TP Lex and Yacc - Musikwissenschaft
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1. for hprec e actions may also be written as 4 or single statement e Turbo Pascal declarations 64 73 may be put at the beginning of the rules section They will be treated as local declarations of the actions routine 32 3 14 Implementation Restrictions As with TP Lex internal table sizes and the main memory available limit the complexity of source grammars that TP Yacc can handle However the maximum table sizes provided by TP Yacc are large enough to handle quite complex grammars such as the Pascal grammar in the TP Yacc distribution The actual table sizes are shown in the statistics printed by TP Yacc when a compilation is finished The given figures are s states i LRO kernel items t shift and goto transitions and r reductions The default stack size of the generated parsers is yymaxdepth 1024 as declared in the TP Yacc library unit This should be sufficient for any average application but you can change the stack size by including a corresponding declaration in the definitions part of the Yacc grammar or change the value in the YaccLib unit Note that right recursive grammar rules may increase stack space requirements so it is a good idea to use left recursive rules wherever possible 3 15 Differences from UNIX Yacc Major differences between TP Yacc and UNIX Yacc are listed below e TP Yacc produces output code for Turbo Pascal rather than for C TP Yacc does2. in a corresponding TP Lex program as follows 0 91 return NUM You can also explicitly assign token numbers in the grammar For this purpose the first occurrence of a token identifier in the definitions section may be followed by an unsigned integer F g you may write Wtoken NUM 299 Besides the return value of yylex the lexical analyzer routine may also return an additional semantic value for the recognized token This value is assigned to a variable named yylval and may then be accessed in actions through the i notation see above Section Grammar Rules and Actions The yylval variable is of type YYSType the semantic value type Integer by default its declaration may be found in the yyparse cod file For instance to assign an Integer value to a NUM token in the above example we may write 0 9 begin val yytext yylval code return NUM end This assigns yylval the value of the NUM token using the Turbo Pascal standard procedure val If a parser uses tokens of different types via a token lt name gt definition then the yylval variable will not be of type Integer but instead of a corresponding variant record type which is capable of holding all the different value types declared in the TP Yacc grammar n this case the lexical analyzer must assign a semantic value to the corresponding record component which is named yyname where name stands for the corresponding type identifier E g if token NUM i
3. This can be done using the notation i with lt 0 0 refers to the first value to the left of the current rule 1 to the second and so on Note that in this case the referenced value depends on the actual contents of the parse stack so you have to make sure that the requested values are always where you expect them There are some situations in which TP Yacc cannot easily determine the type of values when a typed parser is used This is true in particular for values in enclosing rules and for the value in an action inside a rule In such cases you may use a type cast to explicitly specify the type of a value The format for such type casts is lt name gt for left hand side values and lt name gt i for right hand side values where name is a type identifier which must occur in a htoken precedence or type definition 3 7 Auxiliary Procedures The third section of a TP Yacc program is optional If it is present it may contain any Turbo Pascal code such as supporting routines or a main program which is tacked on to the end of the output file 3 8 Lexical Analysis For any TP Yacc generated parser the programmer must supply a lexical analyzer routine named yylex which performs the lexical analysis for the parser This routine must be declared as function yylex Integer The yylex routine may either be prepared by hand or by using the lexical analyzer generator TP Lex see Section TP Lex The lexical analy
4. To give you an idea of how this works let us consider our ambigious arithmetic expression grammar without precedences htoken NUM hh expr expr expr expr expr C expr NUM This grammar generates four shift reduce conflicts The description of state 8 reads as follows state 8 x conflicts shift 4 reduce 1 on shift 5 reduce 1 on expr expr expr 1 expr expr _ expr expr expr _ expr 2 shift 4 shift 5 end reduce 1 gt reduce 1 error 28 In this state we have successfully parsed a expression rule 1 When the next symbol is or x we have the choice between the reduction and shifting the symbol Using the default shift reduce disambiguating rule TP Yacc has resolved these conflicts in favour of shift Now let us assume the above precedence definition hleft 24 hleft x which gives x higher precedence than and makes both operators left associative The rightmost terminal in rule 1 is Hence given these precedence definitions the first conflict will be resolved in favour of shift has higher precedence than while the second one is resolved in favour of reduce is left associative Similar conflicts arise in state 7 state 7 xxx conflicts shift 4 reduce 2 on shift 5 reduce 2 on expr expr x expr _ 2 expr expr _ expr expr expr _ expr 2 shift 4 2 2
5. start name which are used in specifying start conditions on rules described below The start keyword may also be abbreviated as hs or 4S Turbo Pascal declarations enclosed between 1 and 4 These will be inserted into the output file at global scope Also any line that does not look like a Lex definition e g starts with blank or tab will be treated as Turbo Pascal code In particular this also allows you to include Turbo Pascal comments in your Lex program The rules section of a TP Lex program contains the actual specification of the lexical ana lyzer routine It may be thought of as a big CASE statement discriminating over the different patterns to be matched and listing the corresponding statements actions to be executed Each rule consists of a regular expression describing the strings to be matched in the input and a corresponding action a Turbo Pascal statement to be executed when the expression matches Expression and statement are delimited with whitespace blanks and or tabs Thus the format of a Lex grammar rule is expression statement Note that the action must be a single Turbo Pascal statement terminated with a semicolon use begin end for compound statements The statement may span multiple lines if the successor lines are indented with at least one blank or tab The action may also be replaced by the character indicating that the action for this rule is the same as that for the next one The T
6. the corresponding commands are plex exprlex pyacc expr ppc386 expr Note that in the Linux version the programs are named plex and pyacc to avoid name clashes with the corresponding UNIX utilities Having compiled expr pas you can execute the expr program and type some expressions to see it work terminate the program with an empty line There is a number of other sample TP Lex and Yacc programs 1 and y files in the distribution including a TP Yacc cross reference utility and a complete parser for Standard Pascal The TP Lex and Yacc programs recognize some options which may be specified anywhere on the command line E g lex o exprlex runs TP Lex with DFA optimization and yacc v expr runs TP Yacc in verbose mode TP Yacc generates a readable description of the generated parser The TP Lex and Yacc programs use the folloving default filename extensions 1 TP Lex input files y TP Yacc input files pas TP Lex and Yacc output files As usual you may overwrite default filename extensions by explicitly specifying suffixes If you ever forget how to run TP Lex and Yacc you can issue the command lex or yacc resp plex or pyacc without arguments to get a short summary of the command line syntax 2 TP Lex This section describes the TP Lex lexical analyzer generator 2 1 Usage lex options lex file 1 output file pas 2 2 Options v Verbose Lex generates a readable de
7. tokens of the target language Any identifier not introduced in a token definition will be treated as a nonterminal symbol As far as TP Yacc is concerned tokens are atomic symbols which do not have an innert structure A lexical analyzer must be provided which takes on the task of tokenizing the input stream and return the individual tokens and literals to the parser see Section Lezical Analysis e precedence definitions Operator symbols terminals may be associated with a precedence by means of a precedence definition which may have one of the following forms hleft symbol wright symbol nonassoc symbol which are used to declare left right and nonassociative operators respectively Each prece dence definition introduces a new precedence level lowest precedence first E g you may write Ynonassoc gt GEQ LEQ NEQ relational operators hleft 262 2 2 OR addition operators hleft x 2 2 AND x multiplication operators x right NOT UMINUS unary operators A terminal identifier introduced in a precedence definition may but need not appear in a token definition as well 14 e type definitions Any terminal or nonterminal grammar symbol may be associated with a type identifier which is used in the processing of semantic values Type tags of the form lt name gt may be used in token and precedence definitions to declare the type of a terminal symbol e g Ytoken Real
8. NUM left lt AddOp gt 242 To declare the type of a nonterminal symbol use a type definition of the form type lt name gt symbol e g type lt Real gt expr In a type definition you may also omit the nonterminals i e you may write htype lt name gt This is useful when a given type is only used with type casts see Section Grammar Rules and Actions and is not associated with a specific nonterminal Turbo Pascal declarations You may also include arbitrary Turbo Pascal code in the defini tions section enclosed in 41 This code will be inserted as global declarations into the output file unchanged 3 6 Grammar Rules and Actions The second part of a TP Yacc grammar contains the grammar rules for the target language Grammar rules have the format name symbol The left hand side of a rule must be an identifier which denotes a nonterminal symbol The right hand side may be an arbitrary possibly empty sequence of nonterminal and terminal symbols including literals enclosed in single quotes The terminating semicolon may also be omitted Different rules for the same left hand side symbols may be written using the character to separate the different alternatives 15 name symbol symbol For instance to specify a simple grammar for arithmetic expressions you may write fLlett 2 z hleft 2 2 htoken NUM hh expr expr expr expr expr expr
9. expr expr expr expr NUM The left definitions at the beginning of the grammar are needed to specify the precedence and associativity of the operator symbols This will be discussed in more detail in Section Ambigious Grammars Grammar rules may contain actions Turbo Pascal statements enclosed in 4 to be executed as the corresponding rules are recognized Furthermore rules may return values and access values returned by other rules These semantic values are written as value of the left hand side nonterminal and i value of the ith right hand side symbol They are kept on a special value stack which is maintained automatically by the parser Values associated with terminal symbols must be set by the lexical analyzer more about this in Section Lexical Analysis Actions of the form 1 can frequently be omitted since it is the default action assumed by TP Yacc for any rule that does not have an explicit action By default the semantic value type provided by Yacc is Integer You can also put a declaration like ht type YYSType Real ht into the definitions section of your Yacc grammar to change the default value type However if you have different value types the preferred method is to use type definitions as discussed in Section Definitions When such type definitions are given TP Yacc handles all the necessary 16 details of the YYSType definition and also provides a fair amount
10. of type checking which makes 1t easier to find type errors in the grammar For instance we may declare the symbols NUM and expr in the example above to be of type Real and then use these values to evaluate an expression as it is parsed hleft 242 2 2 Wleft x token lt Real gt NUM type lt Real gt expr Ah expr expr expr 1 1 3 expr expr 1 3 expr k expr 1 1 3 expr expr 1 1 3 1 C expr 727 2 NUM Note that we omitted the action of the last rule The copy action 1 required by this rule is automatically added by TP Yacc Actions may not only appear at the end but also in the middle of a rule which is useful to perform some processing before a rule is fully parsed Such actions inside a rule are treated as special nonterminals which are associated with an empty right hand side Thus a rule like x y action z vvill be treated as x y act z act action Actions inside a rule may also access values to the left of the action and may return values by assigning to the value The value returned by such an action can then be accessed by other actions using the usual i notation E g we may write xo y 2x 1 z 4 2 3 which has the effect of setting the value of x to 2 the value of y the value of z 17 Sometimes it is desirable to access values in enclosing rules
11. situations in which TP Yacc will not produce a valid parser for a given input language LALR 1 parsers are restricted to one symbol lookahead on which they have to base their parsing decisions If a grammar is ambigious or cannot be parsed unambigiously using one symbol lookahead TP Yacc will generate parsing conflicts when constructing the parse table There are two types of such conflicts shift reduce conflicts when there is both a shift and a reduce action for a given input symbol in a given state and reduce reduce conflicts if there is more than one reduce action for a given input symbol in a given state Note that there never will be a shift shift conflict When a grammar generates parsing conflicts TP Yacc prints out the number of shift reduce and reduce reduce conflicts it encountered when constructing the parse table However TP Yacc will still generate the output code for the parser To resolve parsing conflicts TP Yacc uses the following built in disambiguating rules e in a shift reduce conflict TP Yacc chooses the shift action e in a reduce reduce conflict TP Yacc chooses reduction of the first grammar rule The shift reduce disambiguating rule correctly resolves a type of ambiguity known as the dangling else ambiguity which arises in the syntax of conditional statements of many pro gramming languages as in Pascal htoken IF THEN ELSE hh stmt IF expr THEN stmt IF expr THEN stmt ELSE stmt This gram
12. subscripted expression compare x to x This special case is therefore caught by the first rule in the above example which causes a reduce reduce conflict with rule 3 in expressions like expr 1 SUB expr 2 SUP expr 3 The conflict is resolved in favour of the first rule In both cases discussed above the ambiguities could also be eliminated by rewriting the grammar accordingly although this yields more complicated and less readable grammars This may not always be the case Often ambiguities are also caused by design errors in the grammar Hence if TP Yacc reports any parsing conflicts when constructing the parser you should use the v option to generate the parser description 1st file and check whether TP Yacc resolved the conflicts correctly There is one type of syntactic constructs for which one often deliberately uses an ambigious grammar as a more concise representation for a language that could also be specified unambi giously the syntax of expressions For instance the following is an unambigious grammar for simple arithmetic expressions Wtoken NUM 26 hh expr term expr term term factor term factor factor expr NUM You may check yourself that this grammar gives a higher precedence than and makes both operators left associative The same effect can be achieved with the following ambigious grammar using precedence definitions htoken NUM hleft hleft
13. A RR A 31 3 12 Yace Library aii a Ade a A A ESR SESS 31 3 13 Other Features Sip ie Sa A it ee Se eS 32 3 14 Implementation Restrictions a e s 3 15 Differences from UNIX Yacc 33 1 Introduction This document describes the TP Lex and Yacc compiler generator toolset These tools are de signed especially to help you prepare compilers and similar programs like text processing utilities and command language interpreters with the Turbo Pascal TM programming language TP Lex and Yacc are Turbo Pascal adaptions of the well known UNIX TM utilities Lex and Yacc which were written by M E Lesk and S C Johnson at Bell Laboratories and are used with the C programming language TP Lex and Yacc are intended to be approximately compatible with these programs However they are an independent development of the author based on the techniques described in the famous dragon book of Aho Sethi and Ullman Aho Sethi Ullman Compilers principles techniques and tools Reading Mass Addison Wesley 1986 Version 4 1 of TP Lex and Yacc works with all recent flavours of Turbo Borland Pascal including Delphi and with the Free Pascal Compiler a free Turbo Pascal compatible compiler which currently runs on DOS and Linux other ports are under development Recent informa tion about TP Lex Yacc and the sources are available from the TPLY homepage http www musikwissenschaft
14. P Lex library unit provides various variables and routines which are useful in the programming of actions In particular the yytext string variable holds the text of the matched string and the yyleng Byte variable its length Regular expressions are used to describe the strings to be matched in a grammar rule They are built from the usual constructs describing character classes and sequences and operators specifying repetitions and alternatives The precise format of regular expressions is described in the next section The rules section may also start with some Turbo Pascal declarations enclosed in which are treated as local declarations of the actions routine Finally the auxiliary procedures section may contain arbitrary Turbo Pascal code such as supporting routines or a main program which is simply tacked on to the end of the output file The auxiliary procedures section is optional 2 5 Regular Expressions Table 1 summarizes the format of the regular expressions recognized by TP Lex also compare Aho Sethi Ullman 1986 fig 3 48 c stands for a single character s for a string r for a regular expression and n m for nonnegative integers The operators and have highest precedence followed by concatenation The operator has lowest precedence Parentheses may be used to group expressions and overwrite default precedences The lt gt and operators may only occur once in an expression The usual C like e
15. TP Lex and Yacc The Compiler VVriter s Tools for Turbo Pascal Version 4 1 User Manual Albert Graf Department of Musicinformatics Johannes Gutenberg University Mainz agQmuwiinfa geschichte uni mainz de April 1998 Contents 1 Introduction 2 TP Lex ZA USADOS 0000000200 232 ODptlehs ss lis dO Bad ia qu E E ced DS Aa a ee LS SR n M M O 28 DESCriptiOn ses acca Sa eae e SOA BESSA ae L R AD BB s DAY e SQUECES la m bo ho dey r are n a da y n eh RO b 2 9 RegularzEXpressions xs eo s a sz E AAA ssla 2 6 Start Conditions uu uma sacas red A EEE EE AD RAE as xub MU A EDA 201 LiDrary as A A E e are e y 2 8 Implementation Restrictions 2 9 Differences from UNIX Lex 8 TP Yacc 921 USADOS 0 3 2 sOptions ss tals A oe ead Je RY o Va m s sd SD escriptloh cs acca Soa tke BOR de ae e m UR O um R LAT aes 32 A eh pe a ce ee BOR ee edn ee oN Ot Ot or A 35 efihitlOhso n Be BA a sae ew n z z L a ES Pak OO A 13 3 6 Grammar Rules and Actions 15 3 7 Auxiliary Procedures ee 18 3 87 Lexical Analysis brit et E A AA m ee D 18 3 9 How The Parser Works 20 3 10 Ambigious Grammars 25 3 11 Error Handling suma oir a a ar RB Ra ee A R AD
16. antage of the large memory models in Borland Pascal 7 0 and Delphi and Michael Van Canneyt Michael VanCanneyt fys kuleuven ac be the maintainer of the Linux version of the Free Pascal compiler who is responsible for the Free Pascal port And of course thanks are due to the many TP Lex Yacc users all over the world for their support and comments which helped to improve these programs Getting Started Instructions on how to compile and install TP Lex and Yacc on all supported platforms can be found in the README file contained in the distribution Once you have installed TP Lex and Yacc on your system you can compile your first TP Lex and Yacc program expr Expr is a simple desktop calculator program contained in the distribution which consists of a lexical analyzer in the TP Lex source file exprlex 1 and the parser and main program in the TP Yacc source file expr y To compile these programs issue the commands lex exprlex yacc expr That s itl You nov have the Turbo Pascal sources exprlex pas and expr pas for the expr program Use the Turbo Pascal compiler to compile these programs as follovvs tpc expr Of course the precise compilation command depends on the type of compiler you are using Thus you may have to replace tpc with bpc dec or dcc32 depending on the version of the Turbo Borland Delphi compiler you have and with ppc386 for the Free Pascal compiler If you are using TP Lex and Yacc with Free Pascal under Linux
17. e yacclib pas for further information 3 4 Yacc Source A TP Yacc program consists of three sections separated with the delimiter definitions hh rules hh auxiliary procedures The TP Yacc language is free format whitespace blanks tabs and newlines is ignored except if it serves as a delimiter Comments have the C like format They are treated as whitespace Grammar symbols are denoted by identifiers which have the usual form letter including underscore followed by a sequence of letters and digits upper and lowercase is distinct The TP Yacc language also has some keywords which always start with the character Literals are denoted by characters enclosed in single quotes The usual C like escapes are recognized e n denotes newline e r denotes carriage return e t denotes tab denotes backspace e f denotes form feed e nnn denotes character no nnn in octal base 3 5 Definitions The first section of a TP Yacc grammar serves to define the symbols used in the grammar It may contain the following types of definitions 13 start symbol definition A definition of the form hstart symbol declares the start nonterminal of the grammar if this definition is omitted TP Yacc assumes the left hand side nonterminal of the first grammar rule as the start symbol of the grammar e terminal definitions Definitions of the form htoken symbol are used to declare the terminal symbols
18. einer Friedman 1985 3 12 Yacc Library The TP Yacc library YaccLib unit provides some global declarations used by the parser routine yyparse and some variables and utility routines which may be used to control the actions of 31 the parser and to implement error recovery See the file yacclib pas for a description of these variables and routines You can also modify the Yacc library unit and or the code template in the yyparse cod file to customize TP Yacc to your target applications 3 13 Other Features TP Yacc supports all additional language elements entitled as Old Features Supported But not Encouraged in the UNIX manual which are provided for backward compatibility with older versions of UNIX Yacc e literals delimited by double quotes e multiple character literals Note that these are not treated as character sequences but repre sent single tokens which are given a symbolic integer code just like any other token identifier However they will not be declared in the output file so you have to make sure yourself that the lexical analyzer returns the correct codes for these symbols E g you might explicitly assign token numbers by using a definition like Wtoken 257 at the beginning of the Yacc grammar N may be used instead of 7 i e means 22 Vleft is the same as Zleft etc e other synonyms W lt for left 25 for right fbinary or 2 for Znonassoc Wterm or 0 for token
19. f a lexical analyzer is to translate some parts of the input and leave the rest unchanged you only have to specify the patterns which have to be treated specially If however the lexical analyzer has to absorb its whole input you will have to provide rules that match everything E g you might use the rules a i Ma which match any other character and ignore it Sometimes certain patterns have to be analyzed differently depending on some amount of context in which the pattern appears In such a case the operator is useful For instance the expression a b matches a but only if followed by b Note that the b does not belong to the match rather the lexical analyzer when matching an a will look ahead in the input to see whether it is followed by a b before it declares that it has matched an a Such lookahead may be arbitrarily complex up to the size of the LexLib input buffer E g the pattern a b matches an a which is followed by a b somewhere on the same input line T P Lex also has a means to specify left context which is described in the next section 2 6 Start Conditions a TP Lex provides some features which make it possible to handle left context The character at the beginning of a regular expression may be used to denote the beginning of the line More distant left context can be described conveniently by using start conditions on rules Any rule which is prefixed with the lt gt construct is only valid
20. ferences from UNIX Lex Major differences between TP Lex and UNIX Lex are listed below e TP Lex produces output code for Turbo Pascal rather than for C e Character tables KT are not supported neither are any directives to determine internal table sizes hp n etc e Library routines are named differently from the UNIX version e g the start routine takes the place of the BEGIN macro of UNIX Lex and of course all macros of UNIX Lex ECHO REJECT etc had to be implemented as procedures 11 The TP Lex library unit starts counting line numbers at 0 incrementing the count before a line is read in contrast UNIX Lex initializes yylineno to 1 and increments it after the line end has been read This is motivated by the way in which TP Lex maintains the current line and vvill not affect your programs unless you explicitly reset the yylineno value e g when opening a new input file In such a case you should set yylineno to 0 rather than 1 3 TP Yacc This section describes the TP Yacc compiler compiler 3 1 Usage yacc options yacc filel yl output file pas 3 2 Options v Verbose TP Yacc generates a readable description of the generated parser written to yacc file with new extension 1st d Debug TP Yacc generates parser with debugging output 3 3 Description TP Yacc is a program that lets you prepare parsers from the description of input languages by BNF like grammars You simply s
21. hh expr expr expr expr expr 0 expr 73 NUM Without the precedence definitions this is an ambigious grammar causing a number of shift reduce conflicts The precedence definitions are used to correctly resolve these conflicts conflicts resolved using precedence will not be reported by TP Yacc Each precedence definition introduces a new precedence level lowest precedence first and specifies whether the corresponding operators should be left right or nonassociative nonasso ciative operators cannot be combined at all example relational operators in Pascal TP Yacc uses precedence information to resolve shift reduce conflicts as follows Precedences are associated with each terminal occuring in a precedence definition Furthermore each gram mar rule is given the precedence of its rightmost terminal this default choice can be overwritten using a hprec tag see below To resolve a shift reduce conflict using precedence both the 27 symbol and the rule involved must have been assigned precedences TP Yacc then chooses the parse action as follovvs e If the symbol has higher precedence than the rule shift e If the rule has higher precedence than the symbol reduce If symbol and rule have the same precedence the associativity of the symbol determines the parse action if the symbol is left associative reduce if the symbol is right associative shift if the symbol is non associative error
22. if the lexical analyzer is in the denoted start state For instance the expression lt x gt a can only be matched if the lexical analyzer is in start state x You can have multiple start states in a rule e g lt x y gt a can be matched in start states x or y Start states have to be declared in the definitions section by means of one or more start state definitions see above The lexical analyzer enters a start state through a call to the LexLib routine start E g you may write start x y hh lt x gt a start y lt y gt b start x hh begin start x if yylex 0 then end Upon initialization the lexical analyzer is put into state x It then proceeds in state x until it matches an a which puts it into state y In state y it may match a b which puts it into state x again etc Start conditions are useful when certain constructs have to be analyzed differently depending on some left context such as a special character at the beginning of the line and if multiple lexical analyzers have to work in concert If a rule is not prefixed with a start condition it is valid in all user defined start states as well as in the lexical analyzer s default start state 2 7 Lex Library The TP Lex library LexLib unit provides various variables and routines which are used by Lex generated lexical analyzers and application programs It provides the input and output streams and other internal data structures used by the lexical analyzer rou
23. lso errors will be caught sooner in most cases where UNIX Yacc would carry out an additional default reduction before detecting the error Library routines are named differently from the UNIX version e g the yyerrlab routine takes the place of the YYERROR macro of UNIX Yacc and of course all macros of UNIX Yacc YYERROR YYACCEPT etc had to be implemented as procedures 34
24. mar is ambigious because a nested construct like IF expr 1 THEN IF expr 2 THEN stmt 1 ELSE stmt 2 can be parsed two ways either as IF expr 1 THEN IF expr 2 THEN stmt 1 ELSE stmt 2 or as 25 IF expr 1 THEN IF expr 2 THEN stmt 1 ELSE stmt 2 The first interpretation makes an ELSE belong to the last unmatched IF which also is the interpretation chosen in most programming languages This is also the way that a TP Yacc generated parser will parse the construct since the shift reduce disambiguating rule has the effect of neglecting the reduction of IF expr 2 THEN stmt 1 instead the parser will shift the ELSE symbol which eventually leads to the reduction of IF expr 2 THEN stmt 1 ELSE stmt 2 The reduce reduce disambiguating rule is used to resolve conflicts that arise when there is more than one grammar rule matching a given construct Such ambiguities are often caused by special case constructs which may be given priority by simply listing the more specific rules ahead of the more general ones For instance the following is an excerpt from the grammar describing the input language of the UNIX equation formatter EQN hright SUB SUP hh expr expr SUB expr SUP expr expr SUB expr expr SUP expr Here the SUB and SUP operator symbols denote sub and superscript respectively The rationale behind this example is that an expression involving both sub and superscript is often set differently from a superscripted
25. not support union definitions Instead a value type is declared by specifying the type identifier itself as the tag of a token or htype definition TP Yacc will automatically generate an appropriate variant record type YYSType which is capable of holding values of any of the types used in token and type Type checking is very strict If you use type definitions then any symbol referred to in an action must have a type introduced in a type definition Either the symbol must have been assigned a type in the definitions section or the lt type identifier gt notation must be used The syntax of the type definition has been changed slightly to allow definitions of the form type lt type identifier gt omitting the nonterminals which may be used to declare types which are not assigned to any grammar symbol but are used with the lt gt construct e The parse tables constructed by this Yacc version are slightly greater than those constructed by UNIX Yacc since a reduce action will only be chosen as the default action if it is the only action in the state In difference UNIX Yacc chooses a reduce action as the default action whenever it is the only reduce action of the state even if there are other shift actions 33 This solves a bug in UNIX Yacc that makes the generated parser start error recovery too late with certain types of error productions see also Schreiner Friedman Introduction to compiler construction with UNIX 1985 A
26. ols until it can again perform a legal shift action To prevent a cascade of error messages the parser returns to its normal mode of operation only after it has seen and shifted three legal input symbols Any additional error found after the first shifted symbol restarts error recovery but no error message is printed The TP Yacc library routine yyerrok may be used to reset the parser to its normal mode of operation explicitly For a simple example consider the rule stmt error 4 yyerrok and assume a syntax error occurs while a statement nonterminal stmt is parsed The parser prints an error message then pops its stack until it can shift the token error of the error rule Proceeding in error mode it will skip symbols until it finds a semicolon then reduces by the error rule The call to yyerrok tells the parser that vve have recovered from the error and that it should proceed with the normal parse This kind of panic mode error recovery scheme works well when statements are always terminated with a semicolon The parser simply skips the bad statement and then resumes the parse Implementing a good error recovery scheme can be a difficult task see Aho Sethi Ullman 1986 for a more comprehensive treatment of this topic Schreiner and Friedman have developed a systematic technique to implement error recovery with Yacc which I found quite useful I used it myself to implement error recovery in the TP Yacc parser see Schr
27. order NB For the Linux Free Pascal version the code template is searched in some directory defined at compile time instead of the execution path usually usr lib fpc lexyacc The TP Lex library LexLib unit is required by programs using Lex generated lexical an alyzers you will therefore have to put an appropriate uses clause into your program or unit that contains the lexical analyzer routine The LexLib unit also provides various useful utility routines see the file lexlib pas for further information 2 4 Lex Source A TP Lex program consists of three sections separated with the delimiter definitions hh rules hh auxiliary procedures All sections may be empty The TP Lex language is line oriented definitions and rules are separated by line breaks There is no special notation for comments but Turbo Pascal style comments may be included as Turbo Pascal fragments see below The definitions section may contain the following elements e regular definitions in the format name substitution which serve to abbreviate common subexpressions The name notation causes the corre sponding substitution from the definitions section to be inserted into a regular expression The name must be a legal identifier letter followed by a sequence of letters and digits the underscore counts as a letter upper and lowercase are distinct Regular definitions must be non recursive e start state definitions in the format
28. pecify the grammar for your target language augmented with the Turbo Pascal code necessary to process the syntactic constructs and TP Yacc translates your grammar into the Turbo Pascal code for a corresponding parser subroutine named yyparse TP Yacc parses the source grammar contained in yacc file with default suffix y and writes the constructed parser subroutine to the specified output file with default suffix pas if no output file is specified output goes to yacc file with new suffix pas If any errors are found during compilation error messages are written to the list file yacc file with new suflix 18t The generated parser routine yyparse is declared as function yyparse Integer This routine may be called by your main program to execute the parser The return value of the yyparse routine denotes success or failure of the parser possible return values 0 success 1 unrecoverable syntax error or parse stack overflow 12 Similar to TP Lex the code template for the yyparse routine may be found inthe yyparse cod file The rules for locating this file are analogous to those of TP Lex see Section TP Lez The TP Yacc library YaccLib unit is required by programs using Yacc generated parsers you will therefore have to put an appropriate uses clause into your program or unit that contains the parser routine The YaccLib unit also provides some routines which may be used to control the actions of the parser See the fil
29. pop 1 state uncovering state 0 then goto state 1 on symbol expr 1 0 shift state 5 5 10 NUM shift state 3 3 510 reduce rule 4 pop 1 state uncovering state 5 then goto state 8 on symbol expr 8 510 shift 4 8510 NUM shift 3 3 48510 reduce rule 4 pop 1 state uncovering state 4 then goto state 7 on symbol expr 7 48510 reduce rule 2 pop 3 states uncovering state 5 then goto state 8 on symbol expr 8 510 end reduce rule 1 pop 3 states uncovering state 0 then goto state 1 on symbol expr 1 0 end accept Table 2 Parse of NUM NUM NUM NUM NUM NUM Table 2 traces the corresponding actions of the parser We also show the current state in each move and the remaining states on the stack It is also instructive to see how the parser responds to illegal inputs E g you may try to figure out what the parser does when confronted with NUM or NUM NUM You will find that the parser sooner or later will always run into an error action when confronted with errorneous inputs An LALR parser will never shift an invalid symbol and thus will always find syntax errors as soon as it is possible during a left to right scan of the input TP Yacc provides a debugging option d that may be used to trace the actions performed by the parser When a grammar is compiled with the d option the generated parser will print out the actions as it parses its input 24 3 10 Ambigious Grammars There are
30. rec UMINUS expr NUM Note the use of the UMINUS token which is not an actual input symbol but whose sole purpose it is to give unary minus its proper precedence If we omitted the precedence tag both unary and binary minus would have the same precedence because they are represented by the same input symbol 30 3 11 Error Handling Syntactic error handling is a difficult area in the design of user friendly parsers Usually you will not like to have the parser give up upon the first occurrence of an errorneous input symbol Instead the parser should recover from a syntax error that is it should try to find a place in the input where it can resume the parse TP Yacc provides a general mechanism to implement parsers with error recovery A special predefined error token may be used in grammar rules to indicate positions vvhere syntax errors might occur When the parser runs into an error action i e reads an errorneous input symbol it prints out an error message and starts error recovery by popping its stack until it uncovers a state in which there is a shift action on the error token If there is no such state the parser terminates with return value 1 indicating an unrecoverable syntax error If there is such a state the parser takes the shift on the error token pretending it has seen an imaginary error token in the input and resumes parsing in a special error mode While in error mode the parser quietly skips symb
31. red in order Of course it would have been more efficient to generate a big CASE statement instead but I found that this may cause problems with the encoding of large DFA tables because Turbo Pascal has a quite rigid limit on the code size of individual procedures I decided to use a scheme in which transitions on different symbols to the same state are merged into one single transition specifying a character set and the corresponding next state This keeps the number of transitions in each state quite small and still allows a fairly efficient access to the transition table The TP Lex program has an option o to optimize DFA tables This causes a minimal DFA to be generated using the algorithm described in Aho Sethi Ullman 1986 Although the absolute limit on the number of DFA states that TP Lex can handle is at least 300 TP Lex poses an additional restriction 100 on the number of states in the initial partition of the DFA optimization algorithm Thus you may get a fatal integer set overflow message when using the o option even when TP Lex is able to generate an unoptimized DFA In such cases you will just have to be content with the unoptimized DFA Hopefully this will be fixed in a future version Anyhow using the merged transitions scheme described above TP Lex usually constructs unoptimized DFA s which are not far from being optimal and thus in most cases DFA optimization won t have a great impact on DFA table sizes 2 9 Dif
32. s declared Real 19 Wtoken lt Real gt NUM then the value for token NUM must be assigned to yylval yyReal 3 9 How The Parser Works TP Yacc uses the LALR 1 technique developed by Donald E Knuth and F DeRemer to construct a simple efficient non backtracking bottom up parser for the source grammar The LALR parsing technique is described in detail in Aho Sethi Ullman 1986 It is quite instructive to take a look at the parser description TP Yacc generates from a small sample grammar to get an idea of how the LALR parsing algorithm works We consider the following simplified version of the arithmetic expression grammar htoken NUM hleft hleft x hh expr expr expr expr k expr expr NUM When run with the v option on the above grammar TP Yacc generates the parser descrip tion listed below state O accept _ expr end shift 2 NUM shift 3 error expr goto 1 state 1 accept expr _ end expr expr _ expr 20 state 2 state 3 state 4 state 5 expr end 232 142 expr 70 NUM expr expr expr NUM expr expr 70 NUM expr _ expr accept shift 4 shift 5 error O expr shift 2 shift 3 error goto 6 NUM _ reduce 4 expr shift 2 shift 3 error goto 7 expr shift 2 shift 3 error 4 expr expr 21 state 6 state 7
33. scapes are recognized e n denotes newline EXPRESSION MATCHES EXAMPLE C any non operator character c a Ne character c literally Nx si string s literally any character but newline a b beginning of line abc end of line abc s any character in s abc s any character not in s abc r k zero or more r s ax r one or more r s ar r zero OT one r a r m n m to n occurrences of r a 1 5 rim m occurrences of r a 5 riro r then r ab rilra T Or T alb r T alb ri ra r when followed by r a b lt x gt r r when in start condition z lt x gt abc Table 1 Regular expressions r denotes carriage return e t denotes tab e b denotes backspace e f denotes form feed e nnn denotes character no nnn in octal base You can also use the character to quote characters which would otherwise be interpreted as operator symbols In character classes you may use the character to denote ranges of characters For instance a z denotes the class of all lowercase letters The expressions in a TP Lex program may be ambigious i e there may be inputs which match more than one rule In such a case the lexical analyzer prefers the longest match and if it still has the choice between different rules it picks the first of these If no rule matches the lexical analyzer executes a default action which consists of copying the input character to the output unchanged Thus if the purpose o
34. scription of the generated lexical analyzer written to lex file with new extension 1st o Optimize Lex optimizes DFA tables to produce a minimal DFA 2 3 Description TP Lex is a program generator that is used to generate the Turbo Pascal source code for a lexical analyzer subroutine from the specification of an input language by a regular expression grammar TP Lex parses the source grammar contained in lex file with default suffix 1 and writes the constructed lexical analyzer subroutine to the specified output file with default suffix pas if no output file is specified output goes to lex file with new suffix pas If any errors are found during compilation error messages are written to the list file lex file with new suffix 1st The generated output file contains a lexical analyzer routine yylex implemented as function yylex Integer This routine has to be called by your main program to execute the lexical analyzer The return value of the yylex routine usually denotes the number of a token recognized by the lexical analyzer see the return routine in the LexLib unit At end of file the yylex routine normally returns 0 The code template for the yylex routine may be found in the yylex cod file This file is needed by TP Lex when it constructs the output file It must be present either in the current directory or in the directory from which TP Lex was executed TP Lex searches these directories in the indicated
35. shift 5 end reduce 2 gt reduce 2 error Here we have successfully parsed a expression which may be followed by another or operator Since is left associative and has higher precedence than both conflicts will be resolved in favour of reduce Of course you can also have different operators on the same precedence level For instance consider the following extended version of the arithmetic expression grammar token NUM Aleft dleft 3 af 29 hh expr expr expr expr expr expr expr expr expr C expr NUM This puts all addition operators on the first and all multiplication operators on the second precedence level All operators are left associative for instance 5 3 2 will be parsed as 5 9 2 By default TP Yacc assigns each rule the precedence of its rightmost terminal This is a sensible decision in most cases Occasionally it may be necessary to overwrite this default choice and explicitly assign a precedence to a rule This can be done by putting a precedence tag of the form prec symbol at the end of the corresponding rule which gives the rule the precedence of the specified symbol For instance to extend the expression grammar with a unary minus operator giving it highest precedence you may write htoken NUM left 242 7 hleft 2 2 hright UMINUS hh expr expr expr expr expr expr expr expr expr expr hp
36. state 8 state 9 expr expr expr expr 2 232 5 expr expr expr expr expr expr 232 end 27 142 expr goto 8 70 expr _ expr _ expr expr _ expr shift 9 shift 4 shift 5 error expr x expr 2 expr _ expr expr _ expr reduce 2 expr expr 1 expr _ expr expr _ expr shift 4 reduce 1 reduce 1 reduce 1 error 70 expr 4 3 reduce 3 22 Each state of the parser corresponds to a certain prefix of the input which has already been seen The parser description lists the grammar rules wich are parsed in each state and indicates the portion of each rule which has already been parsed by an underscore n state the start state of the parser the parsed rule is accept expr end This is not an actual grammar rule but a starting rule automatically added by TP Yacc In general it has the format accept X end where X is the start nonterminal of the grammar and end is a pseudo token denoting end of input the end symbol is used by the parser to determine when it has successfully parsed the input The description of the start rule in state 0 accept _ expr end with the underscore positioned before the expr symbol indicates that we are at the beginning of the parse and are ready to parse an expression nonterminal expr The parser maintains a stack to keep track of states visi
37. ted during the parse There are two basic kinds of actions in each state shift which reads an input symbol and pushes the corresponding next state on top of the stack and reduce which pops a number of states from the stack corresponding to the number of right hand side symbols of the rule used in the reduction and consults the goto entries of the uncovered state to find the transition corresponding to the left hand side symbol of the reduced rule In each step of the parse the parser is in a given state the state on top of its stack and may consult the current lookahead symbol the next symbol in the input to determine the parse action shift or reduce to perform The parser terminates as soon as it reaches state 1 and reads in the endmarker indicated by the accept action on end in state 1 Sometimes the parser may also carry out an action without inspecting the current lookahead token This is the case e g in state 3 where the only action is reduction by rule 4 reduce 4 The default action in a state can also be error indicating that any other input represents a syntax error In case of such an error the parser will start syntactic error recovery as described in Section Error Handling Now let us see how the parser responds to a given input We consider the input string 2 5 3 which is presented to the parser as the token sequence 23 STATE STACK LOOKAHEAD ACTION 0 NUM shift state 3 3 0 reduce rule 4
38. tine and supplies some variables and utility routines which may be used by actions and application programs Refer to the file lexlib pas for a closer description You can also modify the Lex library unit and or the code template in the yylex cod file to customize TP Lex to your target applications E g you might wish to optimize the code of the lexical analyzer for some special application make the analyzer read from write to memory instead of files etc 2 8 Implementation Restrictions Internal table sizes and the main memory available limit the complexity of source grammars that TP Lex can handle There is currently no possibility to change internal table sizes apart 10 from modifying the sources of TP Lex itself but the maximum table sizes provided by TP Lex seem to be large enough to handle most realistic applications The actual table sizes depend on the particular implementation they are much larger than the defaults if TP Lex has been compiled with one of the 32 bit compilers such as Delphi 2 or Free Pascal and are shown in the statistics printed by TP Lex when a compilation is finished The units given there are p positions i e items in the position table used to construct the DFA s DFA states and t transitions of the generated DFA As implemented the generated DFA table is stored as a typed array constant which is inserted into the yylex cod code template The transitions in each state are sto
39. uni mainz de ag tply For information about the Free Pascal Compiler please refer to http www freepascal org TP Lex and Yacc like any other tools of this kind are not intended for novices or casual pro grammers they require extensive programming experience as well as a thorough understanding of the principles of parser design and implementation to be put to work successfully But if you are a seasoned Turbo Pascal programmer with some background in compiler design and formal language theory you will almost certainly find TP Lex and Yacc to be a powerful extension of your Turbo Pascal toolset This manual tells you how to get started with the TP Lex and Yacc programs and provides a short description of these programs Some knowledge about the C versions of Lex and Yacc will be useful although not strictly necessary For further reading you may also refer to e Aho Sethi and Ullman Compilers principles techniques and tools Reading Mass Addison Wesley 1986 e Johnson S C Yacc yet another compiler compiler CSTR 32 Bell Telephone Laborato ries 1974 Lesk M E Lex a lexical analyser generator CSTR 39 Bell Telephone Laboratories 1975 Schreiner Friedman Introduction to compiler construction with UNIX Prentice Hall 1985 e The Unix Programmer s Manual Sections Lex and Yacc Credits I would like to thank Berend de Boer berend pobox com who adapted TP Lex and Yacc to take adv
40. zer must be included in your main program behind the parser subroutine the yyparse code template includes a forward definition of the yylex routine such that the parser can access the lexical analyzer For instance you may put the lexical analyzer routine into the auxiliary procedures section of your TP Yacc grammar either directly or by using the the Turbo Pascal include directive 1 The parser repeatedly calls the yylex routine to tokenize the input stream and obtain the individual lexical items in the input For any literal character the yylex routine has to return the corresponding character code For the other symbolic terminals of the input language the lexical analyzer must return corresponding integer codes These are assigned automatically by TP Yacc in the order in which token definitions appear in the definitions section of the source grammar The lexical analyzer can access these values through corresponding integer constants which are declared by TP Yacc in the output file 18 For instance if htoken NUM is the first definition in the Yacc grammar then TP Yacc will create a corresponding constant declaration const NUM 257 in the output file TP Yacc automatically assigns symbolic token numbers starting at 257 1 thru 255 are reserved for character literals 0 denotes end of file and 256 is reserved for the special error token which will be discussed in Section Error Handling This definition may then be used e g
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