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| Comment: | Updates to the "lemon.html" document received from Andy Goth. |
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| User & Date: | drh 2017-09-20 09:09:34.192 |
Context
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2017-09-20
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| 10:47 | Improved resolution of large integer values in "CAST(x AS NUMERIC)". (check-in: 7f2bd4ff45 user: drh tags: trunk) | |
| 09:09 | Updates to the "lemon.html" document received from Andy Goth. (check-in: 5b2002f3df user: drh tags: trunk) | |
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2017-09-18
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| 18:17 | Add the sqlite3_mmap_warm() function as an extension in the ext/misc/mmapwarm.c source file. (check-in: 1b2de41453 user: drh tags: trunk) | |
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Changes to doc/lemon.html.
1 2 3 4 | <html> <head> <title>The Lemon Parser Generator</title> </head> | | | | | | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | <html> <head> <title>The Lemon Parser Generator</title> </head> <body bgcolor='white'> <h1 align='center'>The Lemon Parser Generator</h1> <p>Lemon is an LALR(1) parser generator for C. It does the same job as "bison" and "yacc". But Lemon is not a bison or yacc clone. Lemon uses a different grammar syntax which is designed to reduce the number of coding errors. Lemon also uses a parsing engine that is faster than yacc and bison and which is both reentrant and threadsafe. (Update: Since the previous sentence was written, bison has also been updated so that it too can generate a reentrant and threadsafe parser.) Lemon also implements features that can be used to eliminate resource leaks, making it suitable for use in long-running programs such as graphical user interfaces or embedded controllers.</p> <p>This document is an introduction to the Lemon parser generator.</p> <h2>Security Note</h2> |
| ︙ | ︙ | |||
54 55 56 57 58 59 60 | <li>A parser template file. </ul> Typically, only the grammar specification is supplied by the programmer. Lemon comes with a default parser template which works fine for most applications. But the user is free to substitute a different parser template if desired.</p> | | | | 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 |
<li>A parser template file.
</ul>
Typically, only the grammar specification is supplied by the programmer.
Lemon comes with a default parser template which works fine for most
applications. But the user is free to substitute a different parser
template if desired.</p>
<p>Depending on command-line options, Lemon will generate up to
three output files.
<ul>
<li>C code to implement the parser.
<li>A header file defining an integer ID for each terminal symbol.
<li>An information file that describes the states of the generated parser
automaton.
</ul>
By default, all three of these output files are generated.
|
| ︙ | ︙ | |||
86 87 88 89 90 91 92 | <h3>Command Line Options</h3> <p>The behavior of Lemon can be modified using command-line options. You can obtain a list of the available command-line options together with a brief explanation of what each does by typing <pre> | | | > | > > | | | | 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 | <h3>Command Line Options</h3> <p>The behavior of Lemon can be modified using command-line options. You can obtain a list of the available command-line options together with a brief explanation of what each does by typing <pre> lemon "-?" </pre> As of this writing, the following command-line options are supported: <ul> <li><b>-b</b> Show only the basis for each parser state in the report file. <li><b>-c</b> Do not compress the generated action tables. The parser will be a little larger and slower, but it will detect syntax errors sooner. <li><b>-D<i>name</i></b> Define C preprocessor macro <i>name</i>. This macro is usable by "<tt><a href='#pifdef'>%ifdef</a></tt>" and "<tt><a href='#pifdef'>%ifndef</a></tt>" lines in the grammar file. <li><b>-g</b> Do not generate a parser. Instead write the input grammar to standard output with all comments, actions, and other extraneous text removed. <li><b>-l</b> Omit "#line" directives in the generated parser C code. <li><b>-m</b> Cause the output C source code to be compatible with the "makeheaders" program. <li><b>-p</b> Display all conflicts that are resolved by <a href='#precrules'>precedence rules</a>. <li><b>-q</b> Suppress generation of the report file. <li><b>-r</b> Do not sort or renumber the parser states as part of optimization. <li><b>-s</b> Show parser statistics before existing. |
| ︙ | ︙ | |||
161 162 163 164 165 166 167 | be parsed. This is accomplished by calling the following function once for each token: <pre> Parse(pParser, hTokenID, sTokenData, pArg); </pre> The first argument to the Parse() routine is the pointer returned by ParseAlloc(). | | | | | | | | | | < > | | | | | | | | | | 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 |
be parsed. This is accomplished by calling the following function
once for each token:
<pre>
Parse(pParser, hTokenID, sTokenData, pArg);
</pre>
The first argument to the Parse() routine is the pointer returned by
ParseAlloc().
The second argument is a small positive integer that tells the parser the
type of the next token in the data stream.
There is one token type for each terminal symbol in the grammar.
The gram.h file generated by Lemon contains #define statements that
map symbolic terminal symbol names into appropriate integer values.
A value of 0 for the second argument is a special flag to the
parser to indicate that the end of input has been reached.
The third argument is the value of the given token. By default,
the type of the third argument is "void*", but the grammar will
usually redefine this type to be some kind of structure.
Typically the second argument will be a broad category of tokens
such as "identifier" or "number" and the third argument will
be the name of the identifier or the value of the number.</p>
<p>The Parse() function may have either three or four arguments,
depending on the grammar. If the grammar specification file requests
it (via the <tt><a href='#extraarg'>%extra_argument</a></tt> directive),
the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer. The parser doesn't do anything
with this argument except to pass it through to action routines.
This is a convenient mechanism for passing state information down
to the action routines without having to use global variables.</p>
<p>A typical use of a Lemon parser might look something like the
following:
<pre>
1 ParseTree *ParseFile(const char *zFilename){
2 Tokenizer *pTokenizer;
3 void *pParser;
4 Token sToken;
5 int hTokenId;
6 ParserState sState;
7
8 pTokenizer = TokenizerCreate(zFilename);
9 pParser = ParseAlloc( malloc );
10 InitParserState(&sState);
11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){
12 Parse(pParser, hTokenId, sToken, &sState);
13 }
14 Parse(pParser, 0, sToken, &sState);
15 ParseFree(pParser, free );
16 TokenizerFree(pTokenizer);
17 return sState.treeRoot;
18 }
</pre>
This example shows a user-written routine that parses a file of
text and returns a pointer to the parse tree.
(All error-handling code is omitted from this example to keep it
simple.)
We assume the existence of some kind of tokenizer which is created
using TokenizerCreate() on line 8 and deleted by TokenizerFree()
on line 16. The GetNextToken() function on line 11 retrieves the
next token from the input file and puts its type in the
integer variable hTokenId. The sToken variable is assumed to be
some kind of structure that contains details about each token,
such as its complete text, what line it occurs on, etc.</p>
<p>This example also assumes the existence of structure of type
ParserState that holds state information about a particular parse.
An instance of such a structure is created on line 6 and initialized
on line 10. A pointer to this structure is passed into the Parse()
routine as the optional 4th argument.
The action routine specified by the grammar for the parser can use
the ParserState structure to hold whatever information is useful and
appropriate. In the example, we note that the treeRoot field of
the ParserState structure is left pointing to the root of the parse
tree.</p>
<p>The core of this example as it relates to Lemon is as follows:
<pre>
ParseFile(){
pParser = ParseAlloc( malloc );
while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){
Parse(pParser, hTokenId, sToken);
}
Parse(pParser, 0, sToken);
ParseFree(pParser, free );
}
</pre>
Basically, what a program has to do to use a Lemon-generated parser
|
| ︙ | ︙ | |||
293 294 295 296 297 298 299 | <p>The main purpose of the grammar specification file for Lemon is to define the grammar for the parser. But the input file also specifies additional information Lemon requires to do its job. Most of the work in using Lemon is in writing an appropriate grammar file.</p> | | | | | | | | | | 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 | <p>The main purpose of the grammar specification file for Lemon is to define the grammar for the parser. But the input file also specifies additional information Lemon requires to do its job. Most of the work in using Lemon is in writing an appropriate grammar file.</p> <p>The grammar file for Lemon is, for the most part, free format. It does not have sections or divisions like yacc or bison. Any declaration can occur at any point in the file. Lemon ignores whitespace (except where it is needed to separate tokens), and it honors the same commenting conventions as C and C++.</p> <h3>Terminals and Nonterminals</h3> <p>A terminal symbol (token) is any string of alphanumeric and/or underscore characters that begins with an uppercase letter. A terminal can contain lowercase letters after the first character, but the usual convention is to make terminals all uppercase. A nonterminal, on the other hand, is any string of alphanumeric and underscore characters than begins with a lowercase letter. Again, the usual convention is to make nonterminals use all lowercase letters.</p> <p>In Lemon, terminal and nonterminal symbols do not need to be declared or identified in a separate section of the grammar file. Lemon is able to generate a list of all terminals and nonterminals by examining the grammar rules, and it can always distinguish a terminal from a nonterminal by checking the case of the first character of the name.</p> <p>Yacc and bison allow terminal symbols to have either alphanumeric |
| ︙ | ︙ | |||
335 336 337 338 339 340 341 | Each grammar rule consists of a nonterminal symbol followed by the special symbol "::=" and then a list of terminals and/or nonterminals. The rule is terminated by a period. The list of terminals and nonterminals on the right-hand side of the rule can be empty. Rules can occur in any order, except that the left-hand side of the first rule is assumed to be the start symbol for the grammar (unless | > | | 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 | Each grammar rule consists of a nonterminal symbol followed by the special symbol "::=" and then a list of terminals and/or nonterminals. The rule is terminated by a period. The list of terminals and nonterminals on the right-hand side of the rule can be empty. Rules can occur in any order, except that the left-hand side of the first rule is assumed to be the start symbol for the grammar (unless specified otherwise using the <tt><a href='#start_symbol'>%start_symbol</a></tt> directive described below.) A typical sequence of grammar rules might look something like this: <pre> expr ::= expr PLUS expr. expr ::= expr TIMES expr. expr ::= LPAREN expr RPAREN. expr ::= VALUE. </pre> |
| ︙ | ︙ | |||
378 379 380 381 382 383 384 | rule and say "$7" when you really mean "$8".</p> <p>Lemon avoids the need to count grammar symbols by assigning symbolic names to each symbol in a grammar rule and then using those symbolic names in the action. In yacc or bison, one would write this: <pre> | | | 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 |
rule and say "$7" when you really mean "$8".</p>
<p>Lemon avoids the need to count grammar symbols by assigning symbolic
names to each symbol in a grammar rule and then using those symbolic
names in the action.
In yacc or bison, one would write this:
<pre>
expr -> expr PLUS expr { $$ = $1 + $3; };
</pre>
But in Lemon, the same rule becomes the following:
<pre>
expr(A) ::= expr(B) PLUS expr(C). { A = B+C; }
</pre>
In the Lemon rule, any symbol in parentheses after a grammar rule
symbol becomes a place holder for that symbol in the grammar rule.
|
| ︙ | ︙ | |||
418 419 420 421 422 423 424 | <p>Lemon resolves parsing ambiguities in exactly the same way as yacc and bison. A shift-reduce conflict is resolved in favor of the shift, and a reduce-reduce conflict is resolved by reducing whichever rule comes first in the grammar file.</p> <p>Just like in | | | | | | | | | 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 | <p>Lemon resolves parsing ambiguities in exactly the same way as yacc and bison. A shift-reduce conflict is resolved in favor of the shift, and a reduce-reduce conflict is resolved by reducing whichever rule comes first in the grammar file.</p> <p>Just like in yacc and bison, Lemon allows a measure of control over the resolution of parsing conflicts using precedence rules. A precedence value can be assigned to any terminal symbol using the <tt><a href='#pleft'>%left</a></tt>, <tt><a href='#pright'>%right</a></tt> or <tt><a href='#pnonassoc'>%nonassoc</a></tt> directives. Terminal symbols mentioned in earlier directives have a lower precedence than terminal symbols mentioned in later directives. For example:</p> <p><pre> %left AND. %left OR. %nonassoc EQ NE GT GE LT LE. %left PLUS MINUS. |
| ︙ | ︙ | |||
501 502 503 504 505 506 507 |
<ul>
<li> If either the token to be shifted or the rule to be reduced
lacks precedence information, then resolve in favor of the
shift, but report a parsing conflict.
<li> If the precedence of the token to be shifted is greater than
the precedence of the rule to reduce, then resolve in favor
of the shift. No parsing conflict is reported.
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | < | | | 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 |
<ul>
<li> If either the token to be shifted or the rule to be reduced
lacks precedence information, then resolve in favor of the
shift, but report a parsing conflict.
<li> If the precedence of the token to be shifted is greater than
the precedence of the rule to reduce, then resolve in favor
of the shift. No parsing conflict is reported.
<li> If the precedence of the token to be shifted is less than the
precedence of the rule to reduce, then resolve in favor of the
reduce action. No parsing conflict is reported.
<li> If the precedences are the same and the shift token is
right-associative, then resolve in favor of the shift.
No parsing conflict is reported.
<li> If the precedences are the same and the shift token is
left-associative, then resolve in favor of the reduce.
No parsing conflict is reported.
<li> Otherwise, resolve the conflict by doing the shift, and
report a parsing conflict.
</ul>
Reduce-reduce conflicts are resolved this way:
<ul>
<li> If either reduce rule
lacks precedence information, then resolve in favor of the
rule that appears first in the grammar, and report a parsing
conflict.
<li> If both rules have precedence and the precedence is different,
then resolve the dispute in favor of the rule with the highest
precedence, and do not report a conflict.
<li> Otherwise, resolve the conflict by reducing by the rule that
appears first in the grammar, and report a parsing conflict.
</ul>
<h3>Special Directives</h3>
<p>The input grammar to Lemon consists of grammar rules and special
directives. We've described all the grammar rules, so now we'll
talk about the special directives.</p>
<p>Directives in Lemon can occur in any order. You can put them before
the grammar rules, or after the grammar rules, or in the midst of the
grammar rules. It doesn't matter. The relative order of
directives used to assign precedence to terminals is important, but
other than that, the order of directives in Lemon is arbitrary.</p>
<p>Lemon supports the following special directives:
<ul>
<li><tt><a href='#pcode'>%code</a></tt>
<li><tt><a href='#default_destructor'>%default_destructor</a></tt>
<li><tt><a href='#default_type'>%default_type</a></tt>
<li><tt><a href='#destructor'>%destructor</a></tt>
<li><tt><a href='#pifdef'>%endif</a></tt>
<li><tt><a href='#extraarg'>%extra_argument</a></tt>
<li><tt><a href='#pfallback'>%fallback</a></tt>
<li><tt><a href='#pifdef'>%ifdef</a></tt>
<li><tt><a href='#pifdef'>%ifndef</a></tt>
<li><tt><a href='#pinclude'>%include</a></tt>
<li><tt><a href='#pleft'>%left</a></tt>
<li><tt><a href='#pname'>%name</a></tt>
<li><tt><a href='#pnonassoc'>%nonassoc</a></tt>
<li><tt><a href='#parse_accept'>%parse_accept</a></tt>
<li><tt><a href='#parse_failure'>%parse_failure</a></tt>
<li><tt><a href='#pright'>%right</a></tt>
<li><tt><a href='#stack_overflow'>%stack_overflow</a></tt>
<li><tt><a href='#stack_size'>%stack_size</a></tt>
<li><tt><a href='#start_symbol'>%start_symbol</a></tt>
<li><tt><a href='#syntax_error'>%syntax_error</a></tt>
<li><tt><a href='#token_class'>%token_class</a></tt>
<li><tt><a href='#token_destructor'>%token_destructor</a></tt>
<li><tt><a href='#token_prefix'>%token_prefix</a></tt>
<li><tt><a href='#token_type'>%token_type</a></tt>
<li><tt><a href='#ptype'>%type</a></tt>
<li><tt><a href='#pwildcard'>%wildcard</a></tt>
</ul>
Each of these directives will be described separately in the
following sections:</p>
<a name='pcode'></a>
<h4>The <tt>%code</tt> directive</h4>
<p>The <tt>%code</tt> directive is used to specify additional C code that
is added to the end of the main output file. This is similar to
the <tt><a href='#pinclude'>%include</a></tt> directive except that
<tt>%include</tt> is inserted at the beginning of the main output file.</p>
<p><tt>%code</tt> is typically used to include some action routines or perhaps
a tokenizer or even the "main()" function
as part of the output file.</p>
<a name='default_destructor'></a>
<h4>The <tt>%default_destructor</tt> directive</h4>
<p>The <tt>%default_destructor</tt> directive specifies a destructor to
use for non-terminals that do not have their own destructor
specified by a separate <tt>%destructor</tt> directive. See the documentation
on the <tt><a name='#destructor'>%destructor</a></tt> directive below for
additional information.</p>
<p>In some grammars, many different non-terminal symbols have the
same data type and hence the same destructor. This directive is
a convenient way to specify the same destructor for all those
non-terminals using a single statement.</p>
<a name='default_type'></a>
<h4>The <tt>%default_type</tt> directive</h4>
<p>The <tt>%default_type</tt> directive specifies the data type of non-terminal
symbols that do not have their own data type defined using a separate
<tt><a href='#ptype'>%type</a></tt> directive.</p>
<a name='destructor'></a>
<h4>The <tt>%destructor</tt> directive</h4>
<p>The <tt>%destructor</tt> directive is used to specify a destructor for
a non-terminal symbol.
(See also the <tt><a href='#token_destructor'>%token_destructor</a></tt>
directive which is used to specify a destructor for terminal symbols.)</p>
<p>A non-terminal's destructor is called to dispose of the
non-terminal's value whenever the non-terminal is popped from
the stack. This includes all of the following circumstances:
<ul>
<li> When a rule reduces and the value of a non-terminal on
|
| ︙ | ︙ | |||
631 632 633 634 635 636 637 |
<p>Consider an example:
<pre>
%type nt {void*}
%destructor nt { free($$); }
nt(A) ::= ID NUM. { A = malloc( 100 ); }
</pre>
| | | | | | | | | | | | | | | | > | | | | | > | | > | | | | | | | | | | | | | > | > | | | | > | | | | | < | > | | | | | | | | | | > > > > > > > > > > > > | | > > | > | | | > | | | | | | | > | | | > | | 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 |
<p>Consider an example:
<pre>
%type nt {void*}
%destructor nt { free($$); }
nt(A) ::= ID NUM. { A = malloc( 100 ); }
</pre>
This example is a bit contrived, but it serves to illustrate how
destructors work. The example shows a non-terminal named
"nt" that holds values of type "void*". When the rule for
an "nt" reduces, it sets the value of the non-terminal to
space obtained from malloc(). Later, when the nt non-terminal
is popped from the stack, the destructor will fire and call
free() on this malloced space, thus avoiding a memory leak.
(Note that the symbol "$$" in the destructor code is replaced
by the value of the non-terminal.)</p>
<p>It is important to note that the value of a non-terminal is passed
to the destructor whenever the non-terminal is removed from the
stack, unless the non-terminal is used in a C-code action. If
the non-terminal is used by C-code, then it is assumed that the
C-code will take care of destroying it.
More commonly, the value is used to build some
larger structure, and we don't want to destroy it, which is why
the destructor is not called in this circumstance.</p>
<p>Destructors help avoid memory leaks by automatically freeing
allocated objects when they go out of scope.
To do the same using yacc or bison is much more difficult.</p>
<a name='extraarg'></a>
<h4>The <tt>%extra_argument</tt> directive</h4>
The <tt>%extra_argument</tt> directive instructs Lemon to add a 4th parameter
to the parameter list of the Parse() function it generates. Lemon
doesn't do anything itself with this extra argument, but it does
make the argument available to C-code action routines, destructors,
and so forth. For example, if the grammar file contains:</p>
<p><pre>
%extra_argument { MyStruct *pAbc }
</pre></p>
<p>Then the Parse() function generated will have an 4th parameter
of type "MyStruct*" and all action routines will have access to
a variable named "pAbc" that is the value of the 4th parameter
in the most recent call to Parse().</p>
<a name='pfallback'></a>
<h4>The <tt>%fallback</tt> directive</h4>
<p>The <tt>%fallback</tt> directive specifies an alternative meaning for one
or more tokens. The alternative meaning is tried if the original token
would have generated a syntax error.</p>
<p>The <tt>%fallback</tt> directive was added to support robust parsing of SQL
syntax in <a href='https://www.sqlite.org/'>SQLite</a>.
The SQL language contains a large assortment of keywords, each of which
appears as a different token to the language parser. SQL contains so
many keywords that it can be difficult for programmers to keep up with
them all. Programmers will, therefore, sometimes mistakenly use an
obscure language keyword for an identifier. The <tt>%fallback</tt> directive
provides a mechanism to tell the parser: "If you are unable to parse
this keyword, try treating it as an identifier instead."</p>
<p>The syntax of <tt>%fallback</tt> is as follows:
<blockquote>
<tt>%fallback</tt> <i>ID</i> <i>TOKEN...</i> <b>.</b>
</blockquote></p>
<p>In words, the <tt>%fallback</tt> directive is followed by a list of token
names terminated by a period.
The first token name is the fallback token — the
token to which all the other tokens fall back to. The second and subsequent
arguments are tokens which fall back to the token identified by the first
argument.</p>
<a name='pifdef'></a>
<h4>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives</h4>
<p>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives
are similar to #ifdef, #ifndef, and #endif in the C-preprocessor,
just not as general.
Each of these directives must begin at the left margin. No whitespace
is allowed between the "%" and the directive name.</p>
<p>Grammar text in between "<tt>%ifdef MACRO</tt>" and the next nested
"<tt>%endif</tt>" is
ignored unless the "-DMACRO" command-line option is used. Grammar text
betwen "<tt>%ifndef MACRO</tt>" and the next nested "<tt>%endif</tt>" is
included except when the "-DMACRO" command-line option is used.</p>
<p>Note that the argument to <tt>%ifdef</tt> and <tt>%ifndef</tt> must
be a single preprocessor symbol name, not a general expression.
There is no "<tt>%else</tt>" directive.</p>
<a name='pinclude'></a>
<h4>The <tt>%include</tt> directive</h4>
<p>The <tt>%include</tt> directive specifies C code that is included at the
top of the generated parser. You can include any text you want —
the Lemon parser generator copies it blindly. If you have multiple
<tt>%include</tt> directives in your grammar file, their values are concatenated
so that all <tt>%include</tt> code ultimately appears near the top of the
generated parser, in the same order as it appeared in the grammar.</p>
<p>The <tt>%include</tt> directive is very handy for getting some extra #include
preprocessor statements at the beginning of the generated parser.
For example:</p>
<p><pre>
%include {#include <unistd.h>}
</pre></p>
<p>This might be needed, for example, if some of the C actions in the
grammar call functions that are prototyped in unistd.h.</p>
<a name='pleft'></a>
<h4>The <tt>%left</tt> directive</h4>
The <tt>%left</tt> directive is used (along with the
<tt><a href='#pright'>%right</a></tt> and
<tt><a href='#pnonassoc'>%nonassoc</a></tt> directives) to declare
precedences of terminal symbols.
Every terminal symbol whose name appears after
a <tt>%left</tt> directive but before the next period (".") is
given the same left-associative precedence value. Subsequent
<tt>%left</tt> directives have higher precedence. For example:</p>
<p><pre>
%left AND.
%left OR.
%nonassoc EQ NE GT GE LT LE.
%left PLUS MINUS.
%left TIMES DIVIDE MOD.
%right EXP NOT.
</pre></p>
<p>Note the period that terminates each <tt>%left</tt>,
<tt>%right</tt> or <tt>%nonassoc</tt>
directive.</p>
<p>LALR(1) grammars can get into a situation where they require
a large amount of stack space if you make heavy use or right-associative
operators. For this reason, it is recommended that you use <tt>%left</tt>
rather than <tt>%right</tt> whenever possible.</p>
<a name='pname'></a>
<h4>The <tt>%name</tt> directive</h4>
<p>By default, the functions generated by Lemon all begin with the
five-character string "Parse". You can change this string to something
different using the <tt>%name</tt> directive. For instance:</p>
<p><pre>
%name Abcde
</pre></p>
<p>Putting this directive in the grammar file will cause Lemon to generate
functions named
<ul>
<li> AbcdeAlloc(),
<li> AbcdeFree(),
<li> AbcdeTrace(), and
<li> Abcde().
</ul>
The <tt>%name</tt> directive allows you to generate two or more different
parsers and link them all into the same executable.</p>
<a name='pnonassoc'></a>
<h4>The <tt>%nonassoc</tt> directive</h4>
<p>This directive is used to assign non-associative precedence to
one or more terminal symbols. See the section on
<a href='#precrules'>precedence rules</a>
or on the <tt><a href='#pleft'>%left</a></tt> directive
for additional information.</p>
<a name='parse_accept'></a>
<h4>The <tt>%parse_accept</tt> directive</h4>
<p>The <tt>%parse_accept</tt> directive specifies a block of C code that is
executed whenever the parser accepts its input string. To "accept"
an input string means that the parser was able to process all tokens
without error.</p>
<p>For example:</p>
<p><pre>
%parse_accept {
printf("parsing complete!\n");
}
</pre></p>
<a name='parse_failure'></a>
<h4>The <tt>%parse_failure</tt> directive</h4>
<p>The <tt>%parse_failure</tt> directive specifies a block of C code that
is executed whenever the parser fails complete. This code is not
executed until the parser has tried and failed to resolve an input
error using is usual error recovery strategy. The routine is
only invoked when parsing is unable to continue.</p>
<p><pre>
%parse_failure {
fprintf(stderr,"Giving up. Parser is hopelessly lost...\n");
}
</pre></p>
<a name='pright'></a>
<h4>The <tt>%right</tt> directive</h4>
<p>This directive is used to assign right-associative precedence to
one or more terminal symbols. See the section on
<a href='#precrules'>precedence rules</a>
or on the <a href='#pleft'>%left</a> directive for additional information.</p>
<a name='stack_overflow'></a>
<h4>The <tt>%stack_overflow</tt> directive</h4>
<p>The <tt>%stack_overflow</tt> directive specifies a block of C code that
is executed if the parser's internal stack ever overflows. Typically
this just prints an error message. After a stack overflow, the parser
will be unable to continue and must be reset.</p>
<p><pre>
%stack_overflow {
fprintf(stderr,"Giving up. Parser stack overflow\n");
}
</pre></p>
<p>You can help prevent parser stack overflows by avoiding the use
of right recursion and right-precedence operators in your grammar.
Use left recursion and and left-precedence operators instead to
encourage rules to reduce sooner and keep the stack size down.
For example, do rules like this:
<pre>
list ::= list element. // left-recursion. Good!
list ::= .
</pre>
Not like this:
<pre>
list ::= element list. // right-recursion. Bad!
list ::= .
</pre></p>
<a name='stack_size'></a>
<h4>The <tt>%stack_size</tt> directive</h4>
<p>If stack overflow is a problem and you can't resolve the trouble
by using left-recursion, then you might want to increase the size
of the parser's stack using this directive. Put an positive integer
after the <tt>%stack_size</tt> directive and Lemon will generate a parse
with a stack of the requested size. The default value is 100.</p>
<p><pre>
%stack_size 2000
</pre></p>
<a name='start_symbol'></a>
<h4>The <tt>%start_symbol</tt> directive</h4>
<p>By default, the start symbol for the grammar that Lemon generates
is the first non-terminal that appears in the grammar file. But you
can choose a different start symbol using the
<tt>%start_symbol</tt> directive.</p>
<p><pre>
%start_symbol prog
</pre></p>
<a name='syntax_error'></a>
<h4>The <tt>%syntax_error</tt> directive</h4>
<p>See <a href='#error_processing'>Error Processing</a>.</p>
<a name='token_class'></a>
<h4>The <tt>%token_class</tt> directive</h4>
<p>Undocumented. Appears to be related to the MULTITERMINAL concept.
<a href='http://sqlite.org/src/fdiff?v1=796930d5fc2036c7&v2=624b24c5dc048e09&sbs=0'>Implementation</a>.</p>
<a name='token_destructor'></a>
<h4>The <tt>%token_destructor</tt> directive</h4>
<p>The <tt>%destructor</tt> directive assigns a destructor to a non-terminal
symbol. (See the description of the
<tt><a href='%destructor'>%destructor</a></tt> directive above.)
The <tt>%token_destructor</tt> directive does the same thing
for all terminal symbols.</p>
<p>Unlike non-terminal symbols which may each have a different data type
for their values, terminals all use the same data type (defined by
the <tt><a href='#token_type'>%token_type</a></tt> directive)
and so they use a common destructor.
Other than that, the token destructor works just like the non-terminal
destructors.</p>
<a name='token_prefix'></a>
<h4>The <tt>%token_prefix</tt> directive</h4>
<p>Lemon generates #defines that assign small integer constants
to each terminal symbol in the grammar. If desired, Lemon will
add a prefix specified by this directive
to each of the #defines it generates.</p>
<p>So if the default output of Lemon looked like this:
<pre>
#define AND 1
#define MINUS 2
#define OR 3
#define PLUS 4
</pre>
You can insert a statement into the grammar like this:
<pre>
%token_prefix TOKEN_
</pre>
to cause Lemon to produce these symbols instead:
<pre>
#define TOKEN_AND 1
#define TOKEN_MINUS 2
#define TOKEN_OR 3
#define TOKEN_PLUS 4
</pre></p>
<a name='token_type'></a><a name='ptype'></a>
<h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4>
<p>These directives are used to specify the data types for values
on the parser's stack associated with terminal and non-terminal
symbols. The values of all terminal symbols must be of the same
type. This turns out to be the same data type as the 3rd parameter
to the Parse() function generated by Lemon. Typically, you will
make the value of a terminal symbol by a pointer to some kind of
token structure. Like this:</p>
<p><pre>
%token_type {Token*}
</pre></p>
<p>If the data type of terminals is not specified, the default value
is "void*".</p>
<p>Non-terminal symbols can each have their own data types. Typically
the data type of a non-terminal is a pointer to the root of a parse tree
structure that contains all information about that non-terminal.
For example:</p>
<p><pre>
%type expr {Expr*}
</pre></p>
<p>Each entry on the parser's stack is actually a union containing
instances of all data types for every non-terminal and terminal symbol.
Lemon will automatically use the correct element of this union depending
on what the corresponding non-terminal or terminal symbol is. But
the grammar designer should keep in mind that the size of the union
will be the size of its largest element. So if you have a single
non-terminal whose data type requires 1K of storage, then your 100
entry parser stack will require 100K of heap space. If you are willing
and able to pay that price, fine. You just need to know.</p>
<a name='pwildcard'></a>
<h4>The <tt>%wildcard</tt> directive</h4>
<p>The <tt>%wildcard</tt> directive is followed by a single token name and a
period. This directive specifies that the identified token should
match any input token.</p>
<p>When the generated parser has the choice of matching an input against
the wildcard token and some other token, the other token is always used.
The wildcard token is only matched if there are no alternatives.</p>
<a name='error_processing'></a>
<h3>Error Processing</h3>
<p>After extensive experimentation over several years, it has been
discovered that the error recovery strategy used by yacc is about
as good as it gets. And so that is what Lemon uses.</p>
<p>When a Lemon-generated parser encounters a syntax error, it
first invokes the code specified by the <tt>%syntax_error</tt> directive, if
any. It then enters its error recovery strategy. The error recovery
strategy is to begin popping the parsers stack until it enters a
state where it is permitted to shift a special non-terminal symbol
named "error". It then shifts this non-terminal and continues
parsing. The <tt>%syntax_error</tt> routine will not be called again
until at least three new tokens have been successfully shifted.</p>
<p>If the parser pops its stack until the stack is empty, and it still
is unable to shift the error symbol, then the
<tt><a href='#parse_failure'>%parse_failure</a></tt> routine
is invoked and the parser resets itself to its start state, ready
to begin parsing a new file. This is what will happen at the very
first syntax error, of course, if there are no instances of the
"error" non-terminal in your grammar.</p>
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