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9 It does the same job as ``bison'' and ``yacc''.
10 But lemon is not another bison or yacc clone. It
11 uses a different grammar syntax which is designed to
12 reduce the number of coding errors. Lemon also uses a more
13 sophisticated parsing engine that is faster than yacc and
14 bison and which is both reentrant and thread-safe.
15 Furthermore, Lemon implements features that can be used
16 to eliminate resource leaks, making is suitable for use
17 in long-running programs such as graphical user interfaces
18 or embedded controllers.</p>
20 <p>This document is an introduction to the Lemon
21 parser generator.</p>
25 <p>The main goal of Lemon is to translate a context free grammar (CFG)
26 for a particular language into C code that implements a parser for
27 that language.
28 The program has two inputs:
29 <ul>
30 <li>The grammar specification.
31 <li>A parser template file.
32 </ul>
33 Typically, only the grammar specification is supplied by the programmer.
34 Lemon comes with a default parser template which works fine for most
35 applications. But the user is free to substitute a different parser
36 template if desired.</p>
38 <p>Depending on command-line options, Lemon will generate between
39 one and three files of outputs.
40 <ul>
41 <li>C code to implement the parser.
42 <li>A header file defining an integer ID for each terminal symbol.
43 <li>An information file that describes the states of the generated parser
44 automaton.
45 </ul>
46 By default, all three of these output files are generated.
47 The header file is suppressed if the ``-m'' command-line option is
48 used and the report file is omitted when ``-q'' is selected.</p>
50 <p>The grammar specification file uses a ``.y'' suffix, by convention.
51 In the examples used in this document, we'll assume the name of the
52 grammar file is ``gram.y''. A typical use of Lemon would be the
53 following command:
54 <pre>
55 lemon gram.y
56 </pre>
57 This command will generate three output files named ``gram.c'',
58 ``gram.h'' and ``gram.out''.
59 The first is C code to implement the parser. The second
60 is the header file that defines numerical values for all
61 terminal symbols, and the last is the report that explains
62 the states used by the parser automaton.</p>
66 <p>The behavior of Lemon can be modified using command-line options.
67 You can obtain a list of the available command-line options together
68 with a brief explanation of what each does by typing
69 <pre>
70 lemon -?
71 </pre>
72 As of this writing, the following command-line options are supported:
73 <ul>
81 </ul>
82 The ``-b'' option reduces the amount of text in the report file by
83 printing only the basis of each parser state, rather than the full
84 configuration.
85 The ``-c'' option suppresses action table compression. Using -c
86 will make the parser a little larger and slower but it will detect
87 syntax errors sooner.
88 The ``-g'' option causes no output files to be generated at all.
89 Instead, the input grammar file is printed on standard output but
90 with all comments, actions and other extraneous text deleted. This
91 is a useful way to get a quick summary of a grammar.
92 The ``-m'' option causes the output C source file to be compatible
93 with the ``makeheaders'' program.
94 Makeheaders is a program that automatically generates header files
95 from C source code. When the ``-m'' option is used, the header
96 file is not output since the makeheaders program will take care
97 of generated all header files automatically.
98 The ``-q'' option suppresses the report file.
99 Using ``-s'' causes a brief summary of parser statistics to be
100 printed. Like this:
101 <pre>
104 </pre>
105 Finally, the ``-x'' option causes Lemon to print its version number
106 and then stops without attempting to read the grammar or generate a parser.</p>
110 <p>Lemon doesn't generate a complete, working program. It only generates
111 a few subroutines that implement a parser. This section describes
112 the interface to those subroutines. It is up to the programmer to
113 call these subroutines in an appropriate way in order to produce a
114 complete system.</p>
116 <p>Before a program begins using a Lemon-generated parser, the program
117 must first create the parser.
118 A new parser is created as follows:
119 <pre>
120 void *pParser = ParseAlloc( malloc );
121 </pre>
122 The ParseAlloc() routine allocates and initializes a new parser and
123 returns a pointer to it.
124 The actual data structure used to represent a parser is opaque --
125 its internal structure is not visible or usable by the calling routine.
126 For this reason, the ParseAlloc() routine returns a pointer to void
127 rather than a pointer to some particular structure.
128 The sole argument to the ParseAlloc() routine is a pointer to the
129 subroutine used to allocate memory. Typically this means ``malloc()''.</p>
131 <p>After a program is finished using a parser, it can reclaim all
132 memory allocated by that parser by calling
133 <pre>
134 ParseFree(pParser, free);
135 </pre>
136 The first argument is the same pointer returned by ParseAlloc(). The
137 second argument is a pointer to the function used to release bulk
138 memory back to the system.</p>
140 <p>After a parser has been allocated using ParseAlloc(), the programmer
141 must supply the parser with a sequence of tokens (terminal symbols) to
142 be parsed. This is accomplished by calling the following function
143 once for each token:
144 <pre>
145 Parse(pParser, hTokenID, sTokenData, pArg);
146 </pre>
147 The first argument to the Parse() routine is the pointer returned by
148 ParseAlloc().
149 The second argument is a small positive integer that tells the parse the
150 type of the next token in the data stream.
151 There is one token type for each terminal symbol in the grammar.
152 The gram.h file generated by Lemon contains #define statements that
153 map symbolic terminal symbol names into appropriate integer values.
154 (A value of 0 for the second argument is a special flag to the
155 parser to indicate that the end of input has been reached.)
156 The third argument is the value of the given token. By default,
157 the type of the third argument is integer, but the grammar will
158 usually redefine this type to be some kind of structure.
159 Typically the second argument will be a broad category of tokens
160 such as ``identifier'' or ``number'' and the third argument will
161 be the name of the identifier or the value of the number.</p>
163 <p>The Parse() function may have either three or four arguments,
164 depending on the grammar. If the grammar specification file request
165 it, the Parse() function will have a fourth parameter that can be
166 of any type chosen by the programmer. The parser doesn't do anything
167 with this argument except to pass it through to action routines.
168 This is a convenient mechanism for passing state information down
169 to the action routines without having to use global variables.</p>
171 <p>A typical use of a Lemon parser might look something like the
172 following:
173 <pre>
174 01 ParseTree *ParseFile(const char *zFilename){
175 02 Tokenizer *pTokenizer;
176 03 void *pParser;
177 04 Token sToken;
178 05 int hTokenId;
179 06 ParserState sState;
180 07
181 08 pTokenizer = TokenizerCreate(zFilename);
182 09 pParser = ParseAlloc( malloc );
183 10 InitParserState(&sState);
184 11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){
185 12 Parse(pParser, hTokenId, sToken, &sState);
186 13 }
188 15 ParseFree(pParser, free );
189 16 TokenizerFree(pTokenizer);
190 17 return sState.treeRoot;
191 18 }
192 </pre>
193 This example shows a user-written routine that parses a file of
194 text and returns a pointer to the parse tree.
195 (We've omitted all error-handling from this example to keep it
196 simple.)
197 We assume the existence of some kind of tokenizer which is created
198 using TokenizerCreate() on line 8 and deleted by TokenizerFree()
200 next token from the input file and puts its type in the
201 integer variable hTokenId. The sToken variable is assumed to be
202 some kind of structure that contains details about each token,
203 such as its complete text, what line it occurs on, etc. </p>
205 <p>This example also assumes the existence of structure of type
206 ParserState that holds state information about a particular parse.
207 An instance of such a structure is created on line 6 and initialized
208 on line 10. A pointer to this structure is passed into the Parse()
209 routine as the optional 4th argument.
210 The action routine specified by the grammar for the parser can use
211 the ParserState structure to hold whatever information is useful and
212 appropriate. In the example, we note that the treeRoot field of
213 the ParserState structure is left pointing to the root of the parse
214 tree.</p>
216 <p>The core of this example as it relates to Lemon is as follows:
217 <pre>
218 ParseFile(){
219 pParser = ParseAlloc( malloc );
220 while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){
221 Parse(pParser, hTokenId, sToken);
222 }
223 Parse(pParser, 0, sToken);
224 ParseFree(pParser, free );
225 }
226 </pre>
227 Basically, what a program has to do to use a Lemon-generated parser
228 is first create the parser, then send it lots of tokens obtained by
229 tokenizing an input source. When the end of input is reached, the
230 Parse() routine should be called one last time with a token type
231 of 0. This step is necessary to inform the parser that the end of
232 input has been reached. Finally, we reclaim memory used by the
233 parser by calling ParseFree().</p>
235 <p>There is one other interface routine that should be mentioned
236 before we move on.
237 The ParseTrace() function can be used to generate debugging output
238 from the parser. A prototype for this routine is as follows:
239 <pre>
240 ParseTrace(FILE *stream, char *zPrefix);
241 </pre>
242 After this routine is called, a short (one-line) message is written
243 to the designated output stream every time the parser changes states
244 or calls an action routine. Each such message is prefaced using
245 the text given by zPrefix. This debugging output can be turned off
250 <p>Programmers who have previously used the yacc or bison parser
251 generator will notice several important differences between yacc and/or
252 bison and Lemon.
253 <ul>
254 <li>In yacc and bison, the parser calls the tokenizer. In Lemon,
255 the tokenizer calls the parser.
256 <li>Lemon uses no global variables. Yacc and bison use global variables
257 to pass information between the tokenizer and parser.
258 <li>Lemon allows multiple parsers to be running simultaneously. Yacc
259 and bison do not.
260 </ul>
261 These differences may cause some initial confusion for programmers
262 with prior yacc and bison experience.
263 But after years of experience using Lemon, I firmly
264 believe that the Lemon way of doing things is better.</p>
268 <p>The main purpose of the grammar specification file for Lemon is
269 to define the grammar for the parser. But the input file also
270 specifies additional information Lemon requires to do its job.
271 Most of the work in using Lemon is in writing an appropriate
272 grammar file.</p>
274 <p>The grammar file for lemon is, for the most part, free format.
275 It does not have sections or divisions like yacc or bison. Any
276 declaration can occur at any point in the file.
277 Lemon ignores whitespace (except where it is needed to separate
278 tokens) and it honors the same commenting conventions as C and C++.</p>
282 <p>A terminal symbol (token) is any string of alphanumeric
283 and underscore characters
284 that begins with an upper case letter.
285 A terminal can contain lowercase letters after the first character,
286 but the usual convention is to make terminals all upper case.
287 A nonterminal, on the other hand, is any string of alphanumeric
288 and underscore characters than begins with a lower case letter.
289 Again, the usual convention is to make nonterminals use all lower
290 case letters.</p>
292 <p>In Lemon, terminal and nonterminal symbols do not need to
293 be declared or identified in a separate section of the grammar file.
294 Lemon is able to generate a list of all terminals and nonterminals
295 by examining the grammar rules, and it can always distinguish a
296 terminal from a nonterminal by checking the case of the first
297 character of the name.</p>
299 <p>Yacc and bison allow terminal symbols to have either alphanumeric
300 names or to be individual characters included in single quotes, like
301 this: ')' or '$'. Lemon does not allow this alternative form for
302 terminal symbols. With Lemon, all symbols, terminals and nonterminals,
303 must have alphanumeric names.</p>
307 <p>The main component of a Lemon grammar file is a sequence of grammar
308 rules.
309 Each grammar rule consists of a nonterminal symbol followed by
310 the special symbol ``::='' and then a list of terminals and/or nonterminals.
311 The rule is terminated by a period.
312 The list of terminals and nonterminals on the right-hand side of the
313 rule can be empty.
314 Rules can occur in any order, except that the left-hand side of the
315 first rule is assumed to be the start symbol for the grammar (unless
317 A typical sequence of grammar rules might look something like this:
318 <pre>
319 expr ::= expr PLUS expr.
320 expr ::= expr TIMES expr.
321 expr ::= LPAREN expr RPAREN.
322 expr ::= VALUE.
323 </pre>
324 </p>
326 <p>There is one non-terminal in this example, ``expr'', and five
327 terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'',
328 ``RPAREN'' and ``VALUE''.</p>
330 <p>Like yacc and bison, Lemon allows the grammar to specify a block
331 of C code that will be executed whenever a grammar rule is reduced
332 by the parser.
333 In Lemon, this action is specified by putting the C code (contained
335 period that closes the rule.
336 For example:
337 <pre>
338 expr ::= expr PLUS expr. { printf("Doing an addition...\n"); }
339 </pre>
340 </p>
342 <p>In order to be useful, grammar actions must normally be linked to
343 their associated grammar rules.
344 In yacc and bison, this is accomplished by embedding a ``$$'' in the
345 action to stand for the value of the left-hand side of the rule and
348 right-hand side of the rule.
349 This idea is very powerful, but it is also very error-prone. The
350 single most common source of errors in a yacc or bison grammar is
351 to miscount the number of symbols on the right-hand side of a grammar
354 <p>Lemon avoids the need to count grammar symbols by assigning symbolic
355 names to each symbol in a grammar rule and then using those symbolic
356 names in the action.
357 In yacc or bison, one would write this:
358 <pre>
360 </pre>
361 But in Lemon, the same rule becomes the following:
362 <pre>
363 expr(A) ::= expr(B) PLUS expr(C). { A = B+C; }
364 </pre>
365 In the Lemon rule, any symbol in parentheses after a grammar rule
366 symbol becomes a place holder for that symbol in the grammar rule.
367 This place holder can then be used in the associated C action to
368 stand for the value of that symbol.<p>
370 <p>The Lemon notation for linking a grammar rule with its reduce
371 action is superior to yacc/bison on several counts.
372 First, as mentioned above, the Lemon method avoids the need to
373 count grammar symbols.
374 Secondly, if a terminal or nonterminal in a Lemon grammar rule
375 includes a linking symbol in parentheses but that linking symbol
376 is not actually used in the reduce action, then an error message
377 is generated.
378 For example, the rule
379 <pre>
380 expr(A) ::= expr(B) PLUS expr(C). { A = B; }
381 </pre>
382 will generate an error because the linking symbol ``C'' is used
383 in the grammar rule but not in the reduce action.</p>
385 <p>The Lemon notation for linking grammar rules to reduce actions
386 also facilitates the use of destructors for reclaiming memory
387 allocated by the values of terminals and nonterminals on the
388 right-hand side of a rule.</p>
392 <p>Lemon resolves parsing ambiguities in exactly the same way as
393 yacc and bison. A shift-reduce conflict is resolved in favor
394 of the shift, and a reduce-reduce conflict is resolved by reducing
395 whichever rule comes first in the grammar file.</p>
397 <p>Just like in
398 yacc and bison, Lemon allows a measure of control
399 over the resolution of paring conflicts using precedence rules.
400 A precedence value can be assigned to any terminal symbol
401 using the %left, %right or %nonassoc directives. Terminal symbols
402 mentioned in earlier directives have a lower precedence that
403 terminal symbols mentioned in later directives. For example:</p>
405 <p><pre>
406 %left AND.
407 %left OR.
408 %nonassoc EQ NE GT GE LT LE.
409 %left PLUS MINUS.
410 %left TIMES DIVIDE MOD.
411 %right EXP NOT.
412 </pre></p>
414 <p>In the preceding sequence of directives, the AND operator is
415 defined to have the lowest precedence. The OR operator is one
416 precedence level higher. And so forth. Hence, the grammar would
417 attempt to group the ambiguous expression
418 <pre>
419 a AND b OR c
420 </pre>
421 like this
422 <pre>
423 a AND (b OR c).
424 </pre>
425 The associativity (left, right or nonassoc) is used to determine
426 the grouping when the precedence is the same. AND is left-associative
427 in our example, so
428 <pre>
429 a AND b AND c
430 </pre>
431 is parsed like this
432 <pre>
433 (a AND b) AND c.
434 </pre>
435 The EXP operator is right-associative, though, so
436 <pre>
437 a EXP b EXP c
438 </pre>
439 is parsed like this
440 <pre>
441 a EXP (b EXP c).
442 </pre>
443 The nonassoc precedence is used for non-associative operators.
444 So
445 <pre>
446 a EQ b EQ c
447 </pre>
448 is an error.</p>
450 <p>The precedence of non-terminals is transferred to rules as follows:
451 The precedence of a grammar rule is equal to the precedence of the
452 left-most terminal symbol in the rule for which a precedence is
453 defined. This is normally what you want, but in those cases where
454 you want to precedence of a grammar rule to be something different,
455 you can specify an alternative precedence symbol by putting the
456 symbol in square braces after the period at the end of the rule and
457 before any C-code. For example:</p>
459 <p><pre>
460 expr = MINUS expr. [NOT]
461 </pre></p>
463 <p>This rule has a precedence equal to that of the NOT symbol, not the
464 MINUS symbol as would have been the case by default.</p>
466 <p>With the knowledge of how precedence is assigned to terminal
467 symbols and individual
468 grammar rules, we can now explain precisely how parsing conflicts
469 are resolved in Lemon. Shift-reduce conflicts are resolved
470 as follows:
471 <ul>
472 <li> If either the token to be shifted or the rule to be reduced
473 lacks precedence information, then resolve in favor of the
474 shift, but report a parsing conflict.
475 <li> If the precedence of the token to be shifted is greater than
476 the precedence of the rule to reduce, then resolve in favor
477 of the shift. No parsing conflict is reported.
478 <li> If the precedence of the token it be shifted is less than the
479 precedence of the rule to reduce, then resolve in favor of the
480 reduce action. No parsing conflict is reported.
481 <li> If the precedences are the same and the shift token is
482 right-associative, then resolve in favor of the shift.
483 No parsing conflict is reported.
484 <li> If the precedences are the same the shift token is
485 left-associative, then resolve in favor of the reduce.
486 No parsing conflict is reported.
487 <li> Otherwise, resolve the conflict by doing the shift and
488 report the parsing conflict.
489 </ul>
490 Reduce-reduce conflicts are resolved this way:
491 <ul>
492 <li> If either reduce rule
493 lacks precedence information, then resolve in favor of the
494 rule that appears first in the grammar and report a parsing
495 conflict.
496 <li> If both rules have precedence and the precedence is different
497 then resolve the dispute in favor of the rule with the highest
498 precedence and do not report a conflict.
499 <li> Otherwise, resolve the conflict by reducing by the rule that
500 appears first in the grammar and report a parsing conflict.
501 </ul>
505 <p>The input grammar to Lemon consists of grammar rules and special
506 directives. We've described all the grammar rules, so now we'll
507 talk about the special directives.</p>
509 <p>Directives in lemon can occur in any order. You can put them before
510 the grammar rules, or after the grammar rules, or in the mist of the
511 grammar rules. It doesn't matter. The relative order of
512 directives used to assign precedence to terminals is important, but
513 other than that, the order of directives in Lemon is arbitrary.</p>
515 <p>Lemon supports the following special directives:
516 <ul>
537 </ul>
538 Each of these directives will be described separately in the
539 following sections:</p>
543 <p>The %code directive is used to specify addition C/C++ code that
544 is added to the end of the main output file. This is similar to
545 the %include directive except that %include is inserted at the
546 beginning of the main output file.</p>
548 <p>%code is typically used to include some action routines or perhaps
549 a tokenizer as part of the output file.</p>
553 <p>The %default_destructor directive specifies a destructor to
554 use for non-terminals that do not have their own destructor
555 specified by a separate %destructor directive. See the documentation
556 on the %destructor directive below for additional information.</p>
558 <p>In some grammers, many different non-terminal symbols have the
559 same datatype and hence the same destructor. This directive is
560 a convenience way to specify the same destructor for all those
561 non-terminals using a single statement.</p>
565 <p>The %default_type directive specifies the datatype of non-terminal
566 symbols that do no have their own datatype defined using a separate
567 %type directive. See the documentation on %type below for addition
568 information.</p>
572 <p>The %destructor directive is used to specify a destructor for
573 a non-terminal symbol.
574 (See also the %token_destructor directive which is used to
575 specify a destructor for terminal symbols.)</p>
577 <p>A non-terminal's destructor is called to dispose of the
578 non-terminal's value whenever the non-terminal is popped from
579 the stack. This includes all of the following circumstances:
580 <ul>
581 <li> When a rule reduces and the value of a non-terminal on
582 the right-hand side is not linked to C code.
583 <li> When the stack is popped during error processing.
584 <li> When the ParseFree() function runs.
585 </ul>
586 The destructor can do whatever it wants with the value of
587 the non-terminal, but its design is to deallocate memory
588 or other resources held by that non-terminal.</p>
590 <p>Consider an example:
591 <pre>
592 %type nt {void*}
593 %destructor nt { free($$); }
594 nt(A) ::= ID NUM. { A = malloc( 100 ); }
595 </pre>
596 This example is a bit contrived but it serves to illustrate how
597 destructors work. The example shows a non-terminal named
598 ``nt'' that holds values of type ``void*''. When the rule for
599 an ``nt'' reduces, it sets the value of the non-terminal to
600 space obtained from malloc(). Later, when the nt non-terminal
601 is popped from the stack, the destructor will fire and call
602 free() on this malloced space, thus avoiding a memory leak.
603 (Note that the symbol ``$$'' in the destructor code is replaced
604 by the value of the non-terminal.)</p>
606 <p>It is important to note that the value of a non-terminal is passed
607 to the destructor whenever the non-terminal is removed from the
608 stack, unless the non-terminal is used in a C-code action. If
609 the non-terminal is used by C-code, then it is assumed that the
610 C-code will take care of destroying it if it should really
611 be destroyed. More commonly, the value is used to build some
612 larger structure and we don't want to destroy it, which is why
613 the destructor is not called in this circumstance.</p>
615 <p>By appropriate use of destructors, it is possible to
616 build a parser using Lemon that can be used within a long-running
617 program, such as a GUI, that will not leak memory or other resources.
618 To do the same using yacc or bison is much more difficult.</p>
622 The %extra_argument directive instructs Lemon to add a 4th parameter
623 to the parameter list of the Parse() function it generates. Lemon
624 doesn't do anything itself with this extra argument, but it does
625 make the argument available to C-code action routines, destructors,
626 and so forth. For example, if the grammar file contains:</p>
628 <p><pre>
629 %extra_argument { MyStruct *pAbc }
630 </pre></p>
633 of type ``MyStruct*'' and all action routines will have access to
634 a variable named ``pAbc'' that is the value of the 4th parameter
635 in the most recent call to Parse().</p>
639 <p>The %include directive specifies C code that is included at the
640 top of the generated parser. You can include any text you want --
641 the Lemon parser generator copies it blindly. If you have multiple
642 %include directives in your grammar file the value of the last
643 %include directive overwrites all the others.</p.
645 <p>The %include directive is very handy for getting some extra #include
646 preprocessor statements at the beginning of the generated parser.
647 For example:</p>
649 <p><pre>
651 </pre></p>
653 <p>This might be needed, for example, if some of the C actions in the
654 grammar call functions that are prototyed in unistd.h.</p>
658 The %left directive is used (along with the %right and
659 %nonassoc directives) to declare precedences of terminal
660 symbols. Every terminal symbol whose name appears after
661 a %left directive but before the next period (``.'') is
662 given the same left-associative precedence value. Subsequent
663 %left directives have higher precedence. For example:</p>
665 <p><pre>
666 %left AND.
667 %left OR.
668 %nonassoc EQ NE GT GE LT LE.
669 %left PLUS MINUS.
670 %left TIMES DIVIDE MOD.
671 %right EXP NOT.
672 </pre></p>
674 <p>Note the period that terminates each %left, %right or %nonassoc
675 directive.</p>
678 a large amount of stack space if you make heavy use or right-associative
679 operators. For this reason, it is recommended that you use %left
680 rather than %right whenever possible.</p>
684 <p>By default, the functions generated by Lemon all begin with the
685 five-character string ``Parse''. You can change this string to something
686 different using the %name directive. For instance:</p>
688 <p><pre>
689 %name Abcde
690 </pre></p>
692 <p>Putting this directive in the grammar file will cause Lemon to generate
693 functions named
694 <ul>
695 <li> AbcdeAlloc(),
696 <li> AbcdeFree(),
697 <li> AbcdeTrace(), and
698 <li> Abcde().
699 </ul>
700 The %name directive allows you to generator two or more different
701 parsers and link them all into the same executable.
702 </p>
706 <p>This directive is used to assign non-associative precedence to
707 one or more terminal symbols. See the section on precedence rules
708 or on the %left directive for additional information.</p>
712 <p>The %parse_accept directive specifies a block of C code that is
713 executed whenever the parser accepts its input string. To ``accept''
714 an input string means that the parser was able to process all tokens
715 without error.</p>
719 <p><pre>
720 %parse_accept {
721 printf("parsing complete!\n");
722 }
723 </pre></p>
728 <p>The %parse_failure directive specifies a block of C code that
729 is executed whenever the parser fails complete. This code is not
730 executed until the parser has tried and failed to resolve an input
731 error using is usual error recovery strategy. The routine is
732 only invoked when parsing is unable to continue.</p>
734 <p><pre>
735 %parse_failure {
736 fprintf(stderr,"Giving up. Parser is hopelessly lost...\n");
737 }
738 </pre></p>
742 <p>This directive is used to assign right-associative precedence to
743 one or more terminal symbols. See the section on precedence rules
744 or on the %left directive for additional information.</p>
748 <p>The %stack_overflow directive specifies a block of C code that
749 is executed if the parser's internal stack ever overflows. Typically
750 this just prints an error message. After a stack overflow, the parser
751 will be unable to continue and must be reset.</p>
753 <p><pre>
754 %stack_overflow {
755 fprintf(stderr,"Giving up. Parser stack overflow\n");
756 }
757 </pre></p>
759 <p>You can help prevent parser stack overflows by avoiding the use
760 of right recursion and right-precedence operators in your grammar.
761 Use left recursion and and left-precedence operators instead, to
762 encourage rules to reduce sooner and keep the stack size down.
763 For example, do rules like this:
764 <pre>
765 list ::= list element. // left-recursion. Good!
766 list ::= .
767 </pre>
768 Not like this:
769 <pre>
770 list ::= element list. // right-recursion. Bad!
771 list ::= .
772 </pre>
776 <p>If stack overflow is a problem and you can't resolve the trouble
777 by using left-recursion, then you might want to increase the size
778 of the parser's stack using this directive. Put an positive integer
779 after the %stack_size directive and Lemon will generate a parse
782 <p><pre>
783 %stack_size 2000
784 </pre></p>
788 <p>By default, the start-symbol for the grammar that Lemon generates
789 is the first non-terminal that appears in the grammar file. But you
790 can choose a different start-symbol using the %start_symbol directive.</p>
792 <p><pre>
793 %start_symbol prog
794 </pre></p>
798 <p>The %destructor directive assigns a destructor to a non-terminal
799 symbol. (See the description of the %destructor directive above.)
800 This directive does the same thing for all terminal symbols.</p>
802 <p>Unlike non-terminal symbols which may each have a different data type
803 for their values, terminals all use the same data type (defined by
804 the %token_type directive) and so they use a common destructor. Other
805 than that, the token destructor works just like the non-terminal
806 destructors.</p>
810 <p>Lemon generates #defines that assign small integer constants
811 to each terminal symbol in the grammar. If desired, Lemon will
812 add a prefix specified by this directive
813 to each of the #defines it generates.
814 So if the default output of Lemon looked like this:
815 <pre>
816 #define AND 1
817 #define MINUS 2
818 #define OR 3
819 #define PLUS 4
820 </pre>
821 You can insert a statement into the grammar like this:
822 <pre>
823 %token_prefix TOKEN_
824 </pre>
825 to cause Lemon to produce these symbols instead:
826 <pre>
827 #define TOKEN_AND 1
828 #define TOKEN_MINUS 2
829 #define TOKEN_OR 3
830 #define TOKEN_PLUS 4
831 </pre>
835 <p>These directives are used to specify the data types for values
836 on the parser's stack associated with terminal and non-terminal
837 symbols. The values of all terminal symbols must be of the same
838 type. This turns out to be the same data type as the 3rd parameter
839 to the Parse() function generated by Lemon. Typically, you will
840 make the value of a terminal symbol by a pointer to some kind of
841 token structure. Like this:</p>
843 <p><pre>
844 %token_type {Token*}
845 </pre></p>
847 <p>If the data type of terminals is not specified, the default value
848 is ``int''.</p>
850 <p>Non-terminal symbols can each have their own data types. Typically
851 the data type of a non-terminal is a pointer to the root of a parse-tree
852 structure that contains all information about that non-terminal.
853 For example:</p>
855 <p><pre>
856 %type expr {Expr*}
857 </pre></p>
859 <p>Each entry on the parser's stack is actually a union containing
860 instances of all data types for every non-terminal and terminal symbol.
861 Lemon will automatically use the correct element of this union depending
862 on what the corresponding non-terminal or terminal symbol is. But
863 the grammar designer should keep in mind that the size of the union
864 will be the size of its largest element. So if you have a single
866 entry parser stack will require 100K of heap space. If you are willing
867 and able to pay that price, fine. You just need to know.</p>
871 <p>After extensive experimentation over several years, it has been
872 discovered that the error recovery strategy used by yacc is about
873 as good as it gets. And so that is what Lemon uses.</p>
875 <p>When a Lemon-generated parser encounters a syntax error, it
876 first invokes the code specified by the %syntax_error directive, if
877 any. It then enters its error recovery strategy. The error recovery
878 strategy is to begin popping the parsers stack until it enters a
879 state where it is permitted to shift a special non-terminal symbol
880 named ``error''. It then shifts this non-terminal and continues
881 parsing. But the %syntax_error routine will not be called again
882 until at least three new tokens have been successfully shifted.</p>
884 <p>If the parser pops its stack until the stack is empty, and it still
885 is unable to shift the error symbol, then the %parse_failed routine
886 is invoked and the parser resets itself to its start state, ready
887 to begin parsing a new file. This is what will happen at the very
888 first syntax error, of course, if there are no instances of the
889 ``error'' non-terminal in your grammar.</p>
891 </body>
892 </html>