SQLite

Check-in [f095341471]
Login

Many hyperlinks are disabled.
Use anonymous login to enable hyperlinks.

Overview
Comment:Updates to the Lemon documentation.
Downloads: Tarball | ZIP archive
Timelines: family | ancestors | descendants | both | trunk
Files: files | file ages | folders
SHA1: f095341471aa822e6d556cb65512ec081c3918da
User & Date: drh 2016-03-19 18:00:44.390
Context
2016-03-19
18:11
Fix exclusive.test so that it works with -DSQLITE_TEMP_STORE=3. (check-in: d7852c6396 user: dan tags: trunk)
18:00
Updates to the Lemon documentation. (check-in: f095341471 user: drh tags: trunk)
17:48
Add the sqlite3rbu_bp_progress() API to the RBU extension. Used to obtain the percentage progress of an RBU update. (check-in: 209e31c729 user: dan tags: trunk)
Changes
Unified Diff Ignore Whitespace Patch
Changes to doc/lemon.html. 
1
2
3
4
5
6
7
8
9
10
11
12
13
14



15
16
17
18
19
20
21
22

<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 or C++.  
It does the same job as ``bison'' and ``yacc''.
But lemon is not another bison or yacc clone.  It
uses a different grammar syntax which is designed to
reduce the number of coding errors.  Lemon also uses a more
sophisticated parsing engine that is faster than yacc and
bison and which is both reentrant and thread-safe.



Furthermore, Lemon implements features that can be used
to eliminate resource leaks, making is 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>









|
|
|

|
|
|
>
>
>
|








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

<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 is 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>
︙︙
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79

80
81
82
83
84
85
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
124
125
126
127
128
129
130
131
132
133
134
135
136

<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.
The header file is suppressed if the ``-m'' command-line option is
used and the report file is omitted when ``-q'' is selected.</p>

<p>The grammar specification file uses a ``.y'' suffix, by convention.
In the examples used in this document, we'll assume the name of the
grammar file is ``gram.y''.  A typical use of Lemon would be the
following command:
<pre>
   lemon gram.y
</pre>
This command will generate three output files named ``gram.c'',
``gram.h'' and ``gram.out''.
The first is C code to implement the parser.  The second
is the header file that defines numerical values for all
terminal symbols, and the last is the report that explains
the states used by the parser automaton.</p>

<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><tt>-b</tt>
<li><tt>-c</tt>
<li><tt>-g</tt>
<li><tt>-m</tt>
<li><tt>-q</tt>
<li><tt>-s</tt>

<li><tt>-x</tt>
</ul>
The ``-b'' option reduces the amount of text in the report file by
printing only the basis of each parser state, rather than the full
configuration.
The ``-c'' option suppresses action table compression.  Using -c
will make the parser a little larger and slower but it will detect
syntax errors sooner.
The ``-g'' option causes no output files to be generated at all.
Instead, the input grammar file is printed on standard output but
with all comments, actions and other extraneous text deleted.  This
is a useful way to get a quick summary of a grammar.



The ``-m'' option causes the output C source file to be compatible
with the ``makeheaders'' program.
Makeheaders is a program that automatically generates header files
from C source code.  When the ``-m'' option is used, the header
file is not output since the makeheaders program will take care

of generated all header files automatically.


The ``-q'' option suppresses the report file.

Using ``-s'' causes a brief summary of parser statistics to be
printed.  Like this:
<pre>
   Parser statistics: 74 terminals, 70 nonterminals, 179 rules
                      340 states, 2026 parser table entries, 0 conflicts


</pre>
Finally, the ``-x'' option causes Lemon to print its version number
and then stops without attempting to read the grammar or generate a parser.</p>


<h3>The Parser Interface</h3>

<p>Lemon doesn't generate a complete, working program.  It only generates
a few subroutines that implement a parser.  This section describes
the interface to those subroutines.  It is up to the programmer to
call these subroutines in an appropriate way in order to produce a
complete system.</p>

<p>Before a program begins using a Lemon-generated parser, the program
must first create the parser.
A new parser is created as follows:
<pre>
   void *pParser = ParseAlloc( malloc );
</pre>
The ParseAlloc() routine allocates and initializes a new parser and
returns a pointer to it.
The actual data structure used to represent a parser is opaque --
its internal structure is not visible or usable by the calling routine.
For this reason, the ParseAlloc() routine returns a pointer to void
rather than a pointer to some particular structure.
The sole argument to the ParseAlloc() routine is a pointer to the
subroutine used to allocate memory.  Typically this means ``malloc()''.</p>

<p>After a program is finished using a parser, it can reclaim all
memory allocated by that parser by calling
<pre>
   ParseFree(pParser, free);
</pre>
The first argument is the same pointer returned by ParseAlloc().  The








|
|

|

|




|
|















|
|
|
|
|
|
>
|
<
<
<
<
<
<
<
<
|
|
<
>
>
>
|
|
<
<
<
>
|
>
>
|
>
|
<
|
|
<
>
>
|
|
<
>

















|




|








43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84








85
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
124
125
126
127
128
129
130
131
132
133
134
135

<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.
The header file is suppressed if the "-m" command-line option is
used and the report file is omitted when "-q" is selected.</p>

<p>The grammar specification file uses a ".y" suffix, by convention.
In the examples used in this document, we'll assume the name of the
grammar file is "gram.y".  A typical use of Lemon would be the
following command:
<pre>
   lemon gram.y
</pre>
This command will generate three output files named "gram.c",
"gram.h" and "gram.out".
The first is C code to implement the parser.  The second
is the header file that defines numerical values for all
terminal symbols, and the last is the report that explains
the states used by the parser automaton.</p>

<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.
<li><b>-D<i>name</i></b>
Define C preprocessor macro <i>name</i>.  This macro is useable by
"%ifdef" 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 int 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.

<li><b>-T<i>file</i></b>
Use <i>file</i> as the template for the generated C-code parser implementation.
<li><b>-x</b>
Print the Lemon version number.

</ul>

<h3>The Parser Interface</h3>

<p>Lemon doesn't generate a complete, working program.  It only generates
a few subroutines that implement a parser.  This section describes
the interface to those subroutines.  It is up to the programmer to
call these subroutines in an appropriate way in order to produce a
complete system.</p>

<p>Before a program begins using a Lemon-generated parser, the program
must first create the parser.
A new parser is created as follows:
<pre>
   void *pParser = ParseAlloc( malloc );
</pre>
The ParseAlloc() routine allocates and initializes a new parser and
returns a pointer to it.
The actual data structure used to represent a parser is opaque &mdash;
its internal structure is not visible or usable by the calling routine.
For this reason, the ParseAlloc() routine returns a pointer to void
rather than a pointer to some particular structure.
The sole argument to the ParseAlloc() routine is a pointer to the
subroutine used to allocate memory.  Typically this means malloc().</p>

<p>After a program is finished using a parser, it can reclaim all
memory allocated by that parser by calling
<pre>
   ParseFree(pParser, free);
</pre>
The first argument is the same pointer returned by ParseAlloc().  The
︙︙
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167

The first argument to the Parse() routine is the pointer returned by
ParseAlloc().
The second argument is a small positive integer that tells the parse 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 integer, 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 <a href='#extraarg'><tt>extra_argument</tt> directive</a>),
the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer.  The parser doesn't do anything








|
|




|








146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166

The first argument to the Parse() routine is the pointer returned by
ParseAlloc().
The second argument is a small positive integer that tells the parse 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 integer, 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 <a href='#extraarg'><tt>extra_argument</tt> directive</a>),
the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer.  The parser doesn't do anything
︙︙
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203

   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.
(We've omitted all error-handling 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,








|








188
189
190
191
192
193
194
195
196
197
198
199
200
201
202

   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,
︙︙
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297

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 underscore characters
that begins with an upper case letter.
A terminal can contain lowercase letters after the first character,
but the usual convention is to make terminals all upper case.
A nonterminal, on the other hand, is any string of alphanumeric
and underscore characters than begins with a lower case letter.
Again, the usual convention is to make nonterminals use all lower
case letters.</p>








|








282
283
284
285
286
287
288
289
290
291
292
293
294
295
296

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 upper case letter.
A terminal can contain lowercase letters after the first character,
but the usual convention is to make terminals all upper case.
A nonterminal, on the other hand, is any string of alphanumeric
and underscore characters than begins with a lower case letter.
Again, the usual convention is to make nonterminals use all lower
case letters.</p>
︙︙
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366

must have alphanumeric names.</p>

<h3>Grammar Rules</h3>

<p>The main component of a Lemon grammar file is a sequence of grammar
rules.
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>%start</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>
</p>

<p>There is one non-terminal in this example, ``expr'', and five
terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'',
``RPAREN'' and ``VALUE''.</p>

<p>Like yacc and bison, Lemon allows the grammar to specify a block
of C code that will be executed whenever a grammar rule is reduced
by the parser.
In Lemon, this action is specified by putting the C code (contained
within curly braces <tt>{...}</tt>) immediately after the
period that closes the rule.
For example:
<pre>
  expr ::= expr PLUS expr.   { printf("Doing an addition...\n"); }
</pre>
</p>

<p>In order to be useful, grammar actions must normally be linked to
their associated grammar rules.
In yacc and bison, this is accomplished by embedding a ``$$'' in the
action to stand for the value of the left-hand side of the rule and
symbols ``$1'', ``$2'', and so forth to stand for the value of
the terminal or nonterminal at position 1, 2 and so forth on the
right-hand side of the rule.
This idea is very powerful, but it is also very error-prone.  The
single most common source of errors in a yacc or bison grammar is
to miscount the number of symbols on the right-hand side of a grammar
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; };








|















|
|
|















|

|





|








309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365

must have alphanumeric names.</p>

<h3>Grammar Rules</h3>

<p>The main component of a Lemon grammar file is a sequence of grammar
rules.
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>%start</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>
</p>

<p>There is one non-terminal in this example, "expr", and five
terminal symbols or tokens: "PLUS", "TIMES", "LPAREN",
"RPAREN" and "VALUE".</p>

<p>Like yacc and bison, Lemon allows the grammar to specify a block
of C code that will be executed whenever a grammar rule is reduced
by the parser.
In Lemon, this action is specified by putting the C code (contained
within curly braces <tt>{...}</tt>) immediately after the
period that closes the rule.
For example:
<pre>
  expr ::= expr PLUS expr.   { printf("Doing an addition...\n"); }
</pre>
</p>

<p>In order to be useful, grammar actions must normally be linked to
their associated grammar rules.
In yacc and bison, this is accomplished by embedding a "$$" in the
action to stand for the value of the left-hand side of the rule and
symbols "$1", "$2", and so forth to stand for the value of
the terminal or nonterminal at position 1, 2 and so forth on the
right-hand side of the rule.
This idea is very powerful, but it is also very error-prone.  The
single most common source of errors in a yacc or bison grammar is
to miscount the number of symbols on the right-hand side of a grammar
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; };
︙︙
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396

397
398
399
400
401
402
403
404
405
406
407



408
409
410
411
412
413
414
415

includes a linking symbol in parentheses but that linking symbol
is not actually used in the reduce action, then an error message
is generated.
For example, the rule
<pre>
  expr(A) ::= expr(B) PLUS expr(C).  { A = B; }
</pre>
will generate an error because the linking symbol ``C'' is used
in the grammar rule but not in the reduce action.</p>

<p>The Lemon notation for linking grammar rules to reduce actions
also facilitates the use of destructors for reclaiming memory
allocated by the values of terminals and nonterminals on the
right-hand side of a rule.</p>


<h3>Precedence Rules</h3>

<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 paring conflicts using precedence rules.
A precedence value can be assigned to any terminal symbol



using the %left, %right or %nonassoc directives.  Terminal symbols
mentioned in earlier directives have a lower precedence that
terminal symbols mentioned in later directives.  For example:</p>

<p><pre>
   %left AND.
   %left OR.
   %nonassoc EQ NE GT GE LT LE.








|







>











>
>
>
|








381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418

includes a linking symbol in parentheses but that linking symbol
is not actually used in the reduce action, then an error message
is generated.
For example, the rule
<pre>
  expr(A) ::= expr(B) PLUS expr(C).  { A = B; }
</pre>
will generate an error because the linking symbol "C" is used
in the grammar rule but not in the reduce action.</p>

<p>The Lemon notation for linking grammar rules to reduce actions
also facilitates the use of destructors for reclaiming memory
allocated by the values of terminals and nonterminals on the
right-hand side of a rule.</p>

<a name='precrules'></a>
<h3>Precedence Rules</h3>

<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 paring conflicts using precedence rules.
A precedence value can be assigned to any terminal symbol
using the 
<a href='#pleft'>%left</a>,
<a href='#pright'>%right</a> or
<a href='#pnonassoc'>%nonassoc</a> directives.  Terminal symbols
mentioned in earlier directives have a lower precedence that
terminal symbols mentioned in later directives.  For example:</p>

<p><pre>
   %left AND.
   %left OR.
   %nonassoc EQ NE GT GE LT LE.
︙︙
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


<p>Lemon supports the following special directives:
<ul>
<li><tt>%code</tt>
<li><tt>%default_destructor</tt>
<li><tt>%default_type</tt>
<li><tt>%destructor</tt>

<li><tt>%extra_argument</tt>



<li><tt>%include</tt>
<li><tt>%left</tt>
<li><tt>%name</tt>
<li><tt>%nonassoc</tt>
<li><tt>%parse_accept</tt>
<li><tt>%parse_failure </tt>
<li><tt>%right</tt>
<li><tt>%stack_overflow</tt>
<li><tt>%stack_size</tt>
<li><tt>%start_symbol</tt>
<li><tt>%syntax_error</tt>

<li><tt>%token_destructor</tt>
<li><tt>%token_prefix</tt>
<li><tt>%token_type</tt>
<li><tt>%type</tt>

</ul>
Each of these directives will be described separately in the
following sections:</p>


<h4>The <tt>%code</tt> directive</h4>

<p>The %code directive is used to specify addition C/C++ code that
is added to the end of the main output file.  This is similar to
the %include directive except that %include is inserted at the
beginning of the main output file.</p>

<p>%code is typically used to include some action routines or perhaps

a tokenizer as part of the output file.</p>


<h4>The <tt>%default_destructor</tt> directive</h4>

<p>The %default_destructor directive specifies a destructor to 
use for non-terminals that do not have their own destructor
specified by a separate %destructor directive.  See the documentation

on the %destructor directive below for additional information.</p>

<p>In some grammers, many different non-terminal symbols have the
same datatype and hence the same destructor.  This directive is
a convenience way to specify the same destructor for all those
non-terminals using a single statement.</p>


<h4>The <tt>%default_type</tt> directive</h4>

<p>The %default_type directive specifies the datatype of non-terminal
symbols that do no have their own datatype defined using a separate
%type directive.  See the documentation on %type below for addition
information.</p>


<h4>The <tt>%destructor</tt> directive</h4>

<p>The %destructor directive is used to specify a destructor for
a non-terminal symbol.
(See also the %token_destructor 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
     the right-hand side is not linked to C code.








>

>
>
>











>




>




>


|

|
|


>
|

>





>
|






>




|
|

>




|
|








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


<p>Lemon supports the following special directives:
<ul>
<li><tt>%code</tt>
<li><tt>%default_destructor</tt>
<li><tt>%default_type</tt>
<li><tt>%destructor</tt>
<li><tt>%endif</tt>
<li><tt>%extra_argument</tt>
<li><tt>%fallback</tt>
<li><tt>%ifdef</tt>
<li><tt>%ifndef</tt>
<li><tt>%include</tt>
<li><tt>%left</tt>
<li><tt>%name</tt>
<li><tt>%nonassoc</tt>
<li><tt>%parse_accept</tt>
<li><tt>%parse_failure </tt>
<li><tt>%right</tt>
<li><tt>%stack_overflow</tt>
<li><tt>%stack_size</tt>
<li><tt>%start_symbol</tt>
<li><tt>%syntax_error</tt>
<li><tt>%token_class</tt>
<li><tt>%token_destructor</tt>
<li><tt>%token_prefix</tt>
<li><tt>%token_type</tt>
<li><tt>%type</tt>
<li><tt>%wildcard</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 %code directive is used to specify addition C code that
is added to the end of the main output file.  This is similar to
the <a href='#pinclude'>%include</a> directive except that %include
is inserted at the beginning of the main output file.</p>

<p>%code 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 %default_destructor directive specifies a destructor to 
use for non-terminals that do not have their own destructor
specified by a separate %destructor directive.  See the documentation
on the <a name='#destructor'>%destructor</a> directive below for
additional information.</p>

<p>In some grammers, many different non-terminal symbols have the
same datatype and hence the same destructor.  This directive is
a convenience 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 %default_type directive specifies the datatype of non-terminal
symbols that do no have their own datatype defined using a separate
<a href='#ptype'>%type</a> directive.  
</p>

<a name='destructor'></a>
<h4>The <tt>%destructor</tt> directive</h4>

<p>The %destructor directive is used to specify a destructor for
a non-terminal symbol.
(See also the <a href='#token_destructor'>%token_destructor</a>
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
     the right-hand side is not linked to C code.
︙︙
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
627
628
629
630
631
632
633
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

<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 if it should really
be destroyed.  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>By appropriate use of destructors, it is possible to
build a parser using Lemon that can be used within a long-running
program, such as a GUI, that will not leak memory or other resources.
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 %extra_argument 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>

















































<h4>The <tt>%include</tt> directive</h4>

<p>The %include 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
%include directives in your grammar file the value of the last
%include directive overwrites all the others.</p.


<p>The %include 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 &lt;unistd.h&gt;}
</pre></p>

<p>This might be needed, for example, if some of the C actions in the
grammar call functions that are prototyed in unistd.h.</p>


<h4>The <tt>%left</tt> directive</h4>

The %left directive is used (along with the %right and
%nonassoc directives) to declare precedences of terminal
symbols.  Every terminal symbol whose name appears after
a %left directive but before the next period (``.'') is
given the same left-associative precedence value.  Subsequent
%left 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 %left, %right or %nonassoc
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 %left
rather than %right whenever possible.</p>


<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 %name 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 %name directive allows you to generator two or more different
parsers and link them all into the same executable.
</p>


<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 precedence rules

or on the %left directive for additional information.</p>


<h4>The <tt>%parse_accept</tt> directive</h4>

<p>The %parse_accept 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>


<h4>The <tt>%parse_failure</tt> directive</h4>

<p>The %parse_failure 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>


<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 precedence rules

or on the %left directive for additional information.</p>


<h4>The <tt>%stack_overflow</tt> directive</h4>

<p>The %stack_overflow 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>









|
|



|






|
|



|
|
<
















|
|


>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>





|
|
>












>


|
|
|
|




















>



|


















>



|
>
|

>



|











|














>



|
>
|

>








613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
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

<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 %extra_argument 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 %fallback 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>The %fallback 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 %fallback directive
provides a mechanism to tell the parser:  "If you are unable to parse
this keyword, try treating it as an identifier instead."

<p>The syntax of %fallback is as follows:

<blockquote>
<tt>%fallback</tt>  <i>ID</i> <i>TOKEN...</i> <b>.</b>
</blockquote>

<p>In words, the %fallback 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.

<a name='pifdef'></a>
<h4>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives.</h4>

<p>The %ifdef, %ifndef, and %endif 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>Grammar text in between "%ifdef MACRO" and the next nested "%endif" is
ignored unless the "-DMACRO" command-line option is used.  Grammar text
betwen "%ifndef MACRO" and the next nested "%endif" is included except when
the "-DMACRO" command-line option is used.

<p>Note that the argument to %ifdef and %ifndef must be a single 
preprocessor symbol name, not a general expression.  There is no "%else"
directive.


<a name='pinclude'></a>
<h4>The <tt>%include</tt> directive</h4>

<p>The %include 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
%include directives in your grammar file, their values are concatenated
so that all %include code ultimately appears near the top of the
generated parser, in the same order as it appeared in the grammer.</p>

<p>The %include 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 &lt;unistd.h&gt;}
</pre></p>

<p>This might be needed, for example, if some of the C actions in the
grammar call functions that are prototyed in unistd.h.</p>

<a name='pleft'></a>
<h4>The <tt>%left</tt> directive</h4>

The %left directive is used (along with the <a href='#pright'>%right</a> and
<a href='#pnonassoc'>%nonassoc</a> directives) to declare precedences of 
terminal symbols.  Every terminal symbol whose name appears after
a %left directive but before the next period (".") is
given the same left-associative precedence value.  Subsequent
%left 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 %left, %right or %nonassoc
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 %left
rather than %right 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 %name 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 %name directive allows you to generator 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 <a href='#pleft'>%left</a> directive for additional information.</p>

<a name='parse_accept'></a>
<h4>The <tt>%parse_accept</tt> directive</h4>

<p>The %parse_accept 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 %parse_failure 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 %stack_overflow 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>
︙︙
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

</pre>
Not like this:
<pre>
   list ::= element list.      // right-recursion.  Bad!
   list ::= .
</pre>


<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 %stack_size 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>


<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 %start_symbol directive.</p>

<p><pre>
   %start_symbol  prog
</pre></p>


<h4>The <tt>%token_destructor</tt> directive</h4>

<p>The %destructor directive assigns a destructor to a non-terminal
symbol.  (See the description of the %destructor directive above.)
This 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 %token_type directive) and so they use a common destructor.  Other
than that, the token destructor works just like the non-terminal
destructors.</p>


<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.
So if the default output of Lemon looked like this:








>












>










>












>








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

</pre>
Not like this:
<pre>
   list ::= element list.      // right-recursion.  Bad!
   list ::= .
</pre>

<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 %stack_size 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 %start_symbol directive.</p>

<p><pre>
   %start_symbol  prog
</pre></p>

<a name='token_destructor'></a>
<h4>The <tt>%token_destructor</tt> directive</h4>

<p>The %destructor directive assigns a destructor to a non-terminal
symbol.  (See the description of the %destructor directive above.)
This 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 %token_type 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.
So if the default output of Lemon looked like this:
︙︙
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

<pre>
    #define TOKEN_AND        1
    #define TOKEN_MINUS      2
    #define TOKEN_OR         3
    #define TOKEN_PLUS       4
</pre>


<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 ``int''.</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>












<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 %syntax_error 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.  But the %syntax_error 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 %parse_failed 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>

</body>
</html>








>















|




















>
>
>
>
>
>
>
>
>
>
>











|








|




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

<pre>
    #define TOKEN_AND        1
    #define TOKEN_MINUS      2
    #define TOKEN_OR         3
    #define TOKEN_PLUS       4
</pre>

<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 "int".</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 %wildcard directive is followed by a single token name and a
period.  This directive specifies that the identified token should 
match any input token.

<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 other alternatives.

<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 %syntax_error 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.  But the %syntax_error 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 %parse_failed 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>

</body>
</html>