1 \input texinfo @c -*-texinfo-*-
3 @setfilename ../../info/wisent
4 @set TITLE Wisent Parser Development
5 @set AUTHOR Eric M. Ludlam, David Ponce, and Richard Y. Kim
6 @settitle @value{TITLE}
8 @c *************************************************************************
10 @c *************************************************************************
12 @c Merge all indexes into a single index for now.
13 @c We can always separate them later into two or more as needed.
20 @c @footnotestyle separate
26 Copyright @copyright{} 1988--1993, 1995, 1998--2004, 2007, 2012
27 Free Software Foundation, Inc.
29 @c Since we are both GNU manuals, we do not need to ack each other here.
31 Some texts are borrowed or adapted from the manual of Bison version
32 1.35. The text in section entitled ``Understanding the automaton'' is
33 adapted from the section ``Understanding Your Parser'' in the manual
34 of Bison version 1.49.
38 Permission is granted to copy, distribute and/or modify this document
39 under the terms of the GNU Free Documentation License, Version 1.3 or
40 any later version published by the Free Software Foundation; with no
41 Invariant Sections, with the Front-Cover texts being ``A GNU Manual,''
42 and with the Back-Cover Texts as in (a) below. A copy of the license
43 is included in the section entitled ``GNU Free Documentation License''.
45 (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
46 modify this GNU manual.''
50 @dircategory Emacs misc features
52 * Wisent: (wisent). Semantic Wisent parser development.
59 @c @setchapternewpage odd
60 @c @setchapternewpage off
65 @author by @value{AUTHOR}
67 @vskip 0pt plus 1 fill
76 @c *************************************************************************
78 @c *************************************************************************
84 Wisent (the European Bison ;-) is an Emacs Lisp implementation of the
85 GNU Compiler Compiler Bison.
87 This manual describes how to use Wisent to develop grammars for
88 programming languages, and how to use grammars to parse language
89 source in Emacs buffers.
91 It also describes how Wisent is used with the @semantic{} tool set
92 described in the @ref{Top, Semantic Manual, Semantic Manual, semantic}.
103 * GNU Free Documentation License::
107 @node Wisent Overview
108 @chapter Wisent Overview
110 @dfn{Wisent} (the European Bison) is an implementation in Emacs Lisp
111 of the GNU Compiler Compiler Bison. Its code is a port of the C code
112 of GNU Bison 1.28 & 1.31.
114 For more details on the basic concepts for understanding Wisent, it is
115 worthwhile to read the @ref{Top, Bison Manual, bison}.
117 @uref{http://www.gnu.org/manual/bison/html_node/index.html}.
120 Wisent can generate compilers compatible with the @semantic{} tool set.
121 See the @ref{Top, Semantic Manual, , semantic}.
123 It benefits from these Bison features:
127 It uses a fast but not so space-efficient encoding for the parse
128 tables, described in Corbett's PhD thesis from Berkeley:
130 @cite{Static Semantics in Compiler Error Recovery}@*
131 June 1985, Report No. UCB/CSD 85/251.
135 For generating the lookahead sets, Wisent uses the well-known
136 technique of F. DeRemer and A. Pennello described in:
138 @cite{Efficient Computation of LALR(1) Look-Ahead Sets}@*
139 October 1982, ACM TOPLAS Vol 4 No 4, 615--49,
140 @uref{http://dx.doi.org/10.1145/69622.357187}.
144 Wisent resolves shift/reduce conflicts using operator precedence and
148 Parser error recovery is accomplished using rules which match the
149 special token @code{error}.
152 Nevertheless there are some fundamental differences between Bison and
157 Wisent is intended to be used in Emacs. It reads and produces Emacs
158 Lisp data structures. All the additional code used in grammars is
162 Contrary to Bison, Wisent does not generate a parser which combines
163 Emacs Lisp code and grammar constructs. They exist separately.
164 Wisent reads the grammar from a Lisp data structure and then generates
165 grammar constructs as tables. Afterward, the derived tables can be
166 included and byte-compiled in separate Emacs Lisp files, and be used
167 at a later time by the Wisent's parser engine.
170 Wisent allows multiple start nonterminals and allows a call to the
171 parsing function to be made for a particular start nonterminal. For
172 example, this is particularly useful to parse a region of an Emacs
173 buffer. @semantic{} heavily depends on the availability of this feature.
177 @chapter Wisent Grammar
179 @cindex context-free grammar
181 In order for Wisent to parse a language, it must be described by a
182 @dfn{context-free grammar}. That is a grammar specified as rules that
183 can be applied regardless of context. For more information, see
184 @ref{Language and Grammar, , , bison}, in the Bison manual.
188 The formal grammar is formulated using @dfn{terminal} and
189 @dfn{nonterminal} items. Terminals can be Emacs Lisp symbols or
190 characters, and nonterminals are symbols only.
193 Terminals (also known as @dfn{tokens}) represent the lexical
194 elements of the language like numbers, strings, etc..
196 For example @samp{PLUS} can represent the operator @samp{+}.
198 Nonterminal symbols are described by rules:
202 RESULT @equiv{} COMPONENTS@dots{}
206 @samp{RESULT} is a nonterminal that this rule describes and
207 @samp{COMPONENTS} are various terminals and nonterminals that are put
208 together by this rule.
210 For example, this rule:
214 exp @equiv{} exp PLUS exp
218 Says that two groupings of type @samp{exp}, with a @samp{PLUS} token
219 in between, can be combined into a larger grouping of type @samp{exp}.
224 * Compiling a grammar::
229 @section Grammar format
231 @cindex grammar format
232 To be acceptable by Wisent a context-free grammar must respect a
233 particular format. That is, must be represented as an Emacs Lisp list
236 @code{(@var{terminals} @var{assocs} . @var{non-terminals})}
240 Is the list of terminal symbols used in the grammar.
242 @cindex associativity
244 Specify the associativity of @var{terminals}. It is @code{nil} when
245 there is no associativity defined, or an alist of
246 @w{@code{(@var{assoc-type} . @var{assoc-value})}} elements.
248 @var{assoc-type} must be one of the @code{default-prec},
249 @code{nonassoc}, @code{left} or @code{right} symbols. When
250 @var{assoc-type} is @code{default-prec}, @var{assoc-value} must be
251 @code{nil} or @code{t} (the default). Otherwise it is a list of
252 tokens which must have been previously declared in @var{terminals}.
254 For details, see @ref{Contextual Precedence, , , bison}, in the
258 Is the list of nonterminal definitions. Each definition has the form:
260 @code{(@var{nonterm} . @var{rules})}
262 Where @var{nonterm} is the nonterminal symbol defined and
263 @var{rules} the list of rules that describe this nonterminal. Each
266 @code{(@var{components} [@var{precedence}] [@var{action}])}
272 Is a list of various terminals and nonterminals that are put together
279 (exp ((exp ?+ exp)) ;; exp: exp '+' exp
284 Says that two groupings of type @samp{exp}, with a @samp{+} token in
285 between, can be combined into a larger grouping of type @samp{exp}.
287 @cindex grammar coding conventions
288 By convention, a nonterminal symbol should be in lower case, such as
289 @samp{exp}, @samp{stmt} or @samp{declaration}. Terminal symbols
290 should be upper case to distinguish them from nonterminals: for
291 example, @samp{INTEGER}, @samp{IDENTIFIER}, @samp{IF} or
292 @samp{RETURN}. A terminal symbol that represents a particular keyword
293 in the language is conventionally the same as that keyword converted
294 to upper case. The terminal symbol @code{error} is reserved for error
297 @cindex middle-rule actions
298 Scattered among the components can be @dfn{middle-rule} actions.
299 Usually only @var{action} is provided (@pxref{action}).
301 If @var{components} in a rule is @code{nil}, it means that the rule
302 can match the empty string. For example, here is how to define a
303 comma-separated sequence of zero or more @samp{exp} groupings:
307 (expseq (nil) ;; expseq: ;; empty
308 ((expseq1)) ;; | expseq1
311 (expseq1 ((exp)) ;; expseq1: exp
312 ((expseq1 ?, exp)) ;; | expseq1 ',' exp
317 @cindex precedence level
319 Assign the rule the precedence of the given terminal item, overriding
320 the precedence that would be deduced for it, that is the one of the
321 last terminal in it. Notice that only terminals declared in
322 @var{assocs} have a precedence level. The altered rule precedence
323 then affects how conflicts involving that rule are resolved.
325 @var{precedence} is an optional vector of one terminal item.
327 Here is how @var{precedence} solves the problem of unary minus.
328 First, declare a precedence for a fictitious terminal symbol named
329 @code{UMINUS}. There are no tokens of this type, but the symbol
330 serves to stand for its precedence:
334 ((default-prec t) ;; This is the default
340 Now the precedence of @code{UMINUS} can be used in specific rules:
344 (exp @dots{} ;; exp: @dots{}
345 ((exp ?- exp)) ;; | exp '-' exp
347 ((?- exp) [UMINUS]) ;; | '-' exp %prec UMINUS
353 If you forget to append @code{[UMINUS]} to the rule for unary minus,
354 Wisent silently assumes that minus has its usual precedence. This
355 kind of problem can be tricky to debug, since one typically discovers
356 the mistake only by testing the code.
358 Using @code{(default-prec nil)} declaration makes it easier to
359 discover this kind of problem systematically. It causes rules that
360 lack a @var{precedence} modifier to have no precedence, even if the
361 last terminal symbol mentioned in their components has a declared
364 If @code{(default-prec nil)} is in effect, you must specify
365 @var{precedence} for all rules that participate in precedence conflict
366 resolution. Then you will see any shift/reduce conflict until you
367 tell Wisent how to resolve it, either by changing your grammar or by
368 adding an explicit precedence. This will probably add declarations to
369 the grammar, but it helps to protect against incorrect rule
372 The effect of @code{(default-prec nil)} can be reversed by giving
373 @code{(default-prec t)}, which is the default.
375 For more details, see @ref{Contextual Precedence, , , bison}, in the
378 It is important to understand that @var{assocs} declarations defines
379 associativity but also assign a precedence level to terminals. All
380 terminals declared in the same @code{left}, @code{right} or
381 @code{nonassoc} association get the same precedence level. The
382 precedence level is increased at each new association.
384 On the other hand, @var{precedence} explicitly assign the precedence
385 level of the given terminal to a rule.
387 @cindex semantic actions
388 @item @anchor{action}action
389 An action is an optional Emacs Lisp function call, like this:
393 The result of an action determines the semantic value of a rule.
395 From an implementation standpoint, the function call will be embedded
396 in a lambda expression, and several useful local variables will be
402 Where @var{n} is a positive integer. Like in Bison, the value of
403 @code{$@var{n}} is the semantic value of the @var{n}th element of
404 @var{components}, starting from 1. It can be of any Lisp data
407 @vindex $region@var{n}
409 Where @var{n} is a positive integer. For each @code{$@var{n}}
410 variable defined there is a corresponding @code{$region@var{n}}
411 variable. Its value is a pair @code{(@var{start-pos} .
412 @var{end-pos})} that represent the start and end positions (in the
413 lexical input stream) of the @code{$@var{n}} value. It can be
414 @code{nil} when the component positions are not available, like for an
415 empty string component for example.
419 Its value is the leftmost and rightmost positions of input data
420 matched by all @var{components} in the rule. This is a pair
421 @code{(@var{leftmost-pos} . @var{rightmost-pos})}. It can be
422 @code{nil} when components positions are not available.
426 This variable is initialized with the nonterminal symbol
427 (@var{nonterm}) the rule belongs to. It could be useful to improve
428 error reporting or debugging. It is also used to automatically
429 provide incremental re-parse entry points for @semantic{} tags
430 (@pxref{Wisent Semantic}).
434 The value of @code{$action} is the symbolic name of the current
435 semantic action (@pxref{Debugging actions}).
438 When an action is not specified a default value is supplied, it is
439 @code{(identity $1)}. This means that the default semantic value of a
440 rule is the value of its first component. Excepted for a rule
441 matching the empty string, for which the default action is to return
449 @cindex grammar example
450 Here is an example to parse simple infix arithmetic expressions. See
451 @ref{Infix Calc, , , bison}, in the Bison manual for details.
459 ;; Terminal associativity & precedence
470 (format "%s %s" $1 $2))
484 (string-to-number $1))
506 In the bison-like @dfn{WY} format (@pxref{Wisent Semantic}) the
507 grammar looks like this:
513 %nonassoc '=' ;; comparison
516 %left NEG ;; negation--unary minus
517 %right '^' ;; exponentiation
524 (format "%s %s" $1 $2)
538 (string-to-number $1)
561 @node Compiling a grammar
562 @section Compiling a grammar
565 After providing a context-free grammar in a suitable format, it must
566 be translated into a set of tables (an @dfn{automaton}) that will be
567 used to derive the parser. Like Bison, Wisent translates grammars that
568 must be @dfn{LALR(1)}.
570 @cindex LALR(1) grammar
571 @cindex look-ahead token
572 A grammar is @acronym{LALR(1)} if it is possible to tell how to parse
573 any portion of an input string with just a single token of look-ahead:
574 the @dfn{look-ahead token}. See @ref{Language and Grammar, , ,
575 bison}, in the Bison manual for more information.
577 @cindex grammar compilation
578 Grammar translation (compilation) is achieved by the function:
580 @cindex compiling a grammar
581 @vindex wisent-single-start-flag
582 @findex wisent-compile-grammar
583 @defun wisent-compile-grammar grammar &optional start-list
584 Compile @var{grammar} and return an @acronym{LALR(1)} automaton.
586 Optional argument @var{start-list} is a list of start symbols
587 (nonterminals). If @code{nil} the first nonterminal defined in the
588 grammar is the default start symbol. If @var{start-list} contains
589 only one element, it defines the start symbol. If @var{start-list}
590 contains more than one element, all are defined as potential start
591 symbols, unless @code{wisent-single-start-flag} is non-@code{nil}. In
592 that case the first element of @var{start-list} defines the start
593 symbol and others are ignored.
595 The @acronym{LALR(1)} automaton is a vector of the form:
597 @code{[@var{actions gotos starts functions}]}
601 A state/token matrix telling the parser what to do at every state
602 based on the current look-ahead token. That is shift, reduce, accept
603 or error. See also @ref{Wisent Parsing}.
606 A state/nonterminal matrix telling the parser the next state to go to
607 after reducing with each rule.
610 An alist which maps the allowed start symbols (nonterminals) to
611 lexical tokens that will be first shifted into the parser stack.
614 An obarray of semantic action symbols. A semantic action is actually
615 an Emacs Lisp function (lambda expression).
622 Normally, a grammar should produce an automaton where at each state
623 the parser has only one action to do (@pxref{Wisent Parsing}).
625 @cindex ambiguous grammar
626 In certain cases, a grammar can produce an automaton where, at some
627 states, there are more than one action possible. Such a grammar is
628 @dfn{ambiguous}, and generates @dfn{conflicts}.
630 @cindex deterministic automaton
631 The parser can't be driven by an automaton which isn't completely
632 @dfn{deterministic}, that is which contains conflicts. It is
633 necessary to resolve the conflicts to eliminate them. Wisent resolves
634 conflicts like Bison does.
636 @cindex grammar conflicts
637 @cindex conflicts resolution
638 There are two sorts of conflicts:
641 @cindex shift/reduce conflicts
642 @item shift/reduce conflicts
643 When either a shift or a reduction would be valid at the same state.
645 Such conflicts are resolved by choosing to shift, unless otherwise
646 directed by operator precedence declarations.
647 See @ref{Shift/Reduce , , , bison}, in the Bison manual for more
650 @cindex reduce/reduce conflicts
651 @item reduce/reduce conflicts
652 That occurs if there are two or more rules that apply to the same
653 sequence of input. This usually indicates a serious error in the
656 Such conflicts are resolved by choosing to use the rule that appears
657 first in the grammar, but it is very risky to rely on this. Every
658 reduce/reduce conflict must be studied and usually eliminated. See
659 @ref{Reduce/Reduce , , , bison}, in the Bison manual for more
664 * Grammar Debugging::
665 * Understanding the automaton::
668 @node Grammar Debugging
669 @subsection Grammar debugging
671 @cindex grammar debugging
672 @cindex grammar verbose description
673 To help writing a new grammar, @code{wisent-compile-grammar} can
674 produce a verbose report containing a detailed description of the
675 grammar and parser (equivalent to what Bison reports with the
676 @option{--verbose} option).
678 To enable the verbose report you can set to non-@code{nil} the
681 @vindex wisent-verbose-flag
682 @deffn Option wisent-verbose-flag
683 non-@code{nil} means to report verbose information on generated parser.
686 Or interactively use the command:
688 @findex wisent-toggle-verbose-flag
689 @deffn Command wisent-toggle-verbose-flag
690 Toggle whether to report verbose information on generated parser.
693 The verbose report is printed in the temporary buffer
694 @code{*wisent-log*} when running interactively, or in file
695 @file{wisent.output} when running in batch mode. Different
696 reports are separated from each other by a line like this:
700 *** Wisent @var{source-file} - 2002-06-27 17:33
704 where @var{source-file} is the name of the Emacs Lisp file from which
705 the grammar was read. See @ref{Understanding the automaton}, for
706 details on the verbose report.
710 To help debugging the grammar compiler itself, you can set this
711 variable to print the content of some internal data structures:
713 @vindex wisent-debug-flag
714 @defvar wisent-debug-flag
715 non-@code{nil} means enable some debug stuff.
719 @node Understanding the automaton
720 @subsection Understanding the automaton
722 @cindex understanding the automaton
723 This section (took from the manual of Bison 1.49) describes how to use
724 the verbose report printed by @code{wisent-compile-grammar} to
725 understand the generated automaton, to tune or fix a grammar.
727 We will use the following example:
731 (let ((wisent-verbose-flag t)) ;; Print a verbose report!
732 (wisent-compile-grammar
733 '((NUM STR) ; %token NUM STR
735 ((left ?+ ?-) ; %left '+' '-';
736 (left ?*)) ; %left '*'
739 ((exp ?+ exp)) ; exp '+' exp
740 ((exp ?- exp)) ; | exp '-' exp
741 ((exp ?* exp)) ; | exp '*' exp
742 ((exp ?/ exp)) ; | exp '/' exp
750 'nil) ; no %start declarations
755 When evaluating the above expression, grammar compilation first issues
756 the following two clear messages:
760 Grammar contains 1 useless nonterminals and 1 useless rules
761 Grammar contains 7 shift/reduce conflicts
765 The @samp{*wisent-log*} buffer details things!
767 The first section reports conflicts that were solved using precedence
768 and/or associativity:
772 Conflict in state 7 between rule 1 and token '+' resolved as reduce.
773 Conflict in state 7 between rule 1 and token '-' resolved as reduce.
774 Conflict in state 7 between rule 1 and token '*' resolved as shift.
775 Conflict in state 8 between rule 2 and token '+' resolved as reduce.
776 Conflict in state 8 between rule 2 and token '-' resolved as reduce.
777 Conflict in state 8 between rule 2 and token '*' resolved as shift.
778 Conflict in state 9 between rule 3 and token '+' resolved as reduce.
779 Conflict in state 9 between rule 3 and token '-' resolved as reduce.
780 Conflict in state 9 between rule 3 and token '*' resolved as reduce.
784 The next section reports useless tokens, nonterminal and rules (note
785 that useless tokens might be used by the scanner):
789 Useless nonterminals:
794 Terminals which are not used:
805 The next section lists states that still have conflicts:
809 State 7 contains 1 shift/reduce conflict.
810 State 8 contains 1 shift/reduce conflict.
811 State 9 contains 1 shift/reduce conflict.
812 State 10 contains 4 shift/reduce conflicts.
816 The next section reproduces the grammar used:
831 And reports the uses of the symbols:
835 Terminals, with rules where they appear
847 Nonterminals, with rules where they appear
850 on left: 1 2 3 4 5, on right: 1 2 3 4
854 The report then details the automaton itself, describing each state
855 with it set of @dfn{items}, also known as @dfn{pointed rules}. Each
856 item is a production rule together with a point (marked by @samp{.})
857 that the input cursor.
863 NUM shift, and go to state 1
869 State 0 corresponds to being at the very beginning of the parsing, in
870 the initial rule, right before the start symbol (@samp{exp}). When
871 the parser returns to this state right after having reduced a rule
872 that produced an @samp{exp}, it jumps to state 2. If there is no such
873 transition on a nonterminal symbol, and the lookahead is a @samp{NUM},
874 then this token is shifted on the parse stack, and the control flow
875 jumps to state 1. Any other lookahead triggers a parse error.
883 exp -> NUM . (rule 5)
885 $default reduce using rule 5 (exp)
889 the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead
890 (@samp{$default}), the parser will reduce it. If it was coming from
891 state 0, then, after this reduction it will return to state 0, and
892 will jump to state 2 (@samp{exp: go to state 2}).
898 exp -> exp . '+' exp (rule 1)
899 exp -> exp . '-' exp (rule 2)
900 exp -> exp . '*' exp (rule 3)
901 exp -> exp . '/' exp (rule 4)
903 $EOI shift, and go to state 11
904 '+' shift, and go to state 3
905 '-' shift, and go to state 4
906 '*' shift, and go to state 5
907 '/' shift, and go to state 6
911 In state 2, the automaton can only shift a symbol. For instance,
912 because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
913 @samp{+}, it will be shifted on the parse stack, and the automaton
914 control will jump to state 3, corresponding to the item
915 @samp{exp -> exp . '+' exp}:
921 exp -> exp '+' . exp (rule 1)
923 NUM shift, and go to state 1
929 Since there is no default action, any other token than those listed
930 above will trigger a parse error.
932 The interpretation of states 4 to 6 is straightforward:
938 exp -> exp '-' . exp (rule 2)
940 NUM shift, and go to state 1
948 exp -> exp '*' . exp (rule 3)
950 NUM shift, and go to state 1
958 exp -> exp '/' . exp (rule 4)
960 NUM shift, and go to state 1
966 As was announced in beginning of the report, @samp{State 7 contains 1
967 shift/reduce conflict.}:
973 exp -> exp . '+' exp (rule 1)
974 exp -> exp '+' exp . (rule 1)
975 exp -> exp . '-' exp (rule 2)
976 exp -> exp . '*' exp (rule 3)
977 exp -> exp . '/' exp (rule 4)
979 '*' shift, and go to state 5
980 '/' shift, and go to state 6
982 '/' [reduce using rule 1 (exp)]
983 $default reduce using rule 1 (exp)
987 Indeed, there are two actions associated to the lookahead @samp{/}:
988 either shifting (and going to state 6), or reducing rule 1. The
989 conflict means that either the grammar is ambiguous, or the parser
990 lacks information to make the right decision. Indeed the grammar is
991 ambiguous, as, since we did not specify the precedence of @samp{/},
992 the sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM
993 / NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM +
994 NUM) / NUM}, which corresponds to reducing rule 1.
996 Because in @acronym{LALR(1)} parsing a single decision can be made,
997 Wisent arbitrarily chose to disable the reduction, see
998 @ref{Conflicts}. Discarded actions are reported in between square
1001 Note that all the previous states had a single possible action: either
1002 shifting the next token and going to the corresponding state, or
1003 reducing a single rule. In the other cases, i.e., when shifting
1004 @emph{and} reducing is possible or when @emph{several} reductions are
1005 possible, the lookahead is required to select the action. State 7 is
1006 one such state: if the lookahead is @samp{*} or @samp{/} then the
1007 action is shifting, otherwise the action is reducing rule 1. In other
1008 words, the first two items, corresponding to rule 1, are not eligible
1009 when the lookahead is @samp{*}, since we specified that @samp{*} has
1010 higher precedence that @samp{+}. More generally, some items are
1011 eligible only with some set of possible lookaheads.
1013 States 8 to 10 are similar:
1019 exp -> exp . '+' exp (rule 1)
1020 exp -> exp . '-' exp (rule 2)
1021 exp -> exp '-' exp . (rule 2)
1022 exp -> exp . '*' exp (rule 3)
1023 exp -> exp . '/' exp (rule 4)
1025 '*' shift, and go to state 5
1026 '/' shift, and go to state 6
1028 '/' [reduce using rule 2 (exp)]
1029 $default reduce using rule 2 (exp)
1035 exp -> exp . '+' exp (rule 1)
1036 exp -> exp . '-' exp (rule 2)
1037 exp -> exp . '*' exp (rule 3)
1038 exp -> exp '*' exp . (rule 3)
1039 exp -> exp . '/' exp (rule 4)
1041 '/' shift, and go to state 6
1043 '/' [reduce using rule 3 (exp)]
1044 $default reduce using rule 3 (exp)
1050 exp -> exp . '+' exp (rule 1)
1051 exp -> exp . '-' exp (rule 2)
1052 exp -> exp . '*' exp (rule 3)
1053 exp -> exp . '/' exp (rule 4)
1054 exp -> exp '/' exp . (rule 4)
1056 '+' shift, and go to state 3
1057 '-' shift, and go to state 4
1058 '*' shift, and go to state 5
1059 '/' shift, and go to state 6
1061 '+' [reduce using rule 4 (exp)]
1062 '-' [reduce using rule 4 (exp)]
1063 '*' [reduce using rule 4 (exp)]
1064 '/' [reduce using rule 4 (exp)]
1065 $default reduce using rule 4 (exp)
1069 Observe that state 10 contains conflicts due to the lack of precedence
1070 of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the
1071 associativity of @samp{/} is not specified.
1073 Finally, the state 11 (plus 12) is named the @dfn{final state}, or the
1074 @dfn{accepting state}:
1080 $EOI shift, and go to state 12
1090 The end of input is shifted @samp{$EOI shift,} and the parser exits
1091 successfully (@samp{go to state 12}, that terminates).
1093 @node Wisent Parsing
1094 @chapter Wisent Parsing
1096 @cindex bottom-up parser
1097 @cindex shift-reduce parser
1098 The Wisent's parser is what is called a @dfn{bottom-up} or
1099 @dfn{shift-reduce} parser which repeatedly:
1104 That is pushes the value of the last lexical token read (the
1105 look-ahead token) into a value stack, and reads a new one.
1109 That is replaces a nonterminal by its semantic value. The values of
1110 the components which form the right hand side of a rule are popped
1111 from the value stack and reduced by the semantic action of this rule.
1112 The result is pushed back on top of value stack.
1115 The parser will stop on:
1120 When all input has been successfully parsed. The semantic value of
1121 the start nonterminal is on top of the value stack.
1123 @cindex syntax error
1125 When a syntax error (an unexpected token in input) has been detected.
1126 At this point the parser issues an error message and either stops or
1127 calls a recovery routine to try to resume parsing.
1130 @cindex table-driven parser
1131 The above elementary actions are driven by the @acronym{LALR(1)}
1132 automaton built by @code{wisent-compile-grammar} from a context-free
1135 The Wisent's parser is entered by calling the function:
1137 @findex wisent-parse
1138 @defun wisent-parse automaton lexer &optional error start
1139 Parse input using the automaton specified in @var{automaton}.
1143 Is an @acronym{LALR(1)} automaton generated by
1144 @code{wisent-compile-grammar} (@pxref{Wisent Grammar}).
1147 Is a function with no argument called by the parser to obtain the next
1148 terminal (token) in input (@pxref{Writing a lexer}).
1151 Is an optional reporting function called when a parse error occurs.
1152 It receives a message string to report. It defaults to the function
1153 @code{wisent-message} (@pxref{Report errors}).
1156 Specify the start symbol (nonterminal) used by the parser as its goal.
1157 It defaults to the start symbol defined in the grammar
1158 (@pxref{Wisent Grammar}).
1162 The following two normal hooks permit to do some useful processing
1163 respectively before to start parsing, and after the parser terminated.
1165 @vindex wisent-pre-parse-hook
1166 @defvar wisent-pre-parse-hook
1167 Normal hook run just before entering the @var{LR} parser engine.
1170 @vindex wisent-post-parse-hook
1171 @defvar wisent-post-parse-hook
1172 Normal hook run just after the @var{LR} parser engine terminated.
1180 * Debugging actions::
1183 @node Writing a lexer
1184 @section What the parser must receive
1186 It is important to understand that the parser does not parse
1187 characters, but lexical tokens, and does not know anything about
1188 characters in text streams!
1190 @cindex lexical analysis
1193 Reading input data to produce lexical tokens is performed by a lexer
1194 (also called a scanner) in a lexical analysis step, before the syntax
1195 analysis step performed by the parser. The parser automatically calls
1196 the lexer when it needs the next token to parse.
1198 @cindex lexical tokens
1199 A Wisent's lexer is an Emacs Lisp function with no argument. It must
1200 return a valid lexical token of the form:
1202 @code{(@var{token-class value} [@var{start} . @var{end}])}
1206 Is a category of lexical token identifying a terminal as specified in
1207 the grammar (@pxref{Wisent Grammar}). It can be a symbol or a character
1211 Is the value of the lexical token. It can be of any valid Emacs Lisp
1216 Are the optional beginning and ending positions of @var{value} in the
1220 When there are no more tokens to read the lexer must return the token
1221 @code{(list wisent-eoi-term)} to each request.
1223 @vindex wisent-eoi-term
1224 @defvar wisent-eoi-term
1225 Predefined constant, End-Of-Input terminal symbol.
1228 @code{wisent-lex} is an example of a lexer that reads lexical tokens
1229 produced by a @semantic{} lexer, and translates them into lexical tokens
1230 suitable to the Wisent parser. See also @ref{Wisent Lex}.
1232 To call the lexer in a semantic action use the function
1233 @code{wisent-lexer}. See also @ref{Actions goodies}.
1235 @node Actions goodies
1236 @section Variables and macros useful in grammar actions.
1238 @vindex wisent-input
1239 @defvar wisent-input
1240 The last token read.
1241 This variable only has meaning in the scope of @code{wisent-parse}.
1244 @findex wisent-lexer
1246 Obtain the next terminal in input.
1249 @findex wisent-region
1250 @defun wisent-region &rest positions
1251 Return the start/end positions of the region including
1252 @var{positions}. Each element of @var{positions} is a pair
1253 @w{@code{(@var{start-pos} . @var{end-pos})}} or @code{nil}. The
1254 returned value is the pair @w{@code{(@var{min-start-pos} .
1255 @var{max-end-pos})}} or @code{nil} if no @var{positions} are
1260 @section The error reporting function
1262 @cindex error reporting
1263 When the parser encounters a syntax error it calls a user-defined
1264 function. It must be an Emacs Lisp function with one argument: a
1265 string containing the message to report.
1267 By default the parser uses this function to report error messages:
1269 @findex wisent-message
1270 @defun wisent-message string &rest args
1271 Print a one-line message if @code{wisent-parse-verbose-flag} is set.
1272 Pass @var{string} and @var{args} arguments to @dfn{message}.
1277 @code{wisent-message} uses the following function to print lexical
1280 @defun wisent-token-to-string token
1281 Return a printed representation of lexical token @var{token}.
1284 The general printed form of a lexical token is:
1286 @w{@code{@var{token}(@var{value})@@@var{location}}}
1289 To control the verbosity of the parser you can set to non-@code{nil}
1292 @vindex wisent-parse-verbose-flag
1293 @deffn Option wisent-parse-verbose-flag
1294 non-@code{nil} means to issue more messages while parsing.
1297 Or interactively use the command:
1299 @findex wisent-parse-toggle-verbose-flag
1300 @deffn Command wisent-parse-toggle-verbose-flag
1301 Toggle whether to issue more messages while parsing.
1304 When the error reporting function is entered the variable
1305 @code{wisent-input} contains the unexpected token as returned by the
1308 The error reporting function can be called from a semantic action too
1309 using the special macro @code{wisent-error}. When called from a
1310 semantic action entered by error recovery (@pxref{Error recovery}) the
1311 value of the variable @code{wisent-recovering} is non-@code{nil}.
1313 @node Error recovery
1314 @section Error recovery
1316 @cindex error recovery
1317 The error recovery mechanism of the Wisent's parser conforms to the
1318 one Bison uses. See @ref{Error Recovery, , , bison}, in the Bison
1322 To recover from a syntax error you must write rules to recognize the
1323 special token @code{error}. This is a terminal symbol that is
1324 automatically defined and reserved for error handling.
1326 When the parser encounters a syntax error, it pops the state stack
1327 until it finds a state that allows shifting the @code{error} token.
1328 After it has been shifted, if the old look-ahead token is not
1329 acceptable to be shifted next, the parser reads tokens and discards
1330 them until it finds a token which is acceptable.
1332 @cindex error recovery strategy
1333 Strategies for error recovery depend on the choice of error rules in
1334 the grammar. A simple and useful strategy is simply to skip the rest
1335 of the current statement if an error is detected:
1339 (statement (( error ?; )) ;; on error, skip until ';' is read
1344 It is also useful to recover to the matching close-delimiter of an
1345 opening-delimiter that has already been parsed:
1349 (primary (( ?@{ expr ?@} ))
1356 @cindex error recovery actions
1357 Note that error recovery rules may have actions, just as any other
1358 rules can. Here are some predefined hooks, variables, functions or
1359 macros, useful in such actions:
1361 @vindex wisent-nerrs
1362 @defvar wisent-nerrs
1363 The number of parse errors encountered so far.
1366 @vindex wisent-recovering
1367 @defvar wisent-recovering
1368 non-@code{nil} means that the parser is recovering.
1369 This variable only has meaning in the scope of @code{wisent-parse}.
1372 @findex wisent-error
1373 @defun wisent-error msg
1374 Call the user supplied error reporting function with message
1375 @var{msg} (@pxref{Report errors}).
1377 For an example of use, @xref{wisent-skip-token}.
1380 @findex wisent-errok
1382 Resume generating error messages immediately for subsequent syntax
1385 The parser suppress error message for syntax errors that happens
1386 shortly after the first, until three consecutive input tokens have
1387 been successfully shifted.
1389 Calling @code{wisent-errok} in an action, make error messages resume
1390 immediately. No error messages will be suppressed if you call it in
1391 an error rule's action.
1393 For an example of use, @xref{wisent-skip-token}.
1396 @findex wisent-clearin
1397 @defun wisent-clearin
1398 Discard the current lookahead token.
1399 This will cause a new lexical token to be read.
1401 In an error rule's action the previous lookahead token is reanalyzed
1402 immediately. @code{wisent-clearin} may be called to clear this token.
1404 For example, suppose that on a parse error, an error handling routine
1405 is called that advances the input stream to some point where parsing
1406 should once again commence. The next symbol returned by the lexical
1407 scanner is probably correct. The previous lookahead token ought to
1408 be discarded with @code{wisent-clearin}.
1410 For an example of use, @xref{wisent-skip-token}.
1413 @findex wisent-abort
1415 Abort parsing and save the lookahead token.
1418 @findex wisent-set-region
1419 @defun wisent-set-region start end
1420 Change the region of text matched by the current nonterminal.
1421 @var{start} and @var{end} are respectively the beginning and end
1422 positions of the region occupied by the group of components associated
1423 to this nonterminal. If @var{start} or @var{end} values are not a
1424 valid positions the region is set to @code{nil}.
1426 For an example of use, @xref{wisent-skip-token}.
1429 @vindex wisent-discarding-token-functions
1430 @defvar wisent-discarding-token-functions
1431 List of functions to be called when discarding a lexical token.
1432 These functions receive the lexical token discarded.
1433 When the parser encounters unexpected tokens, it can discards them,
1434 based on what directed by error recovery rules. Either when the
1435 parser reads tokens until one is found that can be shifted, or when an
1436 semantic action calls the function @code{wisent-skip-token} or
1437 @code{wisent-skip-block}.
1438 For language specific hooks, make sure you define this as a local
1441 For example, in @semantic{}, this hook is set to the function
1442 @code{wisent-collect-unmatched-syntax} to collect unmatched lexical
1443 tokens (@pxref{Useful functions}).
1446 @findex wisent-skip-token
1447 @defun wisent-skip-token
1448 @anchor{wisent-skip-token}
1449 Skip the lookahead token in order to resume parsing.
1451 Must be used in error recovery semantic actions.
1453 It typically looks like this:
1457 (wisent-message "%s: skip %s" $action
1458 (wisent-token-to-string wisent-input))
1460 'wisent-discarding-token-functions wisent-input)
1467 @findex wisent-skip-block
1468 @defun wisent-skip-block
1469 Safely skip a block in order to resume parsing.
1471 Must be used in error recovery semantic actions.
1473 A block is data between an open-delimiter (syntax class @code{(}) and
1474 a matching close-delimiter (syntax class @code{)}):
1478 (a parenthesized block)
1479 [a block between brackets]
1480 @{a block between braces@}
1484 The following example uses @code{wisent-skip-block} to safely skip a
1485 block delimited by @samp{LBRACE} (@code{@{}) and @samp{RBRACE}
1486 (@code{@}}) tokens, when a syntax error occurs in
1487 @samp{other-components}:
1491 (block ((LBRACE other-components RBRACE))
1494 (wisent-skip-block))
1500 @node Debugging actions
1501 @section Debugging semantic actions
1503 @cindex semantic action symbols
1504 Each semantic action is represented by a symbol interned in an
1505 @dfn{obarray} that is part of the @acronym{LALR(1)} automaton
1506 (@pxref{Compiling a grammar}). @code{symbol-function} on a semantic
1507 action symbol return the semantic action lambda expression.
1509 A semantic action symbol name has the form
1510 @code{@var{nonterminal}:@var{index}}, where @var{nonterminal} is the
1511 name of the nonterminal symbol the action belongs to, and @var{index}
1512 is an action sequence number within the scope of @var{nonterminal}.
1513 For example, this nonterminal definition:
1518 line [@code{input:0}]
1520 (format "%s %s" $1 $2) [@code{input:1}]
1525 Will produce two semantic actions, and associated symbols:
1529 A default action that returns @code{$1}.
1532 That returns @code{(format "%s %s" $1 $2)}.
1535 @cindex debugging semantic actions
1536 Debugging uses the Lisp debugger to investigate what is happening
1537 during execution of semantic actions.
1538 Three commands are available to debug semantic actions. They receive
1542 @item The automaton that contains the semantic action.
1544 @item The semantic action symbol.
1547 @findex wisent-debug-on-entry
1548 @deffn Command wisent-debug-on-entry automaton function
1549 Request @var{automaton}'s @var{function} to invoke debugger each time it is called.
1550 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1553 @findex wisent-cancel-debug-on-entry
1554 @deffn Command wisent-cancel-debug-on-entry automaton function
1555 Undo effect of @code{wisent-debug-on-entry} on @var{automaton}'s @var{function}.
1556 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1559 @findex wisent-debug-show-entry
1560 @deffn Command wisent-debug-show-entry automaton function
1561 Show the source of @var{automaton}'s semantic action @var{function}.
1562 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1565 @node Wisent Semantic
1566 @chapter How to use Wisent with Semantic
1569 This section presents how the Wisent's parser can be used to produce
1570 @dfn{tags} for the @semantic{} tool set.
1572 @semantic{} tags form a hierarchy of Emacs Lisp data structures that
1573 describes a program in a way independent of programming languages.
1574 Tags map program declarations, like functions, methods, variables,
1575 data types, classes, includes, grammar rules, etc..
1577 @cindex WY grammar format
1578 To use the Wisent parser with @semantic{} you have to define
1579 your grammar in @dfn{WY} form, a grammar format very close
1580 to the one used by Bison.
1582 Please @inforef{top, Semantic Grammar Framework Manual, grammar-fw}
1583 for more information on @semantic{} grammars.
1590 @node Grammar styles
1591 @section Grammar styles
1593 @cindex grammar styles
1594 @semantic{} parsing heavily depends on how you wrote the grammar.
1595 There are mainly two styles to write a Wisent's grammar intended to be
1596 used with the @semantic{} tool set: the @dfn{Iterative style} and the
1597 @dfn{Bison style}. Each one has pros and cons, and in certain cases
1598 it can be worth a mix of the two styles!
1604 * Start nonterminals::
1605 * Useful functions::
1608 @node Iterative style
1609 @subsection Iterative style
1611 @cindex grammar iterative style
1612 The @dfn{iterative style} is the preferred style to use with @semantic{}.
1613 It relies on an iterative parser back-end mechanism which parses start
1614 nonterminals one at a time and automagically skips unexpected lexical
1617 Compared to rule-based iterative functions (@pxref{Bison style}),
1618 iterative parsers are better in that they can handle obscure errors
1622 Each start nonterminal must produces a @dfn{raw tag} by calling a
1623 @code{TAG}-like grammar macro with appropriate parameters. See also
1624 @ref{Start nonterminals}.
1626 @cindex expanded tag
1627 Then, each parsing iteration automatically translates a raw tag into
1628 @dfn{expanded tags}, updating the raw tag structure with internal
1629 properties and buffer related data.
1631 After parsing completes, it results in a tree of expanded tags.
1633 The following example is a snippet of the iterative style Java grammar
1634 provided in the @semantic{} distribution in the file
1635 @file{semantic/wisent/java-tags.wy}.
1640 ;; Alternate entry points
1641 ;; - Needed by partial re-parse
1642 %start formal_parameter
1644 ;; - Needed by EXPANDFULL clauses
1645 %start formal_parameters
1648 formal_parameter_list
1650 (EXPANDFULL $1 formal_parameters)
1658 | formal_parameter COMMA
1659 | formal_parameter RPAREN
1663 : formal_parameter_modifier_opt type variable_declarator_id
1664 (VARIABLE-TAG $3 $2 nil :typemodifiers $1)
1670 It shows the use of the @code{EXPANDFULL} grammar macro to parse a
1671 @samp{PAREN_BLOCK} which contains a @samp{formal_parameter_list}.
1672 @code{EXPANDFULL} tells to recursively parse @samp{formal_parameters}
1673 inside @samp{PAREN_BLOCK}. The parser iterates until it digested all
1674 available input data inside the @samp{PAREN_BLOCK}, trying to match
1675 any of the @samp{formal_parameters} rules:
1682 @item @samp{formal_parameter COMMA}
1684 @item @samp{formal_parameter RPAREN}
1687 At each iteration it will return a @samp{formal_parameter} raw tag,
1688 or @code{nil} to skip unwanted (single @samp{LPAREN} or @samp{RPAREN}
1689 for example) or unexpected input data. Those raw tags will be
1690 automatically expanded by the iterative back-end parser.
1693 @subsection Bison style
1695 @cindex grammar bison style
1696 What we call the @dfn{Bison style} is the traditional style of Bison's
1697 grammars. Compared to iterative style, it is not straightforward to
1698 use grammars written in Bison style in @semantic{}. Mainly because such
1699 grammars are designed to parse the whole input data in one pass, and
1700 don't use the iterative parser back-end mechanism (@pxref{Iterative
1701 style}). With Bison style the parser is called once to parse the
1702 grammar start nonterminal.
1704 The following example is a snippet of the Bison style Java grammar
1705 provided in the @semantic{} distribution in the file
1706 @file{semantic/wisent/java.wy}.
1710 %start formal_parameter
1713 formal_parameter_list
1714 : formal_parameter_list COMMA formal_parameter
1721 : formal_parameter_modifier_opt type variable_declarator_id
1723 (VARIABLE-TAG $3 $2 :typemodifiers $1)
1729 The first consequence is that syntax errors are not automatically
1730 handled by @semantic{}. Thus, it is necessary to explicitly handle
1731 them at the grammar level, providing error recovery rules to skip
1732 unexpected input data.
1734 The second consequence is that the iterative parser can't do automatic
1735 tag expansion, except for the start nonterminal value. It is
1736 necessary to explicitly expand tags from concerned semantic actions by
1737 calling the grammar macro @code{EXPANDTAG} with a raw tag as
1738 parameter. See also @ref{Start nonterminals}, for incremental
1739 re-parse considerations.
1742 @subsection Mixed style
1744 @cindex grammar mixed style
1749 %start prologue epilogue declaration nonterminal rule
1764 SYMBOL COLON rules SEMI
1765 (TAG $1 'nonterminal :children $3)
1770 (apply 'nconc (nreverse $1))
1783 name type comps prec action elt)
1786 (TAG name 'rule :type type :value comps :prec prec :expr action)
1792 This example shows how iterative and Bison styles can be combined in
1793 the same grammar to obtain a good compromise between grammar
1794 complexity and an efficient parsing strategy in an interactive
1797 @samp{nonterminal} is parsed using iterative style via the main
1798 @samp{grammar} rule. The semantic action uses the @code{TAG} macro to
1799 produce a raw tag, automagically expanded by @semantic{}.
1801 But @samp{rules} part is parsed in Bison style! Why?
1803 Rule delimiters are the colon (@code{:}), that follows the nonterminal
1804 name, and a final semicolon (@code{;}). Unfortunately these
1805 delimiters are not @code{open-paren}/@code{close-paren} type, and the
1806 Emacs' syntactic analyzer can't easily isolate data between them to
1807 produce a @samp{RULES_PART} parenthesis-block-like lexical token.
1808 Consequently it is not possible to use @code{EXPANDFULL} to iterate in
1809 @samp{RULES_PART}, like this:
1814 SYMBOL COLON rules SEMI
1815 (TAG $1 'nonterminal :children $3)
1819 RULES_PART ;; @strong{Map a parenthesis-block-like lexical token}
1820 (EXPANDFULL $1 'rules)
1833 name type comps prec action elt)
1835 (TAG name 'rule :type type :value comps :prec prec :expr action)
1841 In such cases, when it is difficult for Emacs to obtain
1842 parenthesis-block-like lexical tokens, the best solution is to use the
1843 traditional Bison style with error recovery!
1845 In some extreme cases, it can also be convenient to extend the lexer,
1846 to deliver new lexical tokens, to simplify the grammar.
1848 @node Start nonterminals
1849 @subsection Start nonterminals
1851 @cindex start nonterminals
1852 @cindex @code{reparse-symbol} property
1853 When you write a grammar for @semantic{}, it is important to carefully
1854 indicate the start nonterminals. Each one defines an entry point in
1855 the grammar, and after parsing its semantic value is returned to the
1856 back-end iterative engine. Consequently:
1858 @strong{The semantic value of a start nonterminal must be a produced
1859 by a TAG like grammar macro}.
1861 Start nonterminals are declared by @code{%start} statements. When
1862 nothing is specified the first nonterminal that appears in the grammar
1863 is the start nonterminal.
1865 Generally, the following nonterminals must be declared as start
1869 @item The main grammar entry point
1874 @item nonterminals passed to @code{EXPAND}/@code{EXPANDFULL}
1876 These grammar macros recursively parse a part of input data, based on
1877 rules of the given nonterminal.
1879 For example, the following will parse @samp{PAREN_BLOCK} data using
1880 the @samp{formal_parameters} rules:
1884 formal_parameter_list
1886 (EXPANDFULL $1 formal_parameters)
1891 The semantic value of @samp{formal_parameters} becomes the value of
1892 the @code{EXPANDFULL} expression. It is a list of @semantic{} tags
1893 spliced in the tags tree.
1895 Because the automaton must know that @samp{formal_parameters} is a
1896 start symbol, you must declare it like this:
1900 %start formal_parameters
1906 @cindex incremental re-parse
1907 @cindex reparse-symbol
1908 The @code{EXPANDFULL} macro has a side effect it is important to know,
1909 related to the incremental re-parse mechanism of @semantic{}: the
1910 nonterminal symbol parameter passed to @code{EXPANDFULL} also becomes
1911 the @code{reparse-symbol} property of the tag returned by the
1912 @code{EXPANDFULL} expression.
1914 When buffer's data mapped by a tag is modified, @semantic{}
1915 schedules an incremental re-parse of that data, using the tag's
1916 @code{reparse-symbol} property as start nonterminal.
1918 @strong{The rules associated to such start symbols must be carefully
1919 reviewed to ensure that the incremental parser will work!}
1921 Things are a little bit different when the grammar is written in Bison
1924 @strong{The @code{reparse-symbol} property is set to the nonterminal
1925 symbol the rule that explicitly uses @code{EXPANDTAG} belongs to.}
1934 name type comps prec action elt)
1937 (TAG name 'rule :type type :value comps :prec prec :expr action)
1943 Set the @code{reparse-symbol} property of the expanded tag to
1944 @samp{rule}. A important consequence is that:
1946 @strong{Every nonterminal having any rule that calls @code{EXPANDTAG}
1947 in a semantic action, should be declared as a start symbol!}
1949 @node Useful functions
1950 @subsection Useful functions
1952 Here is a description of some predefined functions it might be useful
1953 to know when writing new code to use Wisent in @semantic{}:
1955 @findex wisent-collect-unmatched-syntax
1956 @defun wisent-collect-unmatched-syntax input
1957 Add @var{input} lexical token to the cache of unmatched tokens, in
1958 variable @code{semantic-unmatched-syntax-cache}.
1960 See implementation of the function @code{wisent-skip-token} in
1961 @ref{Error recovery}, for an example of use.
1965 @section The Wisent Lex lexer
1967 @findex semantic-lex
1968 The lexical analysis step of @semantic{} is performed by the general
1969 function @code{semantic-lex}. For more information, @inforef{Writing
1970 Lexers, ,semantic-langdev}.
1972 @code{semantic-lex} produces lexical tokens of the form:
1976 @code{(@var{token-class start} . @var{end})}
1982 Is a symbol that identifies a lexical token class, like @code{symbol},
1983 @code{string}, @code{number}, or @code{PAREN_BLOCK}.
1987 Are the start and end positions of mapped data in the input buffer.
1990 The Wisent's parser doesn't depend on the nature of analyzed input
1991 stream (buffer, string, etc.), and requires that lexical tokens have a
1992 different form (@pxref{Writing a lexer}):
1996 @code{(@var{token-class value} [@var{start} . @var{end}])}
2000 @cindex lexical token mapping
2001 @code{wisent-lex} is the default Wisent's lexer used in @semantic{}.
2003 @vindex wisent-lex-istream
2006 Return the next available lexical token in Wisent's form.
2008 The variable @code{wisent-lex-istream} contains the list of lexical
2009 tokens produced by @code{semantic-lex}. Pop the next token available
2010 and convert it to a form suitable for the Wisent's parser.
2013 Mapping of lexical tokens as produced by @code{semantic-lex} into
2014 equivalent Wisent lexical tokens is straightforward:
2018 (@var{token-class start} . @var{end})
2019 @result{} (@var{token-class value start} . @var{end})
2023 @var{value} is the input @code{buffer-substring} from @var{start} to
2026 @node GNU Free Documentation License
2027 @appendix GNU Free Documentation License
2029 @include doclicense.texi
2042 @c Following comments are for the benefit of ispell.
2044 @c LocalWords: Wisent automagically wisent Wisent's LALR obarray