doc/lemon.html

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   3 <title>The Lemon Parser Generator</title>
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   6 <h1 align='center'>The Lemon Parser Generator</h1>
   7
   8 <p>Lemon is an LALR(1) parser generator for C.
   9 It does the same job as "bison" and "yacc".
  10 But Lemon is not a bison or yacc clone.  Lemon
  11 uses a different grammar syntax which is designed to
  12 reduce the number of coding errors.  Lemon also uses a
  13 parsing engine that is faster than yacc and
  14 bison and which is both reentrant and threadsafe.
  15 (Update: Since the previous sentence was written, bison
  16 has also been updated so that it too can generate a
  17 reentrant and threadsafe parser.)
  18 Lemon also implements features that can be used
  19 to eliminate resource leaks, making it suitable for use
  20 in long-running programs such as graphical user interfaces
  21 or embedded controllers.</p>
  22
  23 <p>This document is an introduction to the Lemon
  24 parser generator.</p>
  25
  26 <h2>Security Note</h2>
  27
  28 <p>The language parser code created by Lemon is very robust and
  29 is well-suited for use in internet-facing applications that need to
  30 safely process maliciously crafted inputs.
  31
  32 <p>The "lemon.exe" command-line tool itself works great when given a valid
  33 input grammar file and almost always gives helpful
  34 error messages for malformed inputs.  However,  it is possible for
  35 a malicious user to craft a grammar file that will cause
  36 lemon.exe to crash.
  37 We do not see this as a problem, as lemon.exe is not intended to be used
  38 with hostile inputs.
  39 To summarize:</p>
  40
  41 <ul>
  42 <li>Parser code generated by lemon &rarr; Robust and secure
  43 <li>The "lemon.exe" command line tool itself &rarr; Not so much
  44 </ul>
  45
  46 <h2>Theory of Operation</h2>
  47
  48 <p>The main goal of Lemon is to translate a context free grammar (CFG)
  49 for a particular language into C code that implements a parser for
  50 that language.
  51 The program has two inputs:
  52 <ul>
  53 <li>The grammar specification.
  54 <li>A parser template file.
  55 </ul>
  56 Typically, only the grammar specification is supplied by the programmer.
  57 Lemon comes with a default parser template which works fine for most
  58 applications.  But the user is free to substitute a different parser
  59 template if desired.</p>
  60
  61 <p>Depending on command-line options, Lemon will generate up to
  62 three output files.
  63 <ul>
  64 <li>C code to implement the parser.
  65 <li>A header file defining an integer ID for each terminal symbol.
  66 <li>An information file that describes the states of the generated parser
  67     automaton.
  68 </ul>
  69 By default, all three of these output files are generated.
  70 The header file is suppressed if the "-m" command-line option is
  71 used and the report file is omitted when "-q" is selected.</p>
  72
  73 <p>The grammar specification file uses a ".y" suffix, by convention.
  74 In the examples used in this document, we'll assume the name of the
  75 grammar file is "gram.y".  A typical use of Lemon would be the
  76 following command:
  77 <pre>
  78    lemon gram.y
  79 </pre>
  80 This command will generate three output files named "gram.c",
  81 "gram.h" and "gram.out".
  82 The first is C code to implement the parser.  The second
  83 is the header file that defines numerical values for all
  84 terminal symbols, and the last is the report that explains
  85 the states used by the parser automaton.</p>
  86
  87 <h3>Command Line Options</h3>
  88
  89 <p>The behavior of Lemon can be modified using command-line options.
  90 You can obtain a list of the available command-line options together
  91 with a brief explanation of what each does by typing
  92 <pre>
  93    lemon "-?"
  94 </pre>
  95 As of this writing, the following command-line options are supported:
  96 <ul>
  97 <li><b>-b</b>
  98 Show only the basis for each parser state in the report file.
  99 <li><b>-c</b>
 100 Do not compress the generated action tables.  The parser will be a
 101 little larger and slower, but it will detect syntax errors sooner.
 102 <li><b>-d</b><i>directory</i>
 103 Write all output files into <i>directory</i>.  Normally, output files
 104 are written into the directory that contains the input grammar file.
 105 <li><b>-D<i>name</i></b>
 106 Define C preprocessor macro <i>name</i>.  This macro is usable by
 107 "<tt><a href='#pifdef'>%ifdef</a></tt>" and
 108 "<tt><a href='#pifdef'>%ifndef</a></tt>" lines
 109 in the grammar file.
 110 <li><b>-g</b>
 111 Do not generate a parser.  Instead write the input grammar to standard
 112 output with all comments, actions, and other extraneous text removed.
 113 <li><b>-l</b>
 114 Omit "#line" directives in the generated parser C code.
 115 <li><b>-m</b>
 116 Cause the output C source code to be compatible with the "makeheaders"
 117 program.
 118 <li><b>-p</b>
 119 Display all conflicts that are resolved by
 120 <a href='#precrules'>precedence rules</a>.
 121 <li><b>-q</b>
 122 Suppress generation of the report file.
 123 <li><b>-r</b>
 124 Do not sort or renumber the parser states as part of optimization.
 125 <li><b>-s</b>
 126 Show parser statistics before existing.
 127 <li><b>-T<i>file</i></b>
 128 Use <i>file</i> as the template for the generated C-code parser implementation.
 129 <li><b>-x</b>
 130 Print the Lemon version number.
 131 </ul>
 132
 133 <h3>The Parser Interface</h3>
 134
 135 <p>Lemon doesn't generate a complete, working program.  It only generates
 136 a few subroutines that implement a parser.  This section describes
 137 the interface to those subroutines.  It is up to the programmer to
 138 call these subroutines in an appropriate way in order to produce a
 139 complete system.</p>
 140
 141 <p>Before a program begins using a Lemon-generated parser, the program
 142 must first create the parser.
 143 A new parser is created as follows:
 144 <pre>
 145    void *pParser = ParseAlloc( malloc );
 146 </pre>
 147 The ParseAlloc() routine allocates and initializes a new parser and
 148 returns a pointer to it.
 149 The actual data structure used to represent a parser is opaque &mdash;
 150 its internal structure is not visible or usable by the calling routine.
 151 For this reason, the ParseAlloc() routine returns a pointer to void
 152 rather than a pointer to some particular structure.
 153 The sole argument to the ParseAlloc() routine is a pointer to the
 154 subroutine used to allocate memory.  Typically this means malloc().</p>
 155
 156 <p>After a program is finished using a parser, it can reclaim all
 157 memory allocated by that parser by calling
 158 <pre>
 159    ParseFree(pParser, free);
 160 </pre>
 161 The first argument is the same pointer returned by ParseAlloc().  The
 162 second argument is a pointer to the function used to release bulk
 163 memory back to the system.</p>
 164
 165 <p>After a parser has been allocated using ParseAlloc(), the programmer
 166 must supply the parser with a sequence of tokens (terminal symbols) to
 167 be parsed.  This is accomplished by calling the following function
 168 once for each token:
 169 <pre>
 170    Parse(pParser, hTokenID, sTokenData, pArg);
 171 </pre>
 172 The first argument to the Parse() routine is the pointer returned by
 173 ParseAlloc().
 174 The second argument is a small positive integer that tells the parser the
 175 type of the next token in the data stream.
 176 There is one token type for each terminal symbol in the grammar.
 177 The gram.h file generated by Lemon contains #define statements that
 178 map symbolic terminal symbol names into appropriate integer values.
 179 A value of 0 for the second argument is a special flag to the
 180 parser to indicate that the end of input has been reached.
 181 The third argument is the value of the given token.  By default,
 182 the type of the third argument is "void*", but the grammar will
 183 usually redefine this type to be some kind of structure.
 184 Typically the second argument will be a broad category of tokens
 185 such as "identifier" or "number" and the third argument will
 186 be the name of the identifier or the value of the number.</p>
 187
 188 <p>The Parse() function may have either three or four arguments,
 189 depending on the grammar.  If the grammar specification file requests
 190 it (via the <tt><a href='#extraarg'>%extra_argument</a></tt> directive),
 191 the Parse() function will have a fourth parameter that can be
 192 of any type chosen by the programmer.  The parser doesn't do anything
 193 with this argument except to pass it through to action routines.
 194 This is a convenient mechanism for passing state information down
 195 to the action routines without having to use global variables.</p>
 196
 197 <p>A typical use of a Lemon parser might look something like the
 198 following:
 199 <pre>
 200     1 ParseTree *ParseFile(const char *zFilename){
 201     2    Tokenizer *pTokenizer;
 202     3    void *pParser;
 203     4    Token sToken;
 204     5    int hTokenId;
 205     6    ParserState sState;
 206     7
 207     8    pTokenizer = TokenizerCreate(zFilename);
 208     9    pParser = ParseAlloc( malloc );
 209    10    InitParserState(&amp;sState);
 210    11    while( GetNextToken(pTokenizer, &amp;hTokenId, &amp;sToken) ){
 211    12       Parse(pParser, hTokenId, sToken, &amp;sState);
 212    13    }
 213    14    Parse(pParser, 0, sToken, &amp;sState);
 214    15    ParseFree(pParser, free );
 215    16    TokenizerFree(pTokenizer);
 216    17    return sState.treeRoot;
 217    18 }
 218 </pre>
 219 This example shows a user-written routine that parses a file of
 220 text and returns a pointer to the parse tree.
 221 (All error-handling code is omitted from this example to keep it
 222 simple.)
 223 We assume the existence of some kind of tokenizer which is created
 224 using TokenizerCreate() on line 8 and deleted by TokenizerFree()
 225 on line 16.  The GetNextToken() function on line 11 retrieves the
 226 next token from the input file and puts its type in the
 227 integer variable hTokenId.  The sToken variable is assumed to be
 228 some kind of structure that contains details about each token,
 229 such as its complete text, what line it occurs on, etc.</p>
 230
 231 <p>This example also assumes the existence of structure of type
 232 ParserState that holds state information about a particular parse.
 233 An instance of such a structure is created on line 6 and initialized
 234 on line 10.  A pointer to this structure is passed into the Parse()
 235 routine as the optional 4th argument.
 236 The action routine specified by the grammar for the parser can use
 237 the ParserState structure to hold whatever information is useful and
 238 appropriate.  In the example, we note that the treeRoot field of
 239 the ParserState structure is left pointing to the root of the parse
 240 tree.</p>
 241
 242 <p>The core of this example as it relates to Lemon is as follows:
 243 <pre>
 244    ParseFile(){
 245       pParser = ParseAlloc( malloc );
 246       while( GetNextToken(pTokenizer,&amp;hTokenId, &amp;sToken) ){
 247          Parse(pParser, hTokenId, sToken);
 248       }
 249       Parse(pParser, 0, sToken);
 250       ParseFree(pParser, free );
 251    }
 252 </pre>
 253 Basically, what a program has to do to use a Lemon-generated parser
 254 is first create the parser, then send it lots of tokens obtained by
 255 tokenizing an input source.  When the end of input is reached, the
 256 Parse() routine should be called one last time with a token type
 257 of 0.  This step is necessary to inform the parser that the end of
 258 input has been reached.  Finally, we reclaim memory used by the
 259 parser by calling ParseFree().</p>
 260
 261 <p>There is one other interface routine that should be mentioned
 262 before we move on.
 263 The ParseTrace() function can be used to generate debugging output
 264 from the parser.  A prototype for this routine is as follows:
 265 <pre>
 266    ParseTrace(FILE *stream, char *zPrefix);
 267 </pre>
 268 After this routine is called, a short (one-line) message is written
 269 to the designated output stream every time the parser changes states
 270 or calls an action routine.  Each such message is prefaced using
 271 the text given by zPrefix.  This debugging output can be turned off
 272 by calling ParseTrace() again with a first argument of NULL (0).</p>
 273
 274 <h3>Differences With YACC and BISON</h3>
 275
 276 <p>Programmers who have previously used the yacc or bison parser
 277 generator will notice several important differences between yacc and/or
 278 bison and Lemon.
 279 <ul>
 280 <li>In yacc and bison, the parser calls the tokenizer.  In Lemon,
 281     the tokenizer calls the parser.
 282 <li>Lemon uses no global variables.  Yacc and bison use global variables
 283     to pass information between the tokenizer and parser.
 284 <li>Lemon allows multiple parsers to be running simultaneously.  Yacc
 285     and bison do not.
 286 </ul>
 287 These differences may cause some initial confusion for programmers
 288 with prior yacc and bison experience.
 289 But after years of experience using Lemon, I firmly
 290 believe that the Lemon way of doing things is better.</p>
 291
 292 <p><i>Updated as of 2016-02-16:</i>
 293 The text above was written in the 1990s.
 294 We are told that Bison has lately been enhanced to support the
 295 tokenizer-calls-parser paradigm used by Lemon, and to obviate the
 296 need for global variables.</p>
 297
 298 <h2>Input File Syntax</h2>
 299
 300 <p>The main purpose of the grammar specification file for Lemon is
 301 to define the grammar for the parser.  But the input file also
 302 specifies additional information Lemon requires to do its job.
 303 Most of the work in using Lemon is in writing an appropriate
 304 grammar file.</p>
 305
 306 <p>The grammar file for Lemon is, for the most part, free format.
 307 It does not have sections or divisions like yacc or bison.  Any
 308 declaration can occur at any point in the file.
 309 Lemon ignores whitespace (except where it is needed to separate
 310 tokens), and it honors the same commenting conventions as C and C++.</p>
 311
 312 <h3>Terminals and Nonterminals</h3>
 313
 314 <p>A terminal symbol (token) is any string of alphanumeric
 315 and/or underscore characters
 316 that begins with an uppercase letter.
 317 A terminal can contain lowercase letters after the first character,
 318 but the usual convention is to make terminals all uppercase.
 319 A nonterminal, on the other hand, is any string of alphanumeric
 320 and underscore characters than begins with a lowercase letter.
 321 Again, the usual convention is to make nonterminals use all lowercase
 322 letters.</p>
 323
 324 <p>In Lemon, terminal and nonterminal symbols do not need to
 325 be declared or identified in a separate section of the grammar file.
 326 Lemon is able to generate a list of all terminals and nonterminals
 327 by examining the grammar rules, and it can always distinguish a
 328 terminal from a nonterminal by checking the case of the first
 329 character of the name.</p>
 330
 331 <p>Yacc and bison allow terminal symbols to have either alphanumeric
 332 names or to be individual characters included in single quotes, like
 333 this: ')' or '$'.  Lemon does not allow this alternative form for
 334 terminal symbols.  With Lemon, all symbols, terminals and nonterminals,
 335 must have alphanumeric names.</p>
 336
 337 <h3>Grammar Rules</h3>
 338
 339 <p>The main component of a Lemon grammar file is a sequence of grammar
 340 rules.
 341 Each grammar rule consists of a nonterminal symbol followed by
 342 the special symbol "::=" and then a list of terminals and/or nonterminals.
 343 The rule is terminated by a period.
 344 The list of terminals and nonterminals on the right-hand side of the
 345 rule can be empty.
 346 Rules can occur in any order, except that the left-hand side of the
 347 first rule is assumed to be the start symbol for the grammar (unless
 348 specified otherwise using the <tt><a href='#start_symbol'>%start_symbol</a></tt>
 349 directive described below.)
 350 A typical sequence of grammar rules might look something like this:
 351 <pre>
 352   expr ::= expr PLUS expr.
 353   expr ::= expr TIMES expr.
 354   expr ::= LPAREN expr RPAREN.
 355   expr ::= VALUE.
 356 </pre>
 357 </p>
 358
 359 <p>There is one non-terminal in this example, "expr", and five
 360 terminal symbols or tokens: "PLUS", "TIMES", "LPAREN",
 361 "RPAREN" and "VALUE".</p>
 362
 363 <p>Like yacc and bison, Lemon allows the grammar to specify a block
 364 of C code that will be executed whenever a grammar rule is reduced
 365 by the parser.
 366 In Lemon, this action is specified by putting the C code (contained
 367 within curly braces <tt>{...}</tt>) immediately after the
 368 period that closes the rule.
 369 For example:
 370 <pre>
 371   expr ::= expr PLUS expr.   { printf("Doing an addition...\n"); }
 372 </pre>
 373 </p>
 374
 375 <p>In order to be useful, grammar actions must normally be linked to
 376 their associated grammar rules.
 377 In yacc and bison, this is accomplished by embedding a "$$" in the
 378 action to stand for the value of the left-hand side of the rule and
 379 symbols "$1", "$2", and so forth to stand for the value of
 380 the terminal or nonterminal at position 1, 2 and so forth on the
 381 right-hand side of the rule.
 382 This idea is very powerful, but it is also very error-prone.  The
 383 single most common source of errors in a yacc or bison grammar is
 384 to miscount the number of symbols on the right-hand side of a grammar
 385 rule and say "$7" when you really mean "$8".</p>
 386
 387 <p>Lemon avoids the need to count grammar symbols by assigning symbolic
 388 names to each symbol in a grammar rule and then using those symbolic
 389 names in the action.
 390 In yacc or bison, one would write this:
 391 <pre>
 392   expr -&gt; expr PLUS expr  { $$ = $1 + $3; };
 393 </pre>
 394 But in Lemon, the same rule becomes the following:
 395 <pre>
 396   expr(A) ::= expr(B) PLUS expr(C).  { A = B+C; }
 397 </pre>
 398 In the Lemon rule, any symbol in parentheses after a grammar rule
 399 symbol becomes a place holder for that symbol in the grammar rule.
 400 This place holder can then be used in the associated C action to
 401 stand for the value of that symbol.<p>
 402
 403 <p>The Lemon notation for linking a grammar rule with its reduce
 404 action is superior to yacc/bison on several counts.
 405 First, as mentioned above, the Lemon method avoids the need to
 406 count grammar symbols.
 407 Secondly, if a terminal or nonterminal in a Lemon grammar rule
 408 includes a linking symbol in parentheses but that linking symbol
 409 is not actually used in the reduce action, then an error message
 410 is generated.
 411 For example, the rule
 412 <pre>
 413   expr(A) ::= expr(B) PLUS expr(C).  { A = B; }
 414 </pre>
 415 will generate an error because the linking symbol "C" is used
 416 in the grammar rule but not in the reduce action.</p>
 417
 418 <p>The Lemon notation for linking grammar rules to reduce actions
 419 also facilitates the use of destructors for reclaiming memory
 420 allocated by the values of terminals and nonterminals on the
 421 right-hand side of a rule.</p>
 422
 423 <a name='precrules'></a>
 424 <h3>Precedence Rules</h3>
 425
 426 <p>Lemon resolves parsing ambiguities in exactly the same way as
 427 yacc and bison.  A shift-reduce conflict is resolved in favor
 428 of the shift, and a reduce-reduce conflict is resolved by reducing
 429 whichever rule comes first in the grammar file.</p>
 430
 431 <p>Just like in
 432 yacc and bison, Lemon allows a measure of control
 433 over the resolution of parsing conflicts using precedence rules.
 434 A precedence value can be assigned to any terminal symbol
 435 using the
 436 <tt><a href='#pleft'>%left</a></tt>,
 437 <tt><a href='#pright'>%right</a></tt> or
 438 <tt><a href='#pnonassoc'>%nonassoc</a></tt> directives.  Terminal symbols
 439 mentioned in earlier directives have a lower precedence than
 440 terminal symbols mentioned in later directives.  For example:</p>
 441
 442 <p><pre>
 443    %left AND.
 444    %left OR.
 445    %nonassoc EQ NE GT GE LT LE.
 446    %left PLUS MINUS.
 447    %left TIMES DIVIDE MOD.
 448    %right EXP NOT.
 449 </pre></p>
 450
 451 <p>In the preceding sequence of directives, the AND operator is
 452 defined to have the lowest precedence.  The OR operator is one
 453 precedence level higher.  And so forth.  Hence, the grammar would
 454 attempt to group the ambiguous expression
 455 <pre>
 456      a AND b OR c
 457 </pre>
 458 like this
 459 <pre>
 460      a AND (b OR c).
 461 </pre>
 462 The associativity (left, right or nonassoc) is used to determine
 463 the grouping when the precedence is the same.  AND is left-associative
 464 in our example, so
 465 <pre>
 466      a AND b AND c
 467 </pre>
 468 is parsed like this
 469 <pre>
 470      (a AND b) AND c.
 471 </pre>
 472 The EXP operator is right-associative, though, so
 473 <pre>
 474      a EXP b EXP c
 475 </pre>
 476 is parsed like this
 477 <pre>
 478      a EXP (b EXP c).
 479 </pre>
 480 The nonassoc precedence is used for non-associative operators.
 481 So
 482 <pre>
 483      a EQ b EQ c
 484 </pre>
 485 is an error.</p>
 486
 487 <p>The precedence of non-terminals is transferred to rules as follows:
 488 The precedence of a grammar rule is equal to the precedence of the
 489 left-most terminal symbol in the rule for which a precedence is
 490 defined.  This is normally what you want, but in those cases where
 491 you want to precedence of a grammar rule to be something different,
 492 you can specify an alternative precedence symbol by putting the
 493 symbol in square braces after the period at the end of the rule and
 494 before any C-code.  For example:</p>
 495
 496 <p><pre>
 497    expr = MINUS expr.  [NOT]
 498 </pre></p>
 499
 500 <p>This rule has a precedence equal to that of the NOT symbol, not the
 501 MINUS symbol as would have been the case by default.</p>
 502
 503 <p>With the knowledge of how precedence is assigned to terminal
 504 symbols and individual
 505 grammar rules, we can now explain precisely how parsing conflicts
 506 are resolved in Lemon.  Shift-reduce conflicts are resolved
 507 as follows:
 508 <ul>
 509 <li> If either the token to be shifted or the rule to be reduced
 510      lacks precedence information, then resolve in favor of the
 511      shift, but report a parsing conflict.
 512 <li> If the precedence of the token to be shifted is greater than
 513      the precedence of the rule to reduce, then resolve in favor
 514      of the shift.  No parsing conflict is reported.
 515 <li> If the precedence of the token to be shifted is less than the
 516      precedence of the rule to reduce, then resolve in favor of the
 517      reduce action.  No parsing conflict is reported.
 518 <li> If the precedences are the same and the shift token is
 519      right-associative, then resolve in favor of the shift.
 520      No parsing conflict is reported.
 521 <li> If the precedences are the same and the shift token is
 522      left-associative, then resolve in favor of the reduce.
 523      No parsing conflict is reported.
 524 <li> Otherwise, resolve the conflict by doing the shift, and
 525      report a parsing conflict.
 526 </ul>
 527 Reduce-reduce conflicts are resolved this way:
 528 <ul>
 529 <li> If either reduce rule
 530      lacks precedence information, then resolve in favor of the
 531      rule that appears first in the grammar, and report a parsing
 532      conflict.
 533 <li> If both rules have precedence and the precedence is different,
 534      then resolve the dispute in favor of the rule with the highest
 535      precedence, and do not report a conflict.
 536 <li> Otherwise, resolve the conflict by reducing by the rule that
 537      appears first in the grammar, and report a parsing conflict.
 538 </ul>
 539
 540 <h3>Special Directives</h3>
 541
 542 <p>The input grammar to Lemon consists of grammar rules and special
 543 directives.  We've described all the grammar rules, so now we'll
 544 talk about the special directives.</p>
 545
 546 <p>Directives in Lemon can occur in any order.  You can put them before
 547 the grammar rules, or after the grammar rules, or in the midst of the
 548 grammar rules.  It doesn't matter.  The relative order of
 549 directives used to assign precedence to terminals is important, but
 550 other than that, the order of directives in Lemon is arbitrary.</p>
 551
 552 <p>Lemon supports the following special directives:
 553 <ul>
 554 <li><tt><a href='#pcode'>%code</a></tt>
 555 <li><tt><a href='#default_destructor'>%default_destructor</a></tt>
 556 <li><tt><a href='#default_type'>%default_type</a></tt>
 557 <li><tt><a href='#destructor'>%destructor</a></tt>
 558 <li><tt><a href='#pifdef'>%endif</a></tt>
 559 <li><tt><a href='#extraarg'>%extra_argument</a></tt>
 560 <li><tt><a href='#pfallback'>%fallback</a></tt>
 561 <li><tt><a href='#pifdef'>%ifdef</a></tt>
 562 <li><tt><a href='#pifdef'>%ifndef</a></tt>
 563 <li><tt><a href='#pinclude'>%include</a></tt>
 564 <li><tt><a href='#pleft'>%left</a></tt>
 565 <li><tt><a href='#pname'>%name</a></tt>
 566 <li><tt><a href='#pnonassoc'>%nonassoc</a></tt>
 567 <li><tt><a href='#parse_accept'>%parse_accept</a></tt>
 568 <li><tt><a href='#parse_failure'>%parse_failure</a></tt>
 569 <li><tt><a href='#pright'>%right</a></tt>
 570 <li><tt><a href='#stack_overflow'>%stack_overflow</a></tt>
 571 <li><tt><a href='#stack_size'>%stack_size</a></tt>
 572 <li><tt><a href='#start_symbol'>%start_symbol</a></tt>
 573 <li><tt><a href='#syntax_error'>%syntax_error</a></tt>
 574 <li><tt><a href='#token_class'>%token_class</a></tt>
 575 <li><tt><a href='#token_destructor'>%token_destructor</a></tt>
 576 <li><tt><a href='#token_prefix'>%token_prefix</a></tt>
 577 <li><tt><a href='#token_type'>%token_type</a></tt>
 578 <li><tt><a href='#ptype'>%type</a></tt>
 579 <li><tt><a href='#pwildcard'>%wildcard</a></tt>
 580 </ul>
 581 Each of these directives will be described separately in the
 582 following sections:</p>
 583
 584 <a name='pcode'></a>
 585 <h4>The <tt>%code</tt> directive</h4>
 586
 587 <p>The <tt>%code</tt> directive is used to specify additional C code that
 588 is added to the end of the main output file.  This is similar to
 589 the <tt><a href='#pinclude'>%include</a></tt> directive except that
 590 <tt>%include</tt> is inserted at the beginning of the main output file.</p>
 591
 592 <p><tt>%code</tt> is typically used to include some action routines or perhaps
 593 a tokenizer or even the "main()" function
 594 as part of the output file.</p>
 595
 596 <a name='default_destructor'></a>
 597 <h4>The <tt>%default_destructor</tt> directive</h4>
 598
 599 <p>The <tt>%default_destructor</tt> directive specifies a destructor to
 600 use for non-terminals that do not have their own destructor
 601 specified by a separate <tt>%destructor</tt> directive.  See the documentation
 602 on the <tt><a name='#destructor'>%destructor</a></tt> directive below for
 603 additional information.</p>
 604
 605 <p>In some grammars, many different non-terminal symbols have the
 606 same data type and hence the same destructor.  This directive is
 607 a convenient way to specify the same destructor for all those
 608 non-terminals using a single statement.</p>
 609
 610 <a name='default_type'></a>
 611 <h4>The <tt>%default_type</tt> directive</h4>
 612
 613 <p>The <tt>%default_type</tt> directive specifies the data type of non-terminal
 614 symbols that do not have their own data type defined using a separate
 615 <tt><a href='#ptype'>%type</a></tt> directive.</p>
 616
 617 <a name='destructor'></a>
 618 <h4>The <tt>%destructor</tt> directive</h4>
 619
 620 <p>The <tt>%destructor</tt> directive is used to specify a destructor for
 621 a non-terminal symbol.
 622 (See also the <tt><a href='#token_destructor'>%token_destructor</a></tt>
 623 directive which is used to specify a destructor for terminal symbols.)</p>
 624
 625 <p>A non-terminal's destructor is called to dispose of the
 626 non-terminal's value whenever the non-terminal is popped from
 627 the stack.  This includes all of the following circumstances:
 628 <ul>
 629 <li> When a rule reduces and the value of a non-terminal on
 630      the right-hand side is not linked to C code.
 631 <li> When the stack is popped during error processing.
 632 <li> When the ParseFree() function runs.
 633 </ul>
 634 The destructor can do whatever it wants with the value of
 635 the non-terminal, but its design is to deallocate memory
 636 or other resources held by that non-terminal.</p>
 637
 638 <p>Consider an example:
 639 <pre>
 640    %type nt {void*}
 641    %destructor nt { free($$); }
 642    nt(A) ::= ID NUM.   { A = malloc( 100 ); }
 643 </pre>
 644 This example is a bit contrived, but it serves to illustrate how
 645 destructors work.  The example shows a non-terminal named
 646 "nt" that holds values of type "void*".  When the rule for
 647 an "nt" reduces, it sets the value of the non-terminal to
 648 space obtained from malloc().  Later, when the nt non-terminal
 649 is popped from the stack, the destructor will fire and call
 650 free() on this malloced space, thus avoiding a memory leak.
 651 (Note that the symbol "$$" in the destructor code is replaced
 652 by the value of the non-terminal.)</p>
 653
 654 <p>It is important to note that the value of a non-terminal is passed
 655 to the destructor whenever the non-terminal is removed from the
 656 stack, unless the non-terminal is used in a C-code action.  If
 657 the non-terminal is used by C-code, then it is assumed that the
 658 C-code will take care of destroying it.
 659 More commonly, the value is used to build some
 660 larger structure, and we don't want to destroy it, which is why
 661 the destructor is not called in this circumstance.</p>
 662
 663 <p>Destructors help avoid memory leaks by automatically freeing
 664 allocated objects when they go out of scope.
 665 To do the same using yacc or bison is much more difficult.</p>
 666
 667 <a name='extraarg'></a>
 668 <h4>The <tt>%extra_argument</tt> directive</h4>
 669
 670 The <tt>%extra_argument</tt> directive instructs Lemon to add a 4th parameter
 671 to the parameter list of the Parse() function it generates.  Lemon
 672 doesn't do anything itself with this extra argument, but it does
 673 make the argument available to C-code action routines, destructors,
 674 and so forth.  For example, if the grammar file contains:</p>
 675
 676 <p><pre>
 677     %extra_argument { MyStruct *pAbc }
 678 </pre></p>
 679
 680 <p>Then the Parse() function generated will have an 4th parameter
 681 of type "MyStruct*" and all action routines will have access to
 682 a variable named "pAbc" that is the value of the 4th parameter
 683 in the most recent call to Parse().</p>
 684
 685 <p>The <tt>%extra_context</tt> directive works the same except that it
 686 is passed in on the ParseAlloc() or ParseInit() routines instead of
 687 on Parse().
 688
 689 <a name='extractx'></a>
 690 <h4>The <tt>%extra_context</tt> directive</h4>
 691
 692 The <tt>%extra_context</tt> directive instructs Lemon to add a 2th parameter
 693 to the parameter list of the ParseAlloc() and ParseInif() functions.  Lemon
 694 doesn't do anything itself with these extra argument, but it does
 695 store the value make it available to C-code action routines, destructors,
 696 and so forth.  For example, if the grammar file contains:</p>
 697
 698 <p><pre>
 699     %extra_context { MyStruct *pAbc }
 700 </pre></p>
 701
 702 <p>Then the ParseAlloc() and ParseInit() functions will have an 2th parameter
 703 of type "MyStruct*" and all action routines will have access to
 704 a variable named "pAbc" that is the value of that 2th parameter.</p>
 705
 706 <p>The <tt>%extra_argument</tt> directive works the same except that it
 707 is passed in on the Parse() routine instead of on ParseAlloc()/ParseInit().
 708
 709 <a name='pfallback'></a>
 710 <h4>The <tt>%fallback</tt> directive</h4>
 711
 712 <p>The <tt>%fallback</tt> directive specifies an alternative meaning for one
 713 or more tokens.  The alternative meaning is tried if the original token
 714 would have generated a syntax error.</p>
 715
 716 <p>The <tt>%fallback</tt> directive was added to support robust parsing of SQL
 717 syntax in <a href='https://www.sqlite.org/'>SQLite</a>.
 718 The SQL language contains a large assortment of keywords, each of which
 719 appears as a different token to the language parser.  SQL contains so
 720 many keywords that it can be difficult for programmers to keep up with
 721 them all.  Programmers will, therefore, sometimes mistakenly use an
 722 obscure language keyword for an identifier.  The <tt>%fallback</tt> directive
 723 provides a mechanism to tell the parser:  "If you are unable to parse
 724 this keyword, try treating it as an identifier instead."</p>
 725
 726 <p>The syntax of <tt>%fallback</tt> is as follows:
 727
 728 <blockquote>
 729 <tt>%fallback</tt> <i>ID</i> <i>TOKEN...</i> <b>.</b>
 730 </blockquote></p>
 731
 732 <p>In words, the <tt>%fallback</tt> directive is followed by a list of token
 733 names terminated by a period.
 734 The first token name is the fallback token &mdash; the
 735 token to which all the other tokens fall back to.  The second and subsequent
 736 arguments are tokens which fall back to the token identified by the first
 737 argument.</p>
 738
 739 <a name='pifdef'></a>
 740 <h4>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives</h4>
 741
 742 <p>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives
 743 are similar to #ifdef, #ifndef, and #endif in the C-preprocessor,
 744 just not as general.
 745 Each of these directives must begin at the left margin.  No whitespace
 746 is allowed between the "%" and the directive name.</p>
 747
 748 <p>Grammar text in between "<tt>%ifdef MACRO</tt>" and the next nested
 749 "<tt>%endif</tt>" is
 750 ignored unless the "-DMACRO" command-line option is used.  Grammar text
 751 betwen "<tt>%ifndef MACRO</tt>" and the next nested "<tt>%endif</tt>" is
 752 included except when the "-DMACRO" command-line option is used.</p>
 753
 754 <p>Note that the argument to <tt>%ifdef</tt> and <tt>%ifndef</tt> must
 755 be a single preprocessor symbol name, not a general expression.
 756 There is no "<tt>%else</tt>" directive.</p>
 757
 758
 759 <a name='pinclude'></a>
 760 <h4>The <tt>%include</tt> directive</h4>
 761
 762 <p>The <tt>%include</tt> directive specifies C code that is included at the
 763 top of the generated parser.  You can include any text you want &mdash;
 764 the Lemon parser generator copies it blindly.  If you have multiple
 765 <tt>%include</tt> directives in your grammar file, their values are concatenated
 766 so that all <tt>%include</tt> code ultimately appears near the top of the
 767 generated parser, in the same order as it appeared in the grammar.</p>
 768
 769 <p>The <tt>%include</tt> directive is very handy for getting some extra #include
 770 preprocessor statements at the beginning of the generated parser.
 771 For example:</p>
 772
 773 <p><pre>
 774    %include {#include &lt;unistd.h&gt;}
 775 </pre></p>
 776
 777 <p>This might be needed, for example, if some of the C actions in the
 778 grammar call functions that are prototyped in unistd.h.</p>
 779
 780 <p>Use the <tt><a href="#pcode">%code</a></tt> directive to add code to
 781 the end of the generated parser.</p>
 782
 783 <a name='pleft'></a>
 784 <h4>The <tt>%left</tt> directive</h4>
 785
 786 The <tt>%left</tt> directive is used (along with the
 787 <tt><a href='#pright'>%right</a></tt> and
 788 <tt><a href='#pnonassoc'>%nonassoc</a></tt> directives) to declare
 789 precedences of terminal symbols.
 790 Every terminal symbol whose name appears after
 791 a <tt>%left</tt> directive but before the next period (".") is
 792 given the same left-associative precedence value.  Subsequent
 793 <tt>%left</tt> directives have higher precedence.  For example:</p>
 794
 795 <p><pre>
 796    %left AND.
 797    %left OR.
 798    %nonassoc EQ NE GT GE LT LE.
 799    %left PLUS MINUS.
 800    %left TIMES DIVIDE MOD.
 801    %right EXP NOT.
 802 </pre></p>
 803
 804 <p>Note the period that terminates each <tt>%left</tt>,
 805 <tt>%right</tt> or <tt>%nonassoc</tt>
 806 directive.</p>
 807
 808 <p>LALR(1) grammars can get into a situation where they require
 809 a large amount of stack space if you make heavy use or right-associative
 810 operators.  For this reason, it is recommended that you use <tt>%left</tt>
 811 rather than <tt>%right</tt> whenever possible.</p>
 812
 813 <a name='pname'></a>
 814 <h4>The <tt>%name</tt> directive</h4>
 815
 816 <p>By default, the functions generated by Lemon all begin with the
 817 five-character string "Parse".  You can change this string to something
 818 different using the <tt>%name</tt> directive.  For instance:</p>
 819
 820 <p><pre>
 821    %name Abcde
 822 </pre></p>
 823
 824 <p>Putting this directive in the grammar file will cause Lemon to generate
 825 functions named
 826 <ul>
 827 <li> AbcdeAlloc(),
 828 <li> AbcdeFree(),
 829 <li> AbcdeTrace(), and
 830 <li> Abcde().
 831 </ul>
 832 The <tt>%name</tt> directive allows you to generate two or more different
 833 parsers and link them all into the same executable.</p>
 834
 835 <a name='pnonassoc'></a>
 836 <h4>The <tt>%nonassoc</tt> directive</h4>
 837
 838 <p>This directive is used to assign non-associative precedence to
 839 one or more terminal symbols.  See the section on
 840 <a href='#precrules'>precedence rules</a>
 841 or on the <tt><a href='#pleft'>%left</a></tt> directive
 842 for additional information.</p>
 843
 844 <a name='parse_accept'></a>
 845 <h4>The <tt>%parse_accept</tt> directive</h4>
 846
 847 <p>The <tt>%parse_accept</tt> directive specifies a block of C code that is
 848 executed whenever the parser accepts its input string.  To "accept"
 849 an input string means that the parser was able to process all tokens
 850 without error.</p>
 851
 852 <p>For example:</p>
 853
 854 <p><pre>
 855    %parse_accept {
 856       printf("parsing complete!\n");
 857    }
 858 </pre></p>
 859
 860 <a name='parse_failure'></a>
 861 <h4>The <tt>%parse_failure</tt> directive</h4>
 862
 863 <p>The <tt>%parse_failure</tt> directive specifies a block of C code that
 864 is executed whenever the parser fails complete.  This code is not
 865 executed until the parser has tried and failed to resolve an input
 866 error using is usual error recovery strategy.  The routine is
 867 only invoked when parsing is unable to continue.</p>
 868
 869 <p><pre>
 870    %parse_failure {
 871      fprintf(stderr,"Giving up.  Parser is hopelessly lost...\n");
 872    }
 873 </pre></p>
 874
 875 <a name='pright'></a>
 876 <h4>The <tt>%right</tt> directive</h4>
 877
 878 <p>This directive is used to assign right-associative precedence to
 879 one or more terminal symbols.  See the section on
 880 <a href='#precrules'>precedence rules</a>
 881 or on the <a href='#pleft'>%left</a> directive for additional information.</p>
 882
 883 <a name='stack_overflow'></a>
 884 <h4>The <tt>%stack_overflow</tt> directive</h4>
 885
 886 <p>The <tt>%stack_overflow</tt> directive specifies a block of C code that
 887 is executed if the parser's internal stack ever overflows.  Typically
 888 this just prints an error message.  After a stack overflow, the parser
 889 will be unable to continue and must be reset.</p>
 890
 891 <p><pre>
 892    %stack_overflow {
 893      fprintf(stderr,"Giving up.  Parser stack overflow\n");
 894    }
 895 </pre></p>
 896
 897 <p>You can help prevent parser stack overflows by avoiding the use
 898 of right recursion and right-precedence operators in your grammar.
 899 Use left recursion and and left-precedence operators instead to
 900 encourage rules to reduce sooner and keep the stack size down.
 901 For example, do rules like this:
 902 <pre>
 903    list ::= list element.      // left-recursion.  Good!
 904    list ::= .
 905 </pre>
 906 Not like this:
 907 <pre>
 908    list ::= element list.      // right-recursion.  Bad!
 909    list ::= .
 910 </pre></p>
 911
 912 <a name='stack_size'></a>
 913 <h4>The <tt>%stack_size</tt> directive</h4>
 914
 915 <p>If stack overflow is a problem and you can't resolve the trouble
 916 by using left-recursion, then you might want to increase the size
 917 of the parser's stack using this directive.  Put an positive integer
 918 after the <tt>%stack_size</tt> directive and Lemon will generate a parse
 919 with a stack of the requested size.  The default value is 100.</p>
 920
 921 <p><pre>
 922    %stack_size 2000
 923 </pre></p>
 924
 925 <a name='start_symbol'></a>
 926 <h4>The <tt>%start_symbol</tt> directive</h4>
 927
 928 <p>By default, the start symbol for the grammar that Lemon generates
 929 is the first non-terminal that appears in the grammar file.  But you
 930 can choose a different start symbol using the
 931 <tt>%start_symbol</tt> directive.</p>
 932
 933 <p><pre>
 934    %start_symbol  prog
 935 </pre></p>
 936
 937 <a name='syntax_error'></a>
 938 <h4>The <tt>%syntax_error</tt> directive</h4>
 939
 940 <p>See <a href='#error_processing'>Error Processing</a>.</p>
 941
 942 <a name='token_class'></a>
 943 <h4>The <tt>%token_class</tt> directive</h4>
 944
 945 <p>Undocumented.  Appears to be related to the MULTITERMINAL concept.
 946 <a href='http://sqlite.org/src/fdiff?v1=796930d5fc2036c7&v2=624b24c5dc048e09&sbs=0'>Implementation</a>.</p>
 947
 948 <a name='token_destructor'></a>
 949 <h4>The <tt>%token_destructor</tt> directive</h4>
 950
 951 <p>The <tt>%destructor</tt> directive assigns a destructor to a non-terminal
 952 symbol.  (See the description of the
 953 <tt><a href='%destructor'>%destructor</a></tt> directive above.)
 954 The <tt>%token_destructor</tt> directive does the same thing
 955 for all terminal symbols.</p>
 956
 957 <p>Unlike non-terminal symbols which may each have a different data type
 958 for their values, terminals all use the same data type (defined by
 959 the <tt><a href='#token_type'>%token_type</a></tt> directive)
 960 and so they use a common destructor.
 961 Other than that, the token destructor works just like the non-terminal
 962 destructors.</p>
 963
 964 <a name='token_prefix'></a>
 965 <h4>The <tt>%token_prefix</tt> directive</h4>
 966
 967 <p>Lemon generates #defines that assign small integer constants
 968 to each terminal symbol in the grammar.  If desired, Lemon will
 969 add a prefix specified by this directive
 970 to each of the #defines it generates.</p>
 971
 972 <p>So if the default output of Lemon looked like this:
 973 <pre>
 974     #define AND              1
 975     #define MINUS            2
 976     #define OR               3
 977     #define PLUS             4
 978 </pre>
 979 You can insert a statement into the grammar like this:
 980 <pre>
 981     %token_prefix    TOKEN_
 982 </pre>
 983 to cause Lemon to produce these symbols instead:
 984 <pre>
 985     #define TOKEN_AND        1
 986     #define TOKEN_MINUS      2
 987     #define TOKEN_OR         3
 988     #define TOKEN_PLUS       4
 989 </pre></p>
 990
 991 <a name='token_type'></a><a name='ptype'></a>
 992 <h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4>
 993
 994 <p>These directives are used to specify the data types for values
 995 on the parser's stack associated with terminal and non-terminal
 996 symbols.  The values of all terminal symbols must be of the same
 997 type.  This turns out to be the same data type as the 3rd parameter
 998 to the Parse() function generated by Lemon.  Typically, you will
 999 make the value of a terminal symbol by a pointer to some kind of
1000 token structure.  Like this:</p>
1001
1002 <p><pre>
1003    %token_type    {Token*}
1004 </pre></p>
1005
1006 <p>If the data type of terminals is not specified, the default value
1007 is "void*".</p>
1008
1009 <p>Non-terminal symbols can each have their own data types.  Typically
1010 the data type of a non-terminal is a pointer to the root of a parse tree
1011 structure that contains all information about that non-terminal.
1012 For example:</p>
1013
1014 <p><pre>
1015    %type   expr  {Expr*}
1016 </pre></p>
1017
1018 <p>Each entry on the parser's stack is actually a union containing
1019 instances of all data types for every non-terminal and terminal symbol.
1020 Lemon will automatically use the correct element of this union depending
1021 on what the corresponding non-terminal or terminal symbol is.  But
1022 the grammar designer should keep in mind that the size of the union
1023 will be the size of its largest element.  So if you have a single
1024 non-terminal whose data type requires 1K of storage, then your 100
1025 entry parser stack will require 100K of heap space.  If you are willing
1026 and able to pay that price, fine.  You just need to know.</p>
1027
1028 <a name='pwildcard'></a>
1029 <h4>The <tt>%wildcard</tt> directive</h4>
1030
1031 <p>The <tt>%wildcard</tt> directive is followed by a single token name and a
1032 period.  This directive specifies that the identified token should
1033 match any input token.</p>
1034
1035 <p>When the generated parser has the choice of matching an input against
1036 the wildcard token and some other token, the other token is always used.
1037 The wildcard token is only matched if there are no alternatives.</p>
1038
1039 <a name='error_processing'></a>
1040 <h3>Error Processing</h3>
1041
1042 <p>After extensive experimentation over several years, it has been
1043 discovered that the error recovery strategy used by yacc is about
1044 as good as it gets.  And so that is what Lemon uses.</p>
1045
1046 <p>When a Lemon-generated parser encounters a syntax error, it
1047 first invokes the code specified by the <tt>%syntax_error</tt> directive, if
1048 any.  It then enters its error recovery strategy.  The error recovery
1049 strategy is to begin popping the parsers stack until it enters a
1050 state where it is permitted to shift a special non-terminal symbol
1051 named "error".  It then shifts this non-terminal and continues
1052 parsing.  The <tt>%syntax_error</tt> routine will not be called again
1053 until at least three new tokens have been successfully shifted.</p>
1054
1055 <p>If the parser pops its stack until the stack is empty, and it still
1056 is unable to shift the error symbol, then the
1057 <tt><a href='#parse_failure'>%parse_failure</a></tt> routine
1058 is invoked and the parser resets itself to its start state, ready
1059 to begin parsing a new file.  This is what will happen at the very
1060 first syntax error, of course, if there are no instances of the
1061 "error" non-terminal in your grammar.</p>
1062
1063 </body>
1064 </html>