticed that this expression represents the structure L 2 1 and not the number 3 The addition would only be performed if the structure was passed as an argument to an appropriate procedure such as is see Section 5 2 The Prolog syntax caters for operators of three main kinds infix prefix and postfix An infix operator appears between its two arguments while a prefix operator precedes its single argument and a postfix operator is written after its single argument C Prolog User s Manual 8 Each operator has a precedence which is a number from 1 to 1200 The precedence is used to disambiguate expressions where the structure of the term denoted is not made explicit through the use of brackets The general rule is that the operator with the highest precedence is the principal fu
47. language The inconvenience caused by this restriction seems in practice to be very slight Usually the restriction is only felt in toy programs 3 4 The Cut Symbol Besides the sequencing of goals and clauses Prolog provides one other very important facility for specifying control information This is the cut symbol written Itis inserted in the program just like a goal but is not to be regarded as part of the logic of the program and should be ignored as far as the declarative semantics is concerned The effect of the cut symbol is as follows When first encountered as a goal cut succeeds immedi ately If backtracking should later return to the cut the effect is to fail the parent goal the goal which matched the head of the clause containing the cut and caused the clause to be activated In other words the cut operation commits the system to all choices made since the parent goal was invoked and causes other alternatives to be discarded The goals thus rendered determinate are the parent goal itself any goals occurring before the cut in the clause containing the cut and any subgoals which were executed during the execution of those preceding goals For example the procedure member X X L C Prolog User s Manual 14 member X Y L member X L can be used to test whether a given term is in a list and the goal member b a b c will succeed The procedure can also be used to extract elements from a list
48. ld be clear by analogy with those for infix operators As an example if not were declared as a prefix operator of type fy then not not P would be a permissible way to write not not P If the type were fx the preceding expression would not be legal although not P would still be a permissible form for not P In C Prolog a functor named name is declared as an operator of type type and precedence pre cedence by calling the evaluable predicate op op precedence type name 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 i e infix prefix or postfix An operator of any kind may be redefined by a new declaration of the same kind This applies equally to operators which are provided as standard in C Prolog namely op 1200 xfx s gt J op 1200 fx C Prolog User s Manual 9 op 1100 xfy D op 1050 xfy gt op 1000 xfy See note below op 900 fy not 4 SPY nospy op 700 xfx is lt gt lt gt op 500 yfx I op 500 fx op 400 yfx I a ds il lt lt gt gt op 300 xfx I op 200 xfy Operator declarations are most usefuly placed in directives at the top of your program files In th
49. ment through backtracking abolish V A Completely remove all clauses for the procedure with name N which should be an atom and arity A which should be an integer The space occupied by retracted or abolished clauses will be recovered when instances of the clause are no longer in use See also erase Section 5 10 which allows a clause to be directly erased via its database reference 5 9 Information about the State of the Program listing Lists in the current output stream all the clauses in the program listing A The argument A may be a predicate specification of the form Name Arity in which case only the clauses for the specified predicate are listed If A is just an atom then the interpreted procedures for all predicates of that name are listed as for listing 0 Finally it is possible for A to be a list of predi cate specifications of either type e g listing concatenate 3 reverse go 0 current atom Atom Generates through backtracking all currently known atoms and returns each one as Atom current functor Name Functor Generates through backtracking all currently known functors and for each one returns its name and most general term as Name and Functor respectively If Name is given only functors with that name are generated current _predicate Name Functor Similar to current functor but it only generates functors corresponding to predicates for which there exists a procedure 5 10 Internal Da
50. mportant to specify a predicate unambiguously the form lt name gt lt arity gt is used for example C Prolog User s Manual 12 concatenate 3 Certain predicates are predefined by procedures supplied by the Prolog system Such predicates are called evaluable predicates 6 As we have seen the goals in the body of a clause are linked by the operator which can be interpreted as conjunction and It is sometimes convenient to use an additional operator stand ing for disjunction or The precedence of is such that it dominates but is dominated by An example is the clause grandfather X Z mother X Y father X Y father Y Z which can be read as For any X Y and Z X has Z as a grandfather if either the mother of X is Y or the father of X is Y and the father of Y is Z Such uses of disjunction can always be eliminated by defining an extra predicate for instance the previous example is equivalent to grandfather X Z parent X Y father Y Z parent X Y mother X Y parent X Y father X Y and so disjunction will not be mentioned further in the following more formal description of the semantics of clauses 3 2 Declarative and Procedural Semantics The semantics of definite clauses should be fairly clear from the informal interpretations already given However it is useful to have a precise definition The declarativesemantics of def
51. n tial quantifier An existential quantification is written YQ meaning there exists a Y such that Q is true where Y is some Prolog variable For example setof X Y X likes Y S would produce the single result X bill dick harry jan tom in contrast to the earlier example C Prolog User s Manual 28 bagof X P Bag This is exactly the same as setof except that the list or alternative lists returned will not be ordered and may contain duplicates The effect of this relaxation is to save considerable time and space in execution The interpreter recognises this as meaning there exists an X such that P is true and treats it as equivalent to call P The use of this explicit existential quantifier outside the setof and bagof con structs is superfluous 5 7 Comparison of Terms These evaluable predicates are meta logical They treat uninstantiated variables as objects with values which may be compared and they never instantiate those variables They should not be used when what you really want is arithmetic comparison Section 5 2 or unification The predicates make reference to a standard total ordering of terms which is as follows variables in a standard order roughly oldest first the order is not related to the names of vari ables Database references roughly in order of age numbers from infinity to infinity atoms in alphabetical i e AS
52. nctor Thus if has a higher precedence than then atb c a b c are equivalent and denote the term a b c Note that the infix form of the term a b c must be written with explicit brackets a b c If there are two operators in the subexpression having the same highest precedence the ambiguity must be resolved from the types of the operators The possible types for an infix operator are xfx xfy yfx With an operator of type xfx it is a requirement that both of the two subexpressions which are the argu ments of the operator must be of LOWER precedence than the operator itself i e their principal func tors must be of lower precedence unless the subexpression is explicitly bracketed which gives it zero precedence With an operator of type xfy only the first or left hand subexpression must be of lower precedence the second can be of the same precedence as the main operator and vice versa for an operator of type yfx For example if the operators and both have type yfx and are of the same precedence then the expression a b c is valid and means a b c i e a b c Note that the expression would be invalid if the operators had type xfx and would mean a b c i e a b c if the types were both xfy The possible types for a prefix operator are fx fy and for a postfix operator they are xf yf The meaning of the types shou
53. nd gives various other information to be described later 4 3 Tracing The following evaluable predicate may be used to commence an exhaustive trace of a program trace Switches debug mode on if it is not on already and ensures that the next time control enters a pro cedure box a message will be produced and you will be asked to interact When stopped at a goal being traced you have a number of options which will be detailed later In particular you can just type END OF LINE carriage return to creep or single step into your program If you continue to creep through your program you will see every entry and exit to from every invocation box However you will notice that you are only given the opportunity to interact on Call and BackTo ports i e a single creep decision may take you through several Exit ports or several Fail ports Nor mally this is desirable as it would be tedious to go through all those ports step by step However if it is not what you want the following evaluable predicate gives full control over the ports at which you are prompted leash Mode Sets the leashing mode to Mode where Mode can be one of the following full prompt on Call Exit BackTo and Fail tight prompt on Call BackTo and Fail half prompt on Call and BackTo loose prompt on Call off never prompt or any other combination of ports as described later The initial leashing mode is half 4 4 Spy Points For programs of any size it
54. o the reconsult when you are at the Call port of the procedure to be re defined 5 Evaluable Predicates This section describes all the evaluable predicates available in C Prolog These predicates are pro vided in advance by the system and they cannot be redefined by the user Any attempt to add clauses or delete clauses to an evaluable predicate fails with an error message and leaves the evaluable predicate unchanged The C Prolog provides a fairly wide range of evaluable predicates to perform the following tasks Input Output Reading in programs Opening and closing files Reading and writing Prolog terms Getting and putting characters Arithmetic Affecting the flow of the execution Classifying and operating on Prolog terms meta logical facilities C Prolog User s Manual 20 Sets Term Comparison Manipulating the Prolog program database Manipulating the additional indexed database Debugging facilities Environmental facilities The evaluable predicates will be described according to this classification Appendix I contains a complete list of the evaluable predicates 5 1 Input and Output A total of 15 I O streams may be open at any one time for input and output This includes a stream that is always available for input and output to the user s terminal A stream to a file F is opened for input by the first see F executed F then becomes the current input stream Similarly a stream to file H is opened for output by the first
55. s available during debugging Much of the information in this chapter is similar but not identical to that of Chapter 8 of Program ming in Prolog 4 1 The Procedure Box Control Flow Model During debugging the interpreter prints out a sequence of goals in various states of instantiation in order to show the state the program has reached in its execution However in order to understand what is occurring it is necessary to understand when and why the interpreter prints out goals As in other programming languages key points of interest are procedure entry and return but in Prolog there is the additional complexity of backtracking One of the major confusions that novice Prolog programmers have to face is the question of what actually happens when a goal fails and the system suddenly starts backtracking The Procedure Box model of Prolog execution views program control flow in terms of movement about the program text This model provides a basis for the debugging mechanism in the inter preter and enables the user to view the behaviour of his program in a consistent way Let us look at an example Prolog procedure C Prolog User s Manual 15 She foe es ee a A ee A Call Exit gt descendant X Y offspring X Y gt descendant X Z lt offspring X Y descendant Y Z lt Fail BackTo x The first clause states that Y is a descendant of X if Y is an offspring of X and
56. set spy points on an evenly spread set of pertinent procedures and then follow the control flow through these by leaping from one to the other s Skip Skip is only valid for Call and BackTo ports It skips over the entire execution of the pro cedure That is you will not see anything until control comes back to this procedure at either the Exit port or the Fail port Skip is particularly useful while creeping since it guarantees that control will be returned after the possibly complex execution within the box If you skip then no message at all will appear until control returns This includes calls to procedures with spy points set they will be masked out during the skip There are two ways of overriding this there is a Quasi skip which does not ignore spy points and the t option after an interrupt will disable the masking Normally however this masking is just what is required q Quasi skip Is like Skip except that it does not mask out spy points If there is a spy point within the execution of the goal then control returns at this point and any action can be performed there The initial skip still guarantees an eventual return of control though when the internal execution is finished f Fail Transfers to the Fail port of the current box This puts your execution in a position where it is about to backtrack out of the current invocation that is you have manually failed the initial goal r Retry Transfers to the
57. t Clause Ref Similar to assert Clause but also unifies Ref with the database reference of the clause asserted asserta C Like assert C except that the new clause becomes the first clause for the procedure concerned asserta Clause Ref Similar to asserta Clause but also unifies Ref with the database reference of the clause asserted assertz C Like assert C except that the new clause becomes the last clause for the procedure concerned assertz Clause Ref Similar to assertz Clause but also unifies Ref with the database reference of the clause asserted clause P Q P must be bound to a non variable term and the program is searched for a clause whose head matches P The head and body of those clauses are unified with P and Q respectively If one of the clauses is a unit clause Q will be unified with true clause Head Body Ref Similar to clause Head Body but also unifies Ref with the database reference of the clause con cerned If Ref is not given at the time of the call Head must be instantiated to a non variable term Thus this predicate can have two different modes of use depending on whether the database refer ence of the clause is known or unknown retract C C Prolog User s Manual 30 The first clause in the program that matches C is erased C must be initially instantiated to a non variable The predicate may be used in a non determinate fashion i e it will successively retract clauses matching the argu
58. tabase This section describes predicates for manipulating an internal indexed database that is kept separate from the normal program database They are intended for more sophisticated database applications and are not really necessary for novice users For normal tasks you should be able to program quite satisfactorily just using assert and retract recorded Key Term Ref The internal database is searched for terms recorded under the key Key These terms are successively unified with Term in the order they occur in the database At the same time Ref is unified with the database reference of the recorded item The key must be given and may be an atom or complex term If it is a complex term only the principal functor is significant recorda Key Term Ref The term Term is recorded in the internal database as the first item for the key Key where Ref is its database reference The key must be given and only its principal functor is significant recordz Key Term Ref The term Term is recorded in the internal database as the last item for the key Key where Ref is its database reference The key must be given and only its principal functor is significant This is a lower level facility required only for complicated database manipulations C Prolog User s Manual 31 erase Ref The recorded item or clause whose database reference is Ref is effectively erased from the internal database or program An erased item will no longer be ac
59. th this convention variables must be distinguished by an initial underline character and the names of atoms and other functors which now have to be written in upper case are implicitly translated into lower case unless enclosed in single quotes For example _VALUE2 is a variable while VALUE2 is NOLC convention notation for the atom which is identical to value2 written in the LC convention The default convention is LC To switch to the no lower case convention call the evaluable predi cate NOLC NOLC To switch back to the full character set convention call the evaluable predicate LC 2 LC Note that the names of these two procedures consist of upper case letters so that they can be referred to on all devices and therefore the names must always be enclosed in single quotes It is recommended that the NOLC convention only be used in emergencies since the standard syn tax is far easier to use and is also easier for other people to read 3 The Meaning of Prolog Programs 3 1 Programs A fundamental unit of a logic program is the literal for instance gives tom apple teacher reverse 1 2 3 L X lt Y A literal is merely a term distinguished by the context in which it appears in the program The principal functor of a literal is called a predicate It corresponds roughly to a verb in natural language or to a procedure name in a conventional programming langua
60. th tools for incrementally build ing programs debugging programs by following their executions and modifying parts of programs without having to start again from scratch The text of a Prolog program is normally created in a number of files using a text editor C Prolog can then be instructed to read in programs from these files this is called Iconsulting the file To change parts of a program being run it is possible to reconsult files containing the changed parts Reconsulting means that definitions for procedures in the file will replace any old definitions for these procedures It is recommended that you make use of a number of different files when writing programs Since you will be editing and consulting reconsulting individual files it is useful to use files to group together related procedures keeping collections of procedures that do different things in different files Thus a Pro log program will consist of a number of files each file containing a number of related procedures When your programs start to grow to a fair size it is also a good idea to have one file which just con tains commands to the interpreter to consult all the other files which form a program You will then be able to consult your entire program by just consulting this single file 1 3 Access to C Prolog In this manual we assume that there is a command on your computer cprolog that invokes C Prolog VAX VMS PDP and DECsystem 10 are trademarks of Digit
61. the second clause states that Z is a descendant of X if Y is an offspring of X and if Z is a descendant of Y In the diagram a box has been drawn around the whole procedure and labelled arrows indicate the control flow in and out of this box There are four such arrows which we shall look at in turn Call This arrow represents initial invocation of the procedure When a descendant goal of the form descendant X Y is required to be satisfied control passes through the Call port of the descendant box with the intention of matching a component clause and then satisfying any subgoals in the body of that clause Notice that this is independent of whether such a match is possible that is first the box is called and then the attempt to match takes place Textually we can imagine moving to the code for descendant when meeting a call to descendant in some other part of the code Exit This arrow represents a successful return from the procedure This occurs when the initial goal has been unified with one of the component clauses and any subgoals have been satisfied Control now passes out of the Exit port of the descendant box Textually we stop following the code for descendant and go back to the place we came from BackTo This arrow indicates that a subsequent goal has failed and that the system has come back to this goal in an attempt to match the goal against another clause Control passes through the BackTo port of the descendant box in an attempt
62. ts arity or number of arguments For example the compound term whose functor is named point of arity 3 with arguments X Y and Z is written point X Y Z An atom is considered to be a functor of arity 0 One may think of a functor as a record type and the arguments of a compound term as the fields of a record Compound terms are usefully pictured as trees For example the term s np john vp v likes np mary would be pictured as the structure s np vp john v np likes mary Sometimes it is convenient to write certain functors as operators 2 ary functors may be declared as infix operators and 1 ary functors as prefix or postfix operators Thus it is possible to write X Y P Q X lt Y X P as optional alternatives to X Y P Q lt X Y X 5 P C Prolog User s Manual 7 Operators are described fully in the next section Lists form an important class of data structures in Prolog They are essentially the same as the lists of LISP a list either is the atom representing the empty list or is a compound term with functor and two arguments which are respectively the head and tail of the list Thus a list of the first three natural numbers is the structure e Die 33 3 0 which could be written using the standard syntax as 2 3 but which is normally written in a special list notation as 1 2 3 The special list notation in the case when the tail of a list is a variable
63. ut They always start with the symbol At top level this is simply written after the prompted which is then effec tively overridden Any required output must be programmed explicitly for example the command member 3 1 2 3 write ok directs the system to check whether 3 belongs to the list 1 2 3 and to output ok if so Execution of a command terminates when all the goals in the command have been successfully executed Other alternative solutions are not sought one may imagine an implicit cut at the end of the command If no solution can be found the system gives as a warning The main use for commands as opposed to questions is to allow files to contain directives which call various procedures but for which you don t want to have the answers printed out In such cases you only want to call the procedures for effect A useful example would be the use of a directive in a file which consults a whole list of other files such as bits bobs mainpart testcases data junk If this directive was contained in the file program then typing the following at top level would be a quick way of loading your entire program program When you are simply interacting with the top level of the Prolog interpreter the distinction between questions and commands is not very important At the top level you should normally only type questions In a file if you wish to exe
64. where you can issue directives to solve goals etc When the evaluable predicate break is called the message Break level 1 will be displayed This signals the start of a break and except for the effect of aborts see below it is as if the interpreter was at top level If break is called within a break then another recursive break is started and the message will say level 2 etc Breaks can be arbitrarily nested Typing the END OF INPUT character will close the break and resume the execution which was suspended starting at the procedure call where the suspension took place To abort the current execution forcing an immediate failure of the directive being executed and a return to the top level of the interpreter call the evaluable predicate abort either from the program or by executing the directive abort within a break In this case no END OF INPUT character is needed to close the break because all break lev els are discarded and the system returns right back to top level The a response to INTERRUPT described above can also be used to force an abort 1 10 Exiting From The Interpreter To exit from C Prolog interpreter you should give the directive C Prolog User s Manual 6 halt This can be issued either at top level or within a break or indeed from within your program If your program is still executing then you should interrupt it and abort to return to top level so that you can call
65. would have complete information about the control flow inside the procedure box Note that the box we have drawn round the procedure should really be seen as an invocation box That is there will be a different box for each different invocation of the procedure Obviously with some thing like a recursive procedure there will be many different Calls and Exits in the control flow but these will be for different invocations Since this might get confusing each invocation box is given a unique integer identifier C Prolog User s Manual 16 4 2 Basic Debugging Predicates The interpreter provides a range of evaluable predicates for control of the debugging facilities The most basic predicates are as follows debug Switches debug mode on It is initially off In order for the full range of control flow informa tion to be available it is necessary to have this on from the start When it is off the system does not remember invocations that are being executed This is because it is expensive and not required for normal running of programs You can switch debug mode on in the middle of execu tion either from within your program or after an interrupt see trace below but information prior to this will just be unavailable nodebug switches debug mode off If there are any spy points set then they will be removed debugging prints onto the terminal information about the current debugging state It shows whether debug mode is on or off a

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