1Parse::Yapp(3) User Contributed Perl Documentation Parse::Yapp(3)
2
6 Parse::Yapp - Perl extension for generating and using LALR parsers.
7
9 yapp -m MyParser grammar_file.yp
10 ...
12 use MyParser;
14 $parser=new MyParser();
16 $value=$parser->YYParse(yylex => \&lexer_sub, yyerror => \&error_sub);
17 $nberr=$parser->YYNberr();
19 $parser->YYData->{DATA}= [ 'Anything', 'You Want' ];
21 $data=$parser->YYData->{DATA}[0];
23
25 Parse::Yapp (Yet Another Perl Parser compiler) is a collection of mod‐
26 ules that let you generate and use yacc like thread safe (reentrant)
27 parsers with perl object oriented interface.
28 The script yapp is a front-end to the Parse::Yapp module and let you
30 easily create a Perl OO parser from an input grammar file.
31 The Grammar file
33 "Comments"
35 Through all your files, comments are either Perl style, introduced
36 by # up to the end of line, or C style, enclosed between /* and
37 */.
38 "Tokens and string literals"
40 Through all the grammar files, two kind of symbols may appear: Non-
41 terminal symbols, called also left-hand-side symbols, which are the
42 names of your rules, and Terminal symbols, called also Tokens.
43 Tokens are the symbols your lexer function will feed your parser
45 with (see below). They are of two flavours: symbolic tokens and
46 string literals.
47 Non-terminals and symbolic tokens share the same identifier syntax:
49 [A-Za-z][A-Za-z0-9_]*
51 String literals are enclosed in single quotes and can contain
53 almost anything. They will be output to your parser file dou‐
54 ble-quoted, making any special character as such. '"', '$' and '@'
55 will be automatically quoted with '\', making their writing more
56 natural. On the other hand, if you need a single quote inside your
57 literal, just quote it with '\'.
58 You cannot have a literal 'error' in your grammar as it would con‐
60 fuse the driver with the error token. Use a symbolic token instead.
61 In case you inadvertently use it, this will produce a warning
62 telling you you should have written it error and will treat it as
63 if it were the error token, which is certainly NOT what you meant.
64 "Grammar file syntax"
66 It is very close to yacc syntax (in fact, Parse::Yapp should com‐
67 pile a clean yacc grammar without any modification, whereas the
68 opposite is not true).
69 This file is divided in three sections, separated by "%%":
71 header section
73 %%
74 rules section
75 %%
76 footer section
77 The Header Section section may optionally contain:
79 * One or more code blocks enclosed inside "%{" and "%}" just like
80 in yacc. They may contain any valid Perl code and will be
81 copied verbatim at the very beginning of the parser module.
82 They are not as useful as they are in yacc, but you can use
83 them, for example, for global variable declarations, though you
84 will notice later that such global variables can be avoided to
85 make a reentrant parser module.
86 * Precedence declarations, introduced by %left, %right and
88 %nonassoc specifying associativity, followed by the list of
89 tokens or litterals having the same precedence and associativ‐
90 ity. The precedence beeing the latter declared will be having
91 the highest level. (see the yacc or bison manuals for a full
92 explanation of how they work, as they are implemented exactly
93 the same way in Parse::Yapp)
94 * %start followed by a rule's left hand side, declaring this rule
96 to be the starting rule of your grammar. The default, when
97 %start is not used, is the first rule in your grammar section.
98 * %token followed by a list of symbols, forcing them to be recog‐
100 nized as tokens, generating a syntax error if used in the left
101 hand side of a rule declaration. Note that in Parse::Yapp, you
102 don't need to declare tokens as in yacc: any symbol not appear‐
103 ing as a left hand side of a rule is considered to be a token.
104 Other yacc declarations or constructs such as %type and %union
105 are parsed but (almost) ignored.
106 * %expect followed by a number, suppress warnings about number of
108 Shift/Reduce conflicts when both numbers match, a la bison.
109 The Rule Section contains your grammar rules:
111 A rule is made of a left-hand-side symbol, followed by a ':'
112 and one or more right-hand-sides separated by '⎪' and termi‐
113 nated by a ';':
114 exp: exp '+' exp
116 ⎪ exp '-' exp
117 ;
118 A right hand side may be empty:
120 input: #empty
122 ⎪ input line
123 ;
124 (if you have more than one empty rhs, Parse::Yapp will issue a
126 warning, as this is usually a mistake, and you will certainly
127 have a reduce/reduce conflict)
128 A rhs may be followed by an optional %prec directive, followed
130 by a token, giving the rule an explicit precedence (see yacc
131 manuals for its precise meaning) and optionnal semantic action
132 code block (see below).
133 exp: '-' exp %prec NEG { -$_[1] }
135 ⎪ exp '+' exp { $_[1] + $_[3] }
136 ⎪ NUM
137 ;
138 Note that in Parse::Yapp, a lhs cannot appear more than once as
140 a rule name (This differs from yacc).
141 "The footer section"
143 may contain any valid Perl code and will be appended at the
144 very end of your parser module. Here you can write your lexer,
145 error report subs and anything relevant to you parser.
146 "Semantic actions"
148 Semantic actions are run every time a reduction occurs in the
149 parsing flow and they must return a semantic value.
150 They are (usually, but see below "In rule actions") written at
152 the very end of the rhs, enclosed with "{ }", and are copied
153 verbatim to your parser file, inside of the rules table.
154 Be aware that matching braces in Perl is much more difficult
156 than in C: inside strings they don't need to match. While in C
157 it is very easy to detect the beginning of a string construct,
158 or a single character, it is much more difficult in Perl, as
159 there are so many ways of writing such literals. So there is no
160 check for that today. If you need a brace in a double-quoted
161 string, just quote it ("\{" or "\}"). For single-quoted
162 strings, you will need to make a comment matching it in th
163 right order. Sorry for the inconvenience.
164 {
166 "{ My string block }".
167 "\{ My other string block \}".
168 qq/ My unmatched brace \} /.
169 # Force the match: {
170 q/ for my closing brace } /
171 q/ My opening brace { /
172 # must be closed: }
173 }
174 All of these constructs should work.
176 In Parse::Yapp, semantic actions are called like normal Perl
178 sub calls, with their arguments passed in @_, and their seman‐
179 tic value are their return values.
180 $_[1] to $_[n] are the parameters just as $1 to $n in yacc,
182 while $_[0] is the parser object itself.
183 Having $_[0] beeing the parser object itself allows you to call
185 parser methods. Thats how the yacc macros are implemented:
186 yyerrok is done by calling $_[0]->YYErrok
188 YYERROR is done by calling $_[0]->YYError
189 YYACCEPT is done by calling $_[0]->YYAccept
190 YYABORT is done by calling $_[0]->YYAbort
191 All those methods explicitly return undef, for convenience.
193 YYRECOVERING is done by calling $_[0]->YYRecovering
195 Four useful methods in error recovery sub
197 $_[0]->YYCurtok
199 $_[0]->YYCurval
200 $_[0]->YYExpect
201 $_[0]->YYLexer
202 return respectivly the current input token that made the parse
204 fail, its semantic value (both can be used to modify their val‐
205 ues too, but know what you are doing ! See Error reporting rou‐
206 tine section for an example), a list which contains the tokens
207 the parser expected when the failure occured and a reference to
208 the lexer routine.
209 Note that if "$_[0]->YYCurtok" is declared as a %nonassoc
211 token, it can be included in "$_[0]->YYExpect" list whenever
212 the input try to use it in an associative way. This is not a
213 bug: the token IS expected to report an error if encountered.
214 To detect such a thing in your error reporting sub, the follow‐
216 ing example should do the trick:
217 grep { $_[0]->YYCurtok eq $_ } $_[0]->YYExpect
219 and do {
220 #Non-associative token used in an associative expression
221 };
222 Accessing semantics values on the left of your reducing rule is
224 done through the method
225 $_[0]->YYSemval( index )
227 where index is an integer. Its value being 1 .. n returns the
229 same values than $_[1] .. $_[n], but -n .. 0 returns values on
230 the left of the rule beeing reduced (It is related to $-n .. $0
231 .. $n in yacc, but you cannot use $_[0] or $_[-n] constructs in
232 Parse::Yapp for obvious reasons)
233 There is also a provision for a user data area in the parser
235 object, accessed by the method:
236 $_[0]->YYData
238 which returns a reference to an anonymous hash, which let you
240 have all of your parsing data held inside the object (see the
241 Calc.yp or ParseYapp.yp files in the distribution for some
242 examples). That's how you can make you parser module reen‐
243 trant: all of your module states and variables are held inside
244 the parser object.
245 Note: unfortunatly, method calls in Perl have a lot of over‐
247 head,
248 and when YYData is used, it may be called a huge number
249 of times. If your are not a *real* purist and efficiency
250 is your concern, you may access directly the user-space
251 in the object: $parser->{USER} wich is a reference to an
252 anonymous hash array, and then benchmark.
253 If no action is specified for a rule, the equivalant of a
255 default action is run, which returns the first parameter:
256 { $_[1] }
258 "In rule actions"
260 It is also possible to embed semantic actions inside of a rule:
261 typedef: TYPE { $type = $_[1] } identlist { ... } ;
263 When the Parse::Yapp's parser encounter such an embedded
265 action, it modifies the grammar as if you wrote (although @x-1
266 is not a legal lhs value):
267 @x-1: /* empty */ { $type = $_[1] };
269 typedef: TYPE @x-1 identlist { ... } ;
270 where x is a sequential number incremented for each "in rule"
272 action, and -1 represents the "dot position" in the rule where
273 the action arises.
274 In such actions, you can use $_[1]..$_[n] variables, which are
276 the semantic values on the left of your action.
277 Be aware that the way Parse::Yapp modifies your grammar because
279 of in rule actions can produce, in some cases, spurious con‐
280 flicts that wouldn't happen otherwise.
281 "Generating the Parser Module"
283 Now that you grammar file is written, you can use yapp on it to
284 generate your parser module:
285 yapp -v Calc.yp
287 will create two files Calc.pm, your parser module, and
289 Calc.output a verbose output of your parser rules, conflicts,
290 warnings, states and summary.
291 What your are missing now is a lexer routine.
293 "The Lexer sub"
295 is called each time the parser need to read the next token.
296 It is called with only one argument that is the parser object
298 itself, so you can access its methods, specially the
299 $_[0]->YYData
301 data area.
303 It is its duty to return the next token and value to the
305 parser. They "must" be returned as a list of two variables,
306 the first one is the token known by the parser (symbolic or
307 literal), the second one beeing anything you want (usualy the
308 content of the token, or the literal value) from a simple
309 scalar value to any complex reference, as the parsing driver
310 never use it but to call semantic actions:
311 ( 'NUMBER', $num )
313 or
314 ( '>=', '>=' )
315 or
316 ( 'ARRAY', [ @values ] )
317 When the lexer reach the end of input, it must return the ''
319 empty token with an undef value:
320 ( '', undef )
322 Note that your lexer should never return 'error' as token
324 value: for the driver, this is the error token used for error
325 recovery and would lead to odd reactions.
326 Now that you have your lexer written, maybe you will need to
328 output meaningful error messages, instead of the default which
329 is to print 'Parse error.' on STDERR.
330 So you will need an Error reporting sub.
332 item "Error reporting routine"
334 If you want one, write it knowing that it is passed as parame‐
336 ter the parser object. So you can share information whith the
337 lexer routine quite easily.
338 You can also use the "$_[0]->YYErrok" method in it, which will
340 resume parsing as if no error occured. Of course, since the
341 invalid token is still invalid, you're supposed to fix the
342 problem by yourself.
343 The method "$_[0]->YYLexer" may help you, as it returns a ref‐
345 erence to the lexer routine, and can be called as
346 ($tok,$val)=&{$_[0]->Lexer}
348 to get the next token and semantic value from the input stream.
350 To make them current for the parser, use:
351 ($_[0]->YYCurtok, $_[0]->YYCurval) = ($tok, $val)
353 and know what you're doing...
355 "Parsing"
357 Now you've got everything to do the parsing.
358 First, use the parser module:
360 use Calc;
362 Then create the parser object:
364 $parser=new Calc;
366 Now, call the YYParse method, telling it where to find the
368 lexer and error report subs:
369 $result=$parser->YYParse(yylex => \&Lexer,
371 yyerror => \&ErrorReport);
372 (assuming Lexer and ErrorReport subs have been written in your
374 current package)
375 The order in which parameters appear is unimportant.
377 Et voila.
379 The YYParse method will do the parse, then return the last
381 semantic value returned, or undef if error recovery cannot
382 recover.
383 If you need to be sure the parse has been successful (in case
385 your last returned semantic value is undef) make a call to:
386 $parser->YYNberr()
388 which returns the total number of time the error reporting sub
390 has been called.
391 "Error Recovery"
393 in Parse::Yapp is implemented the same way it is in yacc.
394 "Debugging Parser"
396 To debug your parser, you can call the YYParse method with a
397 debug parameter:
398 $parser->YYParse( ... , yydebug => value, ... )
400 where value is a bitfield, each bit representing a specific
402 debug output:
403 Bit Value Outputs
405 0x01 Token reading (useful for Lexer debugging)
406 0x02 States information
407 0x04 Driver actions (shifts, reduces, accept...)
408 0x08 Parse Stack dump
409 0x10 Error Recovery tracing
410 To have a full debugging ouput, use
412 debug => 0x1F
414 Debugging output is sent to STDERR, and be aware that it can
416 produce "huge" outputs.
417 "Standalone Parsers"
419 By default, the parser modules generated will need the
420 Parse::Yapp module installed on the system to run. They use the
421 Parse::Yapp::Driver which can be safely shared between parsers
422 in the same script.
423 In the case you'd prefer to have a standalone module generated,
425 use the "-s" switch with yapp: this will automagically copy the
426 driver code into your module so you can use/distribute it with‐
427 out the need of the Parse::Yapp module, making it really a
428 "Standalone Parser".
429 If you do so, please remember to include Parse::Yapp's copy‐
431 right notice in your main module copyright, so others can know
432 about Parse::Yapp module.
433 "Source file line numbers"
435 by default will be included in the generated parser module,
436 which will help to find the guilty line in your source file in
437 case of a syntax error. You can disable this feature by com‐
438 piling your grammar with yapp using the "-n" switch.
439
441 If you find bugs, think of anything that could improve Parse::Yapp or
442 have any questions related to it, feel free to contact the author.
443
445 Francois Desarmenien <francois@fdesar.net>
446
448 yapp(1) perl(1) yacc(1) bison(1).
449
451 The Parse::Yapp module and its related modules and shell scripts are
452 copyright (c) 1998-2001 Francois Desarmenien, France. All rights
453 reserved.
454 You may use and distribute them under the terms of either the GNU Gen‐
456 eral Public License or the Artistic License, as specified in the Perl
457 README file.
458 If you use the "standalone parser" option so people don't need to
460 install Parse::Yapp on their systems in order to run you software, this
461 copyright noticed should be included in your software copyright too,
462 and the copyright notice in the embedded driver should be left
463 untouched.
464perl v5.8.8 2001-02-11 Parse::Yapp(3)