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
23 @documentencoding UTF-8
27 Copyright @copyright{} 1988--1993, 1995, 1998--2004, 2007, 2012--2015
28 Free Software Foundation, Inc.
30 @c Since we are both GNU manuals, we do not need to ack each other here.
32 Some texts are borrowed or adapted from the manual of Bison version
33 1.35. The text in section entitled ``Understanding the automaton'' is
34 adapted from the section ``Understanding Your Parser'' in the manual
35 of Bison version 1.49.
39 Permission is granted to copy, distribute and/or modify this document
40 under the terms of the GNU Free Documentation License, Version 1.3 or
41 any later version published by the Free Software Foundation; with no
42 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
43 and with the Back-Cover Texts as in (a) below. A copy of the license
44 is included in the section entitled ``GNU Free Documentation License''.
46 (a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
47 modify this GNU manual.''
51 @dircategory Emacs misc features
53 * Wisent: (wisent). Semantic Wisent parser development.
60 @c @setchapternewpage odd
61 @c @setchapternewpage off
66 @author by @value{AUTHOR}
68 @vskip 0pt plus 1 fill
77 @c *************************************************************************
79 @c *************************************************************************
85 Wisent (the European Bison ;-) is an Emacs Lisp implementation of the
86 GNU Compiler Compiler Bison.
88 This manual describes how to use Wisent to develop grammars for
89 programming languages, and how to use grammars to parse language
90 source in Emacs buffers.
92 It also describes how Wisent is used with the @semantic{} tool set
93 described in the @ref{Top, Semantic Manual, Semantic Manual, semantic}.
104 * GNU Free Documentation License::
108 @node Wisent Overview
109 @chapter Wisent Overview
111 @dfn{Wisent} (the European Bison) is an implementation in Emacs Lisp
112 of the GNU Compiler Compiler Bison. Its code is a port of the C code
113 of GNU Bison 1.28 & 1.31.
115 For more details on the basic concepts for understanding Wisent, it is
116 worthwhile to read the @ref{Top, Bison Manual, , bison}.
118 Wisent can generate compilers compatible with the @semantic{} tool set.
119 See the @ref{Top, Semantic Manual, , semantic}.
121 It benefits from these Bison features:
125 It uses a fast but not so space-efficient encoding for the parse
126 tables, described in Corbett's PhD thesis from Berkeley:
128 @cite{Static Semantics in Compiler Error Recovery}@*
129 June 1985, Report No. UCB/CSD 85/251.
133 For generating the lookahead sets, Wisent uses the well-known
134 technique of F. DeRemer and T. Pennello described in:
136 @cite{Efficient Computation of LALR(1) Look-Ahead Sets}@*
137 October 1982, ACM TOPLAS Vol 4 No 4, 615--49,
138 @uref{http://dx.doi.org/10.1145/69622.357187}.
142 Wisent resolves shift/reduce conflicts using operator precedence and
146 Parser error recovery is accomplished using rules which match the
147 special token @code{error}.
150 Nevertheless there are some fundamental differences between Bison and
155 Wisent is intended to be used in Emacs. It reads and produces Emacs
156 Lisp data structures. All the additional code used in grammars is
160 Contrary to Bison, Wisent does not generate a parser which combines
161 Emacs Lisp code and grammar constructs. They exist separately.
162 Wisent reads the grammar from a Lisp data structure and then generates
163 grammar constructs as tables. Afterward, the derived tables can be
164 included and byte-compiled in separate Emacs Lisp files, and be used
165 at a later time by the Wisent's parser engine.
168 Wisent allows multiple start nonterminals and allows a call to the
169 parsing function to be made for a particular start nonterminal. For
170 example, this is particularly useful to parse a region of an Emacs
171 buffer. @semantic{} heavily depends on the availability of this feature.
175 @chapter Wisent Grammar
177 @cindex context-free grammar
179 In order for Wisent to parse a language, it must be described by a
180 @dfn{context-free grammar}. That is a grammar specified as rules that
181 can be applied regardless of context. For more information, see
182 @ref{Language and Grammar, , , bison}, in the Bison manual.
186 The formal grammar is formulated using @dfn{terminal} and
187 @dfn{nonterminal} items. Terminals can be Emacs Lisp symbols or
188 characters, and nonterminals are symbols only.
191 Terminals (also known as @dfn{tokens}) represent the lexical
192 elements of the language like numbers, strings, etc..
194 For example @samp{PLUS} can represent the operator @samp{+}.
196 Nonterminal symbols are described by rules:
200 RESULT @equiv{} COMPONENTS@dots{}
204 @samp{RESULT} is a nonterminal that this rule describes and
205 @samp{COMPONENTS} are various terminals and nonterminals that are put
206 together by this rule.
208 For example, this rule:
212 exp @equiv{} exp PLUS exp
216 Says that two groupings of type @samp{exp}, with a @samp{PLUS} token
217 in between, can be combined into a larger grouping of type @samp{exp}.
222 * Compiling a grammar::
227 @section Grammar format
229 @cindex grammar format
230 To be acceptable by Wisent a context-free grammar must respect a
231 particular format. That is, must be represented as an Emacs Lisp list
234 @code{(@var{terminals} @var{assocs} . @var{non-terminals})}
238 Is the list of terminal symbols used in the grammar.
240 @cindex associativity
242 Specify the associativity of @var{terminals}. It is @code{nil} when
243 there is no associativity defined, or an alist of
244 @w{@code{(@var{assoc-type} . @var{assoc-value})}} elements.
246 @var{assoc-type} must be one of the @code{default-prec},
247 @code{nonassoc}, @code{left} or @code{right} symbols. When
248 @var{assoc-type} is @code{default-prec}, @var{assoc-value} must be
249 @code{nil} or @code{t} (the default). Otherwise it is a list of
250 tokens which must have been previously declared in @var{terminals}.
252 For details, see @ref{Contextual Precedence, , , bison}, in the
256 Is the list of nonterminal definitions. Each definition has the form:
258 @code{(@var{nonterm} . @var{rules})}
260 Where @var{nonterm} is the nonterminal symbol defined and
261 @var{rules} the list of rules that describe this nonterminal. Each
264 @code{(@var{components} [@var{precedence}] [@var{action}])}
270 Is a list of various terminals and nonterminals that are put together
277 (exp ((exp ?+ exp)) ;; exp: exp '+' exp
282 Says that two groupings of type @samp{exp}, with a @samp{+} token in
283 between, can be combined into a larger grouping of type @samp{exp}.
285 @cindex grammar coding conventions
286 By convention, a nonterminal symbol should be in lower case, such as
287 @samp{exp}, @samp{stmt} or @samp{declaration}. Terminal symbols
288 should be upper case to distinguish them from nonterminals: for
289 example, @samp{INTEGER}, @samp{IDENTIFIER}, @samp{IF} or
290 @samp{RETURN}. A terminal symbol that represents a particular keyword
291 in the language is conventionally the same as that keyword converted
292 to upper case. The terminal symbol @code{error} is reserved for error
295 @cindex middle-rule actions
296 Scattered among the components can be @dfn{middle-rule} actions.
297 Usually only @var{action} is provided (@pxref{action}).
299 If @var{components} in a rule is @code{nil}, it means that the rule
300 can match the empty string. For example, here is how to define a
301 comma-separated sequence of zero or more @samp{exp} groupings:
305 (expseq (nil) ;; expseq: ;; empty
306 ((expseq1)) ;; | expseq1
309 (expseq1 ((exp)) ;; expseq1: exp
310 ((expseq1 ?, exp)) ;; | expseq1 ',' exp
315 @cindex precedence level
317 Assign the rule the precedence of the given terminal item, overriding
318 the precedence that would be deduced for it, that is the one of the
319 last terminal in it. Notice that only terminals declared in
320 @var{assocs} have a precedence level. The altered rule precedence
321 then affects how conflicts involving that rule are resolved.
323 @var{precedence} is an optional vector of one terminal item.
325 Here is how @var{precedence} solves the problem of unary minus.
326 First, declare a precedence for a fictitious terminal symbol named
327 @code{UMINUS}. There are no tokens of this type, but the symbol
328 serves to stand for its precedence:
332 ((default-prec t) ;; This is the default
338 Now the precedence of @code{UMINUS} can be used in specific rules:
342 (exp @dots{} ;; exp: @dots{}
343 ((exp ?- exp)) ;; | exp '-' exp
345 ((?- exp) [UMINUS]) ;; | '-' exp %prec UMINUS
351 If you forget to append @code{[UMINUS]} to the rule for unary minus,
352 Wisent silently assumes that minus has its usual precedence. This
353 kind of problem can be tricky to debug, since one typically discovers
354 the mistake only by testing the code.
356 Using @code{(default-prec nil)} declaration makes it easier to
357 discover this kind of problem systematically. It causes rules that
358 lack a @var{precedence} modifier to have no precedence, even if the
359 last terminal symbol mentioned in their components has a declared
362 If @code{(default-prec nil)} is in effect, you must specify
363 @var{precedence} for all rules that participate in precedence conflict
364 resolution. Then you will see any shift/reduce conflict until you
365 tell Wisent how to resolve it, either by changing your grammar or by
366 adding an explicit precedence. This will probably add declarations to
367 the grammar, but it helps to protect against incorrect rule
370 The effect of @code{(default-prec nil)} can be reversed by giving
371 @code{(default-prec t)}, which is the default.
373 For more details, see @ref{Contextual Precedence, , , bison}, in the
376 It is important to understand that @var{assocs} declarations defines
377 associativity but also assign a precedence level to terminals. All
378 terminals declared in the same @code{left}, @code{right} or
379 @code{nonassoc} association get the same precedence level. The
380 precedence level is increased at each new association.
382 On the other hand, @var{precedence} explicitly assign the precedence
383 level of the given terminal to a rule.
385 @cindex semantic actions
386 @item @anchor{action}action
387 An action is an optional Emacs Lisp function call, like this:
391 The result of an action determines the semantic value of a rule.
393 From an implementation standpoint, the function call will be embedded
394 in a lambda expression, and several useful local variables will be
400 Where @var{n} is a positive integer. Like in Bison, the value of
401 @code{$@var{n}} is the semantic value of the @var{n}th element of
402 @var{components}, starting from 1. It can be of any Lisp data
405 @vindex $region@var{n}
407 Where @var{n} is a positive integer. For each @code{$@var{n}}
408 variable defined there is a corresponding @code{$region@var{n}}
409 variable. Its value is a pair @code{(@var{start-pos} .
410 @var{end-pos})} that represent the start and end positions (in the
411 lexical input stream) of the @code{$@var{n}} value. It can be
412 @code{nil} when the component positions are not available, like for an
413 empty string component for example.
417 Its value is the leftmost and rightmost positions of input data
418 matched by all @var{components} in the rule. This is a pair
419 @code{(@var{leftmost-pos} . @var{rightmost-pos})}. It can be
420 @code{nil} when components positions are not available.
424 This variable is initialized with the nonterminal symbol
425 (@var{nonterm}) the rule belongs to. It could be useful to improve
426 error reporting or debugging. It is also used to automatically
427 provide incremental re-parse entry points for @semantic{} tags
428 (@pxref{Wisent Semantic}).
432 The value of @code{$action} is the symbolic name of the current
433 semantic action (@pxref{Debugging actions}).
436 When an action is not specified a default value is supplied, it is
437 @code{(identity $1)}. This means that the default semantic value of a
438 rule is the value of its first component. Excepted for a rule
439 matching the empty string, for which the default action is to return
447 @cindex grammar example
448 Here is an example to parse simple infix arithmetic expressions. See
449 @ref{Infix Calc, , , bison}, in the Bison manual for details.
457 ;; Terminal associativity & precedence
468 (format "%s %s" $1 $2))
482 (string-to-number $1))
504 In the bison-like @dfn{WY} format (@pxref{Wisent Semantic}) the
505 grammar looks like this:
511 %nonassoc '=' ;; comparison
514 %left NEG ;; negation--unary minus
515 %right '^' ;; exponentiation
522 (format "%s %s" $1 $2)
536 (string-to-number $1)
559 @node Compiling a grammar
560 @section Compiling a grammar
563 After providing a context-free grammar in a suitable format, it must
564 be translated into a set of tables (an @dfn{automaton}) that will be
565 used to derive the parser. Like Bison, Wisent translates grammars that
566 must be @dfn{LALR(1)}.
568 @cindex LALR(1) grammar
569 @cindex look-ahead token
570 A grammar is @acronym{LALR(1)} if it is possible to tell how to parse
571 any portion of an input string with just a single token of look-ahead:
572 the @dfn{look-ahead token}. See @ref{Language and Grammar, , ,
573 bison}, in the Bison manual for more information.
575 @cindex grammar compilation
576 Grammar translation (compilation) is achieved by the function:
578 @cindex compiling a grammar
579 @vindex wisent-single-start-flag
580 @findex wisent-compile-grammar
581 @defun wisent-compile-grammar grammar &optional start-list
582 Compile @var{grammar} and return an @acronym{LALR(1)} automaton.
584 Optional argument @var{start-list} is a list of start symbols
585 (nonterminals). If @code{nil} the first nonterminal defined in the
586 grammar is the default start symbol. If @var{start-list} contains
587 only one element, it defines the start symbol. If @var{start-list}
588 contains more than one element, all are defined as potential start
589 symbols, unless @code{wisent-single-start-flag} is non-@code{nil}. In
590 that case the first element of @var{start-list} defines the start
591 symbol and others are ignored.
593 The @acronym{LALR(1)} automaton is a vector of the form:
595 @code{[@var{actions gotos starts functions}]}
599 A state/token matrix telling the parser what to do at every state
600 based on the current look-ahead token. That is shift, reduce, accept
601 or error. See also @ref{Wisent Parsing}.
604 A state/nonterminal matrix telling the parser the next state to go to
605 after reducing with each rule.
608 An alist which maps the allowed start symbols (nonterminals) to
609 lexical tokens that will be first shifted into the parser stack.
612 An obarray of semantic action symbols. A semantic action is actually
613 an Emacs Lisp function (lambda expression).
620 Normally, a grammar should produce an automaton where at each state
621 the parser has only one action to do (@pxref{Wisent Parsing}).
623 @cindex ambiguous grammar
624 In certain cases, a grammar can produce an automaton where, at some
625 states, there are more than one action possible. Such a grammar is
626 @dfn{ambiguous}, and generates @dfn{conflicts}.
628 @cindex deterministic automaton
629 The parser can't be driven by an automaton which isn't completely
630 @dfn{deterministic}, that is which contains conflicts. It is
631 necessary to resolve the conflicts to eliminate them. Wisent resolves
632 conflicts like Bison does.
634 @cindex grammar conflicts
635 @cindex conflicts resolution
636 There are two sorts of conflicts:
639 @cindex shift/reduce conflicts
640 @item shift/reduce conflicts
641 When either a shift or a reduction would be valid at the same state.
643 Such conflicts are resolved by choosing to shift, unless otherwise
644 directed by operator precedence declarations.
645 See @ref{Shift/Reduce , , , bison}, in the Bison manual for more
648 @cindex reduce/reduce conflicts
649 @item reduce/reduce conflicts
650 That occurs if there are two or more rules that apply to the same
651 sequence of input. This usually indicates a serious error in the
654 Such conflicts are resolved by choosing to use the rule that appears
655 first in the grammar, but it is very risky to rely on this. Every
656 reduce/reduce conflict must be studied and usually eliminated. See
657 @ref{Reduce/Reduce , , , bison}, in the Bison manual for more
662 * Grammar Debugging::
663 * Understanding the automaton::
666 @node Grammar Debugging
667 @subsection Grammar debugging
669 @cindex grammar debugging
670 @cindex grammar verbose description
671 To help writing a new grammar, @code{wisent-compile-grammar} can
672 produce a verbose report containing a detailed description of the
673 grammar and parser (equivalent to what Bison reports with the
674 @option{--verbose} option).
676 To enable the verbose report you can set to non-@code{nil} the
679 @vindex wisent-verbose-flag
680 @deffn Option wisent-verbose-flag
681 non-@code{nil} means to report verbose information on generated parser.
684 Or interactively use the command:
686 @findex wisent-toggle-verbose-flag
687 @deffn Command wisent-toggle-verbose-flag
688 Toggle whether to report verbose information on generated parser.
691 The verbose report is printed in the temporary buffer
692 @file{*wisent-log*} when running interactively, or in file
693 @file{wisent.output} when running in batch mode. Different
694 reports are separated from each other by a line like this:
698 *** Wisent @var{source-file} - 2002-06-27 17:33
702 where @var{source-file} is the name of the Emacs Lisp file from which
703 the grammar was read. See @ref{Understanding the automaton}, for
704 details on the verbose report.
708 To help debugging the grammar compiler itself, you can set this
709 variable to print the content of some internal data structures:
711 @vindex wisent-debug-flag
712 @defvar wisent-debug-flag
713 non-@code{nil} means enable some debug stuff.
717 @node Understanding the automaton
718 @subsection Understanding the automaton
720 @cindex understanding the automaton
721 This section (took from the manual of Bison 1.49) describes how to use
722 the verbose report printed by @code{wisent-compile-grammar} to
723 understand the generated automaton, to tune or fix a grammar.
725 We will use the following example:
729 (let ((wisent-verbose-flag t)) ;; Print a verbose report!
730 (wisent-compile-grammar
731 '((NUM STR) ; %token NUM STR
733 ((left ?+ ?-) ; %left '+' '-';
734 (left ?*)) ; %left '*'
737 ((exp ?+ exp)) ; exp '+' exp
738 ((exp ?- exp)) ; | exp '-' exp
739 ((exp ?* exp)) ; | exp '*' exp
740 ((exp ?/ exp)) ; | exp '/' exp
748 'nil) ; no %start declarations
753 When evaluating the above expression, grammar compilation first issues
754 the following two clear messages:
758 Grammar contains 1 useless nonterminals and 1 useless rules
759 Grammar contains 7 shift/reduce conflicts
763 The @file{*wisent-log*} buffer details things!
765 The first section reports conflicts that were solved using precedence
766 and/or associativity:
770 Conflict in state 7 between rule 1 and token '+' resolved as reduce.
771 Conflict in state 7 between rule 1 and token '-' resolved as reduce.
772 Conflict in state 7 between rule 1 and token '*' resolved as shift.
773 Conflict in state 8 between rule 2 and token '+' resolved as reduce.
774 Conflict in state 8 between rule 2 and token '-' resolved as reduce.
775 Conflict in state 8 between rule 2 and token '*' resolved as shift.
776 Conflict in state 9 between rule 3 and token '+' resolved as reduce.
777 Conflict in state 9 between rule 3 and token '-' resolved as reduce.
778 Conflict in state 9 between rule 3 and token '*' resolved as reduce.
782 The next section reports useless tokens, nonterminal and rules (note
783 that useless tokens might be used by the scanner):
787 Useless nonterminals:
792 Terminals which are not used:
803 The next section lists states that still have conflicts:
807 State 7 contains 1 shift/reduce conflict.
808 State 8 contains 1 shift/reduce conflict.
809 State 9 contains 1 shift/reduce conflict.
810 State 10 contains 4 shift/reduce conflicts.
814 The next section reproduces the grammar used:
829 And reports the uses of the symbols:
833 Terminals, with rules where they appear
845 Nonterminals, with rules where they appear
848 on left: 1 2 3 4 5, on right: 1 2 3 4
852 The report then details the automaton itself, describing each state
853 with it set of @dfn{items}, also known as @dfn{pointed rules}. Each
854 item is a production rule together with a point (marked by @samp{.})
855 that the input cursor.
861 NUM shift, and go to state 1
867 State 0 corresponds to being at the very beginning of the parsing, in
868 the initial rule, right before the start symbol (@samp{exp}). When
869 the parser returns to this state right after having reduced a rule
870 that produced an @samp{exp}, it jumps to state 2. If there is no such
871 transition on a nonterminal symbol, and the lookahead is a @samp{NUM},
872 then this token is shifted on the parse stack, and the control flow
873 jumps to state 1. Any other lookahead triggers a parse error.
881 exp -> NUM . (rule 5)
883 $default reduce using rule 5 (exp)
887 the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead
888 (@samp{$default}), the parser will reduce it. If it was coming from
889 state 0, then, after this reduction it will return to state 0, and
890 will jump to state 2 (@samp{exp: go to state 2}).
896 exp -> exp . '+' exp (rule 1)
897 exp -> exp . '-' exp (rule 2)
898 exp -> exp . '*' exp (rule 3)
899 exp -> exp . '/' exp (rule 4)
901 $EOI shift, and go to state 11
902 '+' shift, and go to state 3
903 '-' shift, and go to state 4
904 '*' shift, and go to state 5
905 '/' shift, and go to state 6
909 In state 2, the automaton can only shift a symbol. For instance,
910 because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
911 @samp{+}, it will be shifted on the parse stack, and the automaton
912 control will jump to state 3, corresponding to the item
913 @samp{exp -> exp . '+' exp}:
919 exp -> exp '+' . exp (rule 1)
921 NUM shift, and go to state 1
927 Since there is no default action, any other token than those listed
928 above will trigger a parse error.
930 The interpretation of states 4 to 6 is straightforward:
936 exp -> exp '-' . exp (rule 2)
938 NUM shift, and go to state 1
946 exp -> exp '*' . exp (rule 3)
948 NUM shift, and go to state 1
956 exp -> exp '/' . exp (rule 4)
958 NUM shift, and go to state 1
964 As was announced in beginning of the report, @samp{State 7 contains 1
965 shift/reduce conflict.}:
971 exp -> exp . '+' exp (rule 1)
972 exp -> exp '+' exp . (rule 1)
973 exp -> exp . '-' exp (rule 2)
974 exp -> exp . '*' exp (rule 3)
975 exp -> exp . '/' exp (rule 4)
977 '*' shift, and go to state 5
978 '/' shift, and go to state 6
980 '/' [reduce using rule 1 (exp)]
981 $default reduce using rule 1 (exp)
985 Indeed, there are two actions associated to the lookahead @samp{/}:
986 either shifting (and going to state 6), or reducing rule 1. The
987 conflict means that either the grammar is ambiguous, or the parser
988 lacks information to make the right decision. Indeed the grammar is
989 ambiguous, as, since we did not specify the precedence of @samp{/},
990 the sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM
991 / NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM +
992 NUM) / NUM}, which corresponds to reducing rule 1.
994 Because in @acronym{LALR(1)} parsing a single decision can be made,
995 Wisent arbitrarily chose to disable the reduction, see
996 @ref{Conflicts}. Discarded actions are reported in between square
999 Note that all the previous states had a single possible action: either
1000 shifting the next token and going to the corresponding state, or
1001 reducing a single rule. In the other cases, i.e., when shifting
1002 @emph{and} reducing is possible or when @emph{several} reductions are
1003 possible, the lookahead is required to select the action. State 7 is
1004 one such state: if the lookahead is @samp{*} or @samp{/} then the
1005 action is shifting, otherwise the action is reducing rule 1. In other
1006 words, the first two items, corresponding to rule 1, are not eligible
1007 when the lookahead is @samp{*}, since we specified that @samp{*} has
1008 higher precedence that @samp{+}. More generally, some items are
1009 eligible only with some set of possible lookaheads.
1011 States 8 to 10 are similar:
1017 exp -> exp . '+' exp (rule 1)
1018 exp -> exp . '-' exp (rule 2)
1019 exp -> exp '-' exp . (rule 2)
1020 exp -> exp . '*' exp (rule 3)
1021 exp -> exp . '/' exp (rule 4)
1023 '*' shift, and go to state 5
1024 '/' shift, and go to state 6
1026 '/' [reduce using rule 2 (exp)]
1027 $default reduce using rule 2 (exp)
1032 exp -> exp . '+' exp (rule 1)
1033 exp -> exp . '-' exp (rule 2)
1034 exp -> exp . '*' exp (rule 3)
1035 exp -> exp '*' exp . (rule 3)
1036 exp -> exp . '/' exp (rule 4)
1038 '/' shift, and go to state 6
1040 '/' [reduce using rule 3 (exp)]
1041 $default reduce using rule 3 (exp)
1046 exp -> exp . '+' exp (rule 1)
1047 exp -> exp . '-' exp (rule 2)
1048 exp -> exp . '*' exp (rule 3)
1049 exp -> exp . '/' exp (rule 4)
1050 exp -> exp '/' exp . (rule 4)
1052 '+' shift, and go to state 3
1053 '-' shift, and go to state 4
1054 '*' shift, and go to state 5
1055 '/' shift, and go to state 6
1057 '+' [reduce using rule 4 (exp)]
1058 '-' [reduce using rule 4 (exp)]
1059 '*' [reduce using rule 4 (exp)]
1060 '/' [reduce using rule 4 (exp)]
1061 $default reduce using rule 4 (exp)
1065 Observe that state 10 contains conflicts due to the lack of precedence
1066 of @samp{/} wrt @samp{+}, @samp{-}, and @samp{*}, but also because the
1067 associativity of @samp{/} is not specified.
1069 Finally, the state 11 (plus 12) is named the @dfn{final state}, or the
1070 @dfn{accepting state}:
1076 $EOI shift, and go to state 12
1086 The end of input is shifted @samp{$EOI shift,} and the parser exits
1087 successfully (@samp{go to state 12}, that terminates).
1089 @node Wisent Parsing
1090 @chapter Wisent Parsing
1092 @cindex bottom-up parser
1093 @cindex shift-reduce parser
1094 The Wisent's parser is what is called a @dfn{bottom-up} or
1095 @dfn{shift-reduce} parser which repeatedly:
1100 That is pushes the value of the last lexical token read (the
1101 look-ahead token) into a value stack, and reads a new one.
1105 That is replaces a nonterminal by its semantic value. The values of
1106 the components which form the right hand side of a rule are popped
1107 from the value stack and reduced by the semantic action of this rule.
1108 The result is pushed back on top of value stack.
1111 The parser will stop on:
1116 When all input has been successfully parsed. The semantic value of
1117 the start nonterminal is on top of the value stack.
1119 @cindex syntax error
1121 When a syntax error (an unexpected token in input) has been detected.
1122 At this point the parser issues an error message and either stops or
1123 calls a recovery routine to try to resume parsing.
1126 @cindex table-driven parser
1127 The above elementary actions are driven by the @acronym{LALR(1)}
1128 automaton built by @code{wisent-compile-grammar} from a context-free
1131 The Wisent's parser is entered by calling the function:
1133 @findex wisent-parse
1134 @defun wisent-parse automaton lexer &optional error start
1135 Parse input using the automaton specified in @var{automaton}.
1139 Is an @acronym{LALR(1)} automaton generated by
1140 @code{wisent-compile-grammar} (@pxref{Wisent Grammar}).
1143 Is a function with no argument called by the parser to obtain the next
1144 terminal (token) in input (@pxref{Writing a lexer}).
1147 Is an optional reporting function called when a parse error occurs.
1148 It receives a message string to report. It defaults to the function
1149 @code{wisent-message} (@pxref{Report errors}).
1152 Specify the start symbol (nonterminal) used by the parser as its goal.
1153 It defaults to the start symbol defined in the grammar
1154 (@pxref{Wisent Grammar}).
1158 The following two normal hooks permit to do some useful processing
1159 respectively before to start parsing, and after the parser terminated.
1161 @vindex wisent-pre-parse-hook
1162 @defvar wisent-pre-parse-hook
1163 Normal hook run just before entering the @var{LR} parser engine.
1166 @vindex wisent-post-parse-hook
1167 @defvar wisent-post-parse-hook
1168 Normal hook run just after the @var{LR} parser engine terminated.
1176 * Debugging actions::
1179 @node Writing a lexer
1180 @section What the parser must receive
1182 It is important to understand that the parser does not parse
1183 characters, but lexical tokens, and does not know anything about
1184 characters in text streams!
1186 @cindex lexical analysis
1189 Reading input data to produce lexical tokens is performed by a lexer
1190 (also called a scanner) in a lexical analysis step, before the syntax
1191 analysis step performed by the parser. The parser automatically calls
1192 the lexer when it needs the next token to parse.
1194 @cindex lexical tokens
1195 A Wisent's lexer is an Emacs Lisp function with no argument. It must
1196 return a valid lexical token of the form:
1198 @code{(@var{token-class value} [@var{start} . @var{end}])}
1202 Is a category of lexical token identifying a terminal as specified in
1203 the grammar (@pxref{Wisent Grammar}). It can be a symbol or a character
1207 Is the value of the lexical token. It can be of any valid Emacs Lisp
1212 Are the optional beginning and ending positions of @var{value} in the
1216 When there are no more tokens to read the lexer must return the token
1217 @code{(list wisent-eoi-term)} to each request.
1219 @vindex wisent-eoi-term
1220 @defvar wisent-eoi-term
1221 Predefined constant, End-Of-Input terminal symbol.
1224 @code{wisent-lex} is an example of a lexer that reads lexical tokens
1225 produced by a @semantic{} lexer, and translates them into lexical tokens
1226 suitable to the Wisent parser. See also @ref{Wisent Lex}.
1228 To call the lexer in a semantic action use the function
1229 @code{wisent-lexer}. See also @ref{Actions goodies}.
1231 @node Actions goodies
1232 @section Variables and macros useful in grammar actions.
1234 @vindex wisent-input
1235 @defvar wisent-input
1236 The last token read.
1237 This variable only has meaning in the scope of @code{wisent-parse}.
1240 @findex wisent-lexer
1242 Obtain the next terminal in input.
1245 @findex wisent-region
1246 @defun wisent-region &rest positions
1247 Return the start/end positions of the region including
1248 @var{positions}. Each element of @var{positions} is a pair
1249 @w{@code{(@var{start-pos} . @var{end-pos})}} or @code{nil}. The
1250 returned value is the pair @w{@code{(@var{min-start-pos} .
1251 @var{max-end-pos})}} or @code{nil} if no @var{positions} are
1256 @section The error reporting function
1258 @cindex error reporting
1259 When the parser encounters a syntax error it calls a user-defined
1260 function. It must be an Emacs Lisp function with one argument: a
1261 string containing the message to report.
1263 By default the parser uses this function to report error messages:
1265 @findex wisent-message
1266 @defun wisent-message string &rest args
1267 Print a one-line message if @code{wisent-parse-verbose-flag} is set.
1268 Pass @var{string} and @var{args} arguments to @dfn{message}.
1273 @code{wisent-message} uses the following function to print lexical
1276 @defun wisent-token-to-string token
1277 Return a printed representation of lexical token @var{token}.
1280 The general printed form of a lexical token is:
1282 @w{@code{@var{token}(@var{value})@@@var{location}}}
1285 To control the verbosity of the parser you can set to non-@code{nil}
1288 @vindex wisent-parse-verbose-flag
1289 @deffn Option wisent-parse-verbose-flag
1290 non-@code{nil} means to issue more messages while parsing.
1293 Or interactively use the command:
1295 @findex wisent-parse-toggle-verbose-flag
1296 @deffn Command wisent-parse-toggle-verbose-flag
1297 Toggle whether to issue more messages while parsing.
1300 When the error reporting function is entered the variable
1301 @code{wisent-input} contains the unexpected token as returned by the
1304 The error reporting function can be called from a semantic action too
1305 using the special macro @code{wisent-error}. When called from a
1306 semantic action entered by error recovery (@pxref{Error recovery}) the
1307 value of the variable @code{wisent-recovering} is non-@code{nil}.
1309 @node Error recovery
1310 @section Error recovery
1312 @cindex error recovery
1313 The error recovery mechanism of the Wisent's parser conforms to the
1314 one Bison uses. See @ref{Error Recovery, , , bison}, in the Bison
1318 To recover from a syntax error you must write rules to recognize the
1319 special token @code{error}. This is a terminal symbol that is
1320 automatically defined and reserved for error handling.
1322 When the parser encounters a syntax error, it pops the state stack
1323 until it finds a state that allows shifting the @code{error} token.
1324 After it has been shifted, if the old look-ahead token is not
1325 acceptable to be shifted next, the parser reads tokens and discards
1326 them until it finds a token which is acceptable.
1328 @cindex error recovery strategy
1329 Strategies for error recovery depend on the choice of error rules in
1330 the grammar. A simple and useful strategy is simply to skip the rest
1331 of the current statement if an error is detected:
1335 (statement (( error ?; )) ;; on error, skip until ';' is read
1340 It is also useful to recover to the matching close-delimiter of an
1341 opening-delimiter that has already been parsed:
1345 (primary (( ?@{ expr ?@} ))
1352 @cindex error recovery actions
1353 Note that error recovery rules may have actions, just as any other
1354 rules can. Here are some predefined hooks, variables, functions or
1355 macros, useful in such actions:
1357 @vindex wisent-nerrs
1358 @defvar wisent-nerrs
1359 The number of parse errors encountered so far.
1362 @vindex wisent-recovering
1363 @defvar wisent-recovering
1364 non-@code{nil} means that the parser is recovering.
1365 This variable only has meaning in the scope of @code{wisent-parse}.
1368 @findex wisent-error
1369 @defun wisent-error msg
1370 Call the user supplied error reporting function with message
1371 @var{msg} (@pxref{Report errors}).
1373 For an example of use, @xref{wisent-skip-token}.
1376 @findex wisent-errok
1378 Resume generating error messages immediately for subsequent syntax
1381 The parser suppress error message for syntax errors that happens
1382 shortly after the first, until three consecutive input tokens have
1383 been successfully shifted.
1385 Calling @code{wisent-errok} in an action, make error messages resume
1386 immediately. No error messages will be suppressed if you call it in
1387 an error rule's action.
1389 For an example of use, @xref{wisent-skip-token}.
1392 @findex wisent-clearin
1393 @defun wisent-clearin
1394 Discard the current lookahead token.
1395 This will cause a new lexical token to be read.
1397 In an error rule's action the previous lookahead token is reanalyzed
1398 immediately. @code{wisent-clearin} may be called to clear this token.
1400 For example, suppose that on a parse error, an error handling routine
1401 is called that advances the input stream to some point where parsing
1402 should once again commence. The next symbol returned by the lexical
1403 scanner is probably correct. The previous lookahead token ought to
1404 be discarded with @code{wisent-clearin}.
1406 For an example of use, @xref{wisent-skip-token}.
1409 @findex wisent-abort
1411 Abort parsing and save the lookahead token.
1414 @findex wisent-set-region
1415 @defun wisent-set-region start end
1416 Change the region of text matched by the current nonterminal.
1417 @var{start} and @var{end} are respectively the beginning and end
1418 positions of the region occupied by the group of components associated
1419 to this nonterminal. If @var{start} or @var{end} values are not a
1420 valid positions the region is set to @code{nil}.
1422 For an example of use, @xref{wisent-skip-token}.
1425 @vindex wisent-discarding-token-functions
1426 @defvar wisent-discarding-token-functions
1427 List of functions to be called when discarding a lexical token.
1428 These functions receive the lexical token discarded.
1429 When the parser encounters unexpected tokens, it can discards them,
1430 based on what directed by error recovery rules. Either when the
1431 parser reads tokens until one is found that can be shifted, or when an
1432 semantic action calls the function @code{wisent-skip-token} or
1433 @code{wisent-skip-block}.
1434 For language specific hooks, make sure you define this as a local
1437 For example, in @semantic{}, this hook is set to the function
1438 @code{wisent-collect-unmatched-syntax} to collect unmatched lexical
1439 tokens (@pxref{Useful functions}).
1442 @findex wisent-skip-token
1443 @defun wisent-skip-token
1444 @anchor{wisent-skip-token}
1445 Skip the lookahead token in order to resume parsing.
1447 Must be used in error recovery semantic actions.
1449 It typically looks like this:
1453 (wisent-message "%s: skip %s" $action
1454 (wisent-token-to-string wisent-input))
1456 'wisent-discarding-token-functions wisent-input)
1463 @findex wisent-skip-block
1464 @defun wisent-skip-block
1465 Safely skip a block in order to resume parsing.
1467 Must be used in error recovery semantic actions.
1469 A block is data between an open-delimiter (syntax class @code{(}) and
1470 a matching close-delimiter (syntax class @code{)}):
1474 (a parenthesized block)
1475 [a block between brackets]
1476 @{a block between braces@}
1480 The following example uses @code{wisent-skip-block} to safely skip a
1481 block delimited by @samp{LBRACE} (@code{@{}) and @samp{RBRACE}
1482 (@code{@}}) tokens, when a syntax error occurs in
1483 @samp{other-components}:
1487 (block ((LBRACE other-components RBRACE))
1490 (wisent-skip-block))
1496 @node Debugging actions
1497 @section Debugging semantic actions
1499 @cindex semantic action symbols
1500 Each semantic action is represented by a symbol interned in an
1501 @dfn{obarray} that is part of the @acronym{LALR(1)} automaton
1502 (@pxref{Compiling a grammar}). @code{symbol-function} on a semantic
1503 action symbol return the semantic action lambda expression.
1505 A semantic action symbol name has the form
1506 @code{@var{nonterminal}:@var{index}}, where @var{nonterminal} is the
1507 name of the nonterminal symbol the action belongs to, and @var{index}
1508 is an action sequence number within the scope of @var{nonterminal}.
1509 For example, this nonterminal definition:
1514 line [@code{input:0}]
1516 (format "%s %s" $1 $2) [@code{input:1}]
1521 Will produce two semantic actions, and associated symbols:
1525 A default action that returns @code{$1}.
1528 That returns @code{(format "%s %s" $1 $2)}.
1531 @cindex debugging semantic actions
1532 Debugging uses the Lisp debugger to investigate what is happening
1533 during execution of semantic actions.
1534 Three commands are available to debug semantic actions. They receive
1538 @item The automaton that contains the semantic action.
1540 @item The semantic action symbol.
1543 @findex wisent-debug-on-entry
1544 @deffn Command wisent-debug-on-entry automaton function
1545 Request @var{automaton}'s @var{function} to invoke debugger each time it is called.
1546 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1549 @findex wisent-cancel-debug-on-entry
1550 @deffn Command wisent-cancel-debug-on-entry automaton function
1551 Undo effect of @code{wisent-debug-on-entry} on @var{automaton}'s @var{function}.
1552 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1555 @findex wisent-debug-show-entry
1556 @deffn Command wisent-debug-show-entry automaton function
1557 Show the source of @var{automaton}'s semantic action @var{function}.
1558 @var{function} must be a semantic action symbol that exists in @var{automaton}.
1561 @node Wisent Semantic
1562 @chapter How to use Wisent with Semantic
1565 This section presents how the Wisent's parser can be used to produce
1566 @dfn{tags} for the @semantic{} tool set.
1568 @semantic{} tags form a hierarchy of Emacs Lisp data structures that
1569 describes a program in a way independent of programming languages.
1570 Tags map program declarations, like functions, methods, variables,
1571 data types, classes, includes, grammar rules, etc..
1573 @cindex WY grammar format
1574 To use the Wisent parser with @semantic{} you have to define
1575 your grammar in @dfn{WY} form, a grammar format very close
1576 to the one used by Bison.
1578 Please @inforef{top, Semantic Grammar Framework Manual, grammar-fw}
1579 for more information on @semantic{} grammars.
1586 @node Grammar styles
1587 @section Grammar styles
1589 @cindex grammar styles
1590 @semantic{} parsing heavily depends on how you wrote the grammar.
1591 There are mainly two styles to write a Wisent's grammar intended to be
1592 used with the @semantic{} tool set: the @dfn{Iterative style} and the
1593 @dfn{Bison style}. Each one has pros and cons, and in certain cases
1594 it can be worth a mix of the two styles!
1600 * Start nonterminals::
1601 * Useful functions::
1604 @node Iterative style
1605 @subsection Iterative style
1607 @cindex grammar iterative style
1608 The @dfn{iterative style} is the preferred style to use with @semantic{}.
1609 It relies on an iterative parser back-end mechanism which parses start
1610 nonterminals one at a time and automagically skips unexpected lexical
1613 Compared to rule-based iterative functions (@pxref{Bison style}),
1614 iterative parsers are better in that they can handle obscure errors
1618 Each start nonterminal must produces a @dfn{raw tag} by calling a
1619 @code{TAG}-like grammar macro with appropriate parameters. See also
1620 @ref{Start nonterminals}.
1622 @cindex expanded tag
1623 Then, each parsing iteration automatically translates a raw tag into
1624 @dfn{expanded tags}, updating the raw tag structure with internal
1625 properties and buffer related data.
1627 After parsing completes, it results in a tree of expanded tags.
1629 The following example is a snippet of the iterative style Java grammar
1630 provided in the @semantic{} distribution in the file
1631 @file{semantic/wisent/java-tags.wy}.
1636 ;; Alternate entry points
1637 ;; - Needed by partial re-parse
1638 %start formal_parameter
1640 ;; - Needed by EXPANDFULL clauses
1641 %start formal_parameters
1644 formal_parameter_list
1646 (EXPANDFULL $1 formal_parameters)
1654 | formal_parameter COMMA
1655 | formal_parameter RPAREN
1659 : formal_parameter_modifier_opt type variable_declarator_id
1660 (VARIABLE-TAG $3 $2 nil :typemodifiers $1)
1666 It shows the use of the @code{EXPANDFULL} grammar macro to parse a
1667 @samp{PAREN_BLOCK} which contains a @samp{formal_parameter_list}.
1668 @code{EXPANDFULL} tells to recursively parse @samp{formal_parameters}
1669 inside @samp{PAREN_BLOCK}. The parser iterates until it digested all
1670 available input data inside the @samp{PAREN_BLOCK}, trying to match
1671 any of the @samp{formal_parameters} rules:
1678 @item @samp{formal_parameter COMMA}
1680 @item @samp{formal_parameter RPAREN}
1683 At each iteration it will return a @samp{formal_parameter} raw tag,
1684 or @code{nil} to skip unwanted (single @samp{LPAREN} or @samp{RPAREN}
1685 for example) or unexpected input data. Those raw tags will be
1686 automatically expanded by the iterative back-end parser.
1689 @subsection Bison style
1691 @cindex grammar bison style
1692 What we call the @dfn{Bison style} is the traditional style of Bison's
1693 grammars. Compared to iterative style, it is not straightforward to
1694 use grammars written in Bison style in @semantic{}. Mainly because such
1695 grammars are designed to parse the whole input data in one pass, and
1696 don't use the iterative parser back-end mechanism (@pxref{Iterative
1697 style}). With Bison style the parser is called once to parse the
1698 grammar start nonterminal.
1700 The following example is a snippet of the Bison style Java grammar
1701 provided in the @semantic{} distribution in the file
1702 @file{semantic/wisent/java.wy}.
1706 %start formal_parameter
1709 formal_parameter_list
1710 : formal_parameter_list COMMA formal_parameter
1717 : formal_parameter_modifier_opt type variable_declarator_id
1719 (VARIABLE-TAG $3 $2 :typemodifiers $1)
1725 The first consequence is that syntax errors are not automatically
1726 handled by @semantic{}. Thus, it is necessary to explicitly handle
1727 them at the grammar level, providing error recovery rules to skip
1728 unexpected input data.
1730 The second consequence is that the iterative parser can't do automatic
1731 tag expansion, except for the start nonterminal value. It is
1732 necessary to explicitly expand tags from concerned semantic actions by
1733 calling the grammar macro @code{EXPANDTAG} with a raw tag as
1734 parameter. See also @ref{Start nonterminals}, for incremental
1735 re-parse considerations.
1738 @subsection Mixed style
1740 @cindex grammar mixed style
1745 %start prologue epilogue declaration nonterminal rule
1760 SYMBOL COLON rules SEMI
1761 (TAG $1 'nonterminal :children $3)
1766 (apply 'nconc (nreverse $1))
1779 name type comps prec action elt)
1782 (TAG name 'rule :type type :value comps :prec prec :expr action)
1788 This example shows how iterative and Bison styles can be combined in
1789 the same grammar to obtain a good compromise between grammar
1790 complexity and an efficient parsing strategy in an interactive
1793 @samp{nonterminal} is parsed using iterative style via the main
1794 @samp{grammar} rule. The semantic action uses the @code{TAG} macro to
1795 produce a raw tag, automagically expanded by @semantic{}.
1797 But @samp{rules} part is parsed in Bison style! Why?
1799 Rule delimiters are the colon (@code{:}), that follows the nonterminal
1800 name, and a final semicolon (@code{;}). Unfortunately these
1801 delimiters are not @code{open-paren}/@code{close-paren} type, and the
1802 Emacs' syntactic analyzer can't easily isolate data between them to
1803 produce a @samp{RULES_PART} parenthesis-block-like lexical token.
1804 Consequently it is not possible to use @code{EXPANDFULL} to iterate in
1805 @samp{RULES_PART}, like this:
1810 SYMBOL COLON rules SEMI
1811 (TAG $1 'nonterminal :children $3)
1815 RULES_PART ;; @strong{Map a parenthesis-block-like lexical token}
1816 (EXPANDFULL $1 'rules)
1829 name type comps prec action elt)
1831 (TAG name 'rule :type type :value comps :prec prec :expr action)
1837 In such cases, when it is difficult for Emacs to obtain
1838 parenthesis-block-like lexical tokens, the best solution is to use the
1839 traditional Bison style with error recovery!
1841 In some extreme cases, it can also be convenient to extend the lexer,
1842 to deliver new lexical tokens, to simplify the grammar.
1844 @node Start nonterminals
1845 @subsection Start nonterminals
1847 @cindex start nonterminals
1848 @cindex @code{reparse-symbol} property
1849 When you write a grammar for @semantic{}, it is important to carefully
1850 indicate the start nonterminals. Each one defines an entry point in
1851 the grammar, and after parsing its semantic value is returned to the
1852 back-end iterative engine. Consequently:
1854 @strong{The semantic value of a start nonterminal must be a produced
1855 by a TAG like grammar macro}.
1857 Start nonterminals are declared by @code{%start} statements. When
1858 nothing is specified the first nonterminal that appears in the grammar
1859 is the start nonterminal.
1861 Generally, the following nonterminals must be declared as start
1865 @item The main grammar entry point
1870 @item nonterminals passed to @code{EXPAND}/@code{EXPANDFULL}
1872 These grammar macros recursively parse a part of input data, based on
1873 rules of the given nonterminal.
1875 For example, the following will parse @samp{PAREN_BLOCK} data using
1876 the @samp{formal_parameters} rules:
1880 formal_parameter_list
1882 (EXPANDFULL $1 formal_parameters)
1887 The semantic value of @samp{formal_parameters} becomes the value of
1888 the @code{EXPANDFULL} expression. It is a list of @semantic{} tags
1889 spliced in the tags tree.
1891 Because the automaton must know that @samp{formal_parameters} is a
1892 start symbol, you must declare it like this:
1896 %start formal_parameters
1902 @cindex incremental re-parse
1903 @cindex reparse-symbol
1904 The @code{EXPANDFULL} macro has a side effect it is important to know,
1905 related to the incremental re-parse mechanism of @semantic{}: the
1906 nonterminal symbol parameter passed to @code{EXPANDFULL} also becomes
1907 the @code{reparse-symbol} property of the tag returned by the
1908 @code{EXPANDFULL} expression.
1910 When buffer's data mapped by a tag is modified, @semantic{}
1911 schedules an incremental re-parse of that data, using the tag's
1912 @code{reparse-symbol} property as start nonterminal.
1914 @strong{The rules associated to such start symbols must be carefully
1915 reviewed to ensure that the incremental parser will work!}
1917 Things are a little bit different when the grammar is written in Bison
1920 @strong{The @code{reparse-symbol} property is set to the nonterminal
1921 symbol the rule that explicitly uses @code{EXPANDTAG} belongs to.}
1930 name type comps prec action elt)
1933 (TAG name 'rule :type type :value comps :prec prec :expr action)
1939 Set the @code{reparse-symbol} property of the expanded tag to
1940 @samp{rule}. A important consequence is that:
1942 @strong{Every nonterminal having any rule that calls @code{EXPANDTAG}
1943 in a semantic action, should be declared as a start symbol!}
1945 @node Useful functions
1946 @subsection Useful functions
1948 Here is a description of some predefined functions it might be useful
1949 to know when writing new code to use Wisent in @semantic{}:
1951 @findex wisent-collect-unmatched-syntax
1952 @defun wisent-collect-unmatched-syntax input
1953 Add @var{input} lexical token to the cache of unmatched tokens, in
1954 variable @code{semantic-unmatched-syntax-cache}.
1956 See implementation of the function @code{wisent-skip-token} in
1957 @ref{Error recovery}, for an example of use.
1961 @section The Wisent Lex lexer
1963 @findex semantic-lex
1964 The lexical analysis step of @semantic{} is performed by the general
1965 function @code{semantic-lex}. For more information, @inforef{Writing
1966 Lexers, ,semantic-langdev}.
1968 @code{semantic-lex} produces lexical tokens of the form:
1972 @code{(@var{token-class start} . @var{end})}
1978 Is a symbol that identifies a lexical token class, like @code{symbol},
1979 @code{string}, @code{number}, or @code{PAREN_BLOCK}.
1983 Are the start and end positions of mapped data in the input buffer.
1986 The Wisent's parser doesn't depend on the nature of analyzed input
1987 stream (buffer, string, etc.), and requires that lexical tokens have a
1988 different form (@pxref{Writing a lexer}):
1992 @code{(@var{token-class value} [@var{start} . @var{end}])}
1996 @cindex lexical token mapping
1997 @code{wisent-lex} is the default Wisent's lexer used in @semantic{}.
1999 @vindex wisent-lex-istream
2002 Return the next available lexical token in Wisent's form.
2004 The variable @code{wisent-lex-istream} contains the list of lexical
2005 tokens produced by @code{semantic-lex}. Pop the next token available
2006 and convert it to a form suitable for the Wisent's parser.
2009 Mapping of lexical tokens as produced by @code{semantic-lex} into
2010 equivalent Wisent lexical tokens is straightforward:
2014 (@var{token-class start} . @var{end})
2015 @result{} (@var{token-class value start} . @var{end})
2019 @var{value} is the input @code{buffer-substring} from @var{start} to
2022 @node GNU Free Documentation License
2023 @appendix GNU Free Documentation License
2025 @include doclicense.texi
2038 @c Following comments are for the benefit of ispell.
2040 @c LocalWords: Wisent automagically wisent Wisent's LALR obarray