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| 1 <html> |
| 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 lower class 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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