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2: <head>
3: <title>The Lemon Parser Generator</title>
4: </head>
5: <body bgcolor=white>
6: <h1 align=center>The Lemon Parser Generator</h1>
7:
8: <p>Lemon is an LALR(1) parser generator for C or C++.
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>
19:
20: <p>This document is an introduction to the Lemon
21: parser generator.</p>
22:
23: <h2>Theory of Operation</h2>
24:
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>
37:
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>
49:
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>
63:
64: <h3>Command Line Options</h3>
65:
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>
74: <li><tt>-b</tt>
75: <li><tt>-c</tt>
76: <li><tt>-g</tt>
77: <li><tt>-m</tt>
78: <li><tt>-q</tt>
79: <li><tt>-s</tt>
80: <li><tt>-x</tt>
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>
102: Parser statistics: 74 terminals, 70 nonterminals, 179 rules
103: 340 states, 2026 parser table entries, 0 conflicts
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>
107:
108: <h3>The Parser Interface</h3>
109:
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>
115:
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>
130:
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>
139:
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>
162:
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>
170:
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 }
187: 14 Parse(pParser, 0, sToken, &sState);
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()
199: on line 16. The GetNextToken() function on line 11 retrieves the
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>
204:
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>
215:
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>
234:
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
246: by calling ParseTrace() again with a first argument of NULL (0).</p>
247:
248: <h3>Differences With YACC and BISON</h3>
249:
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>
265:
266: <h2>Input File Syntax</h2>
267:
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>
273:
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>
279:
280: <h3>Terminals and Nonterminals</h3>
281:
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>
291:
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>
298:
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>
304:
305: <h3>Grammar Rules</h3>
306:
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
316: specified otherwise using the <tt>%start</tt> directive described below.)
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>
325:
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>
329:
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
334: within curly braces <tt>{...}</tt>) immediately after the
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>
341:
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 ``$$'' in the
345: action to stand for the value of the left-hand side of the rule and
346: symbols ``$1'', ``$2'', and so forth to stand for the value of
347: the terminal or nonterminal at position 1, 2 and so forth on the
348: right-hand side of the rule.
349: This idea is very powerful, but it is also very error-prone. The
350: single most common source of errors in a yacc or bison grammar is
351: to miscount the number of symbols on the right-hand side of a grammar
352: rule and say ``$7'' when you really mean ``$8''.</p>
353:
354: <p>Lemon avoids the need to count grammar symbols by assigning symbolic
355: names to each symbol in a grammar rule and then using those symbolic
356: names in the action.
357: In yacc or bison, one would write this:
358: <pre>
359: expr -> expr PLUS expr { $$ = $1 + $3; };
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>
369:
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>
384:
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>
389:
390: <h3>Precedence Rules</h3>
391:
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>
396:
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>
404:
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>
413:
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>
449:
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>
458:
459: <p><pre>
460: expr = MINUS expr. [NOT]
461: </pre></p>
462:
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>
465:
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 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>
502:
503: <h3>Special Directives</h3>
504:
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>
508:
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>
514:
515: <p>Lemon supports the following special directives:
516: <ul>
517: <li><tt>%code</tt>
518: <li><tt>%default_destructor</tt>
519: <li><tt>%default_type</tt>
520: <li><tt>%destructor</tt>
521: <li><tt>%extra_argument</tt>
522: <li><tt>%include</tt>
523: <li><tt>%left</tt>
524: <li><tt>%name</tt>
525: <li><tt>%nonassoc</tt>
526: <li><tt>%parse_accept</tt>
527: <li><tt>%parse_failure </tt>
528: <li><tt>%right</tt>
529: <li><tt>%stack_overflow</tt>
530: <li><tt>%stack_size</tt>
531: <li><tt>%start_symbol</tt>
532: <li><tt>%syntax_error</tt>
533: <li><tt>%token_destructor</tt>
534: <li><tt>%token_prefix</tt>
535: <li><tt>%token_type</tt>
536: <li><tt>%type</tt>
537: </ul>
538: Each of these directives will be described separately in the
539: following sections:</p>
540:
541: <h4>The <tt>%code</tt> directive</h4>
542:
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>
547:
548: <p>%code is typically used to include some action routines or perhaps
549: a tokenizer as part of the output file.</p>
550:
551: <h4>The <tt>%default_destructor</tt> directive</h4>
552:
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>
557:
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>
562:
563: <h4>The <tt>%default_type</tt> directive</h4>
564:
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>
569:
570: <h4>The <tt>%destructor</tt> directive</h4>
571:
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>
576:
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>
589:
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>
605:
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>
614:
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>
619:
620: <h4>The <tt>%extra_argument</tt> directive</h4>
621:
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>
627:
628: <p><pre>
629: %extra_argument { MyStruct *pAbc }
630: </pre></p>
631:
632: <p>Then the Parse() function generated will have an 4th parameter
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>
636:
637: <h4>The <tt>%include</tt> directive</h4>
638:
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.
644:
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>
648:
649: <p><pre>
650: %include {#include <unistd.h>}
651: </pre></p>
652:
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>
655:
656: <h4>The <tt>%left</tt> directive</h4>
657:
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>
664:
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>
673:
674: <p>Note the period that terminates each %left, %right or %nonassoc
675: directive.</p>
676:
677: <p>LALR(1) grammars can get into a situation where they require
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>
681:
682: <h4>The <tt>%name</tt> directive</h4>
683:
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>
687:
688: <p><pre>
689: %name Abcde
690: </pre></p>
691:
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>
703:
704: <h4>The <tt>%nonassoc</tt> directive</h4>
705:
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>
709:
710: <h4>The <tt>%parse_accept</tt> directive</h4>
711:
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>
716:
717: <p>For example:</p>
718:
719: <p><pre>
720: %parse_accept {
721: printf("parsing complete!\n");
722: }
723: </pre></p>
724:
725:
726: <h4>The <tt>%parse_failure</tt> directive</h4>
727:
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>
733:
734: <p><pre>
735: %parse_failure {
736: fprintf(stderr,"Giving up. Parser is hopelessly lost...\n");
737: }
738: </pre></p>
739:
740: <h4>The <tt>%right</tt> directive</h4>
741:
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>
745:
746: <h4>The <tt>%stack_overflow</tt> directive</h4>
747:
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>
752:
753: <p><pre>
754: %stack_overflow {
755: fprintf(stderr,"Giving up. Parser stack overflow\n");
756: }
757: </pre></p>
758:
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>
773:
774: <h4>The <tt>%stack_size</tt> directive</h4>
775:
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
780: with a stack of the requested size. The default value is 100.</p>
781:
782: <p><pre>
783: %stack_size 2000
784: </pre></p>
785:
786: <h4>The <tt>%start_symbol</tt> directive</h4>
787:
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>
791:
792: <p><pre>
793: %start_symbol prog
794: </pre></p>
795:
796: <h4>The <tt>%token_destructor</tt> directive</h4>
797:
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>
801:
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>
807:
808: <h4>The <tt>%token_prefix</tt> directive</h4>
809:
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>
832:
833: <h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4>
834:
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>
842:
843: <p><pre>
844: %token_type {Token*}
845: </pre></p>
846:
847: <p>If the data type of terminals is not specified, the default value
848: is ``int''.</p>
849:
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>
854:
855: <p><pre>
856: %type expr {Expr*}
857: </pre></p>
858:
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
865: non-terminal whose data type requires 1K of storage, then your 100
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>
868:
869: <h3>Error Processing</h3>
870:
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>
874:
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>
883:
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>
890:
891: </body>
892: </html>
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