1pt::peg::export::peg(n)          Parser Tools          pt::peg::export::peg(n)
2
3
4
5______________________________________________________________________________
6

NAME

8       pt::peg::export::peg - PEG Export Plugin. Write PEG format
9

SYNOPSIS

11       package require Tcl  8.5
12
13       package require pt::peg::export::peg  ?1?
14
15       package require pt::peg::to::peg
16
17       export serial configuration
18
19______________________________________________________________________________
20

DESCRIPTION

22       Are  you  lost ?  Do you have trouble understanding this document ?  In
23       that case please read the overview  provided  by  the  Introduction  to
24       Parser  Tools.  This document is the entrypoint to the whole system the
25       current package is a part of.
26
27       This package implements the parsing expression  grammar  export  plugin
28       for the generation of PEG markup.
29
30       It  resides in the Export section of the Core Layer of Parser Tools and
31       is intended to be used by pt::peg::export, the export manager,  sitting
32       between it and the corresponding core conversion functionality provided
33       by pt::peg::to::peg.
34
35       IMAGE: arch_core_eplugins
36
37       While the direct use of this package with a regular interpreter is pos‐
38       sible, this is strongly disrecommended and requires a number of contor‐
39       tions to provide the expected environment.  The proper way to use  this
40       functionality depends on the situation:
41
42       [1]    In  an  untrusted  environment  the proper access is through the
43              package pt::peg::export and the export manager objects  it  pro‐
44              vides.
45
46       [2]    In   a  trusted  environment  however  simply  use  the  package
47              pt::peg::to::peg and access the  core  conversion  functionality
48              directly.
49

API

51       The  API  provided  by  this package satisfies the specification of the
52       Plugin API found in the Parser Tools Export API specification.
53
54       export serial configuration
55              This command takes the  canonical  serialization  of  a  parsing
56              expression  grammar,  as  specified in section PEG serialization
57              format, and contained in serial, the  configuration,  a  dictio‐
58              nary,  and  generates PEG markup encoding the grammar.  The cre‐
59              ated string is then returned as the result of the command.
60

CONFIGURATION

62       The PEG export plugin recognizes the following configuration  variables
63       and changes its behaviour as they specify.
64
65       string template
66              If this configuration variable is set it is assumed to contain a
67              string into which to put the generated text and other configura‐
68              tion  data.  The  various locations are expected to be specified
69              with the following placeholders:
70
71              @user@ To be replaced with the value of the configuration  vari‐
72                     able user.
73
74              @format@
75                     To be replaced with the the constant PEG.
76
77              @file@ To  be replaced with the value of the configuration vari‐
78                     able file.
79
80              @name@ To be replaced with the value of the configuration  vari‐
81                     able name.
82
83              @code@ To be replaced with the generated text.
84
85       If  this  configuration  variable is not set, or empty, then the plugin
86       falls back to a standard template, which is defined as "@code@".
87
88       Note that this plugin may ignore the standard  configuration  variables
89       user, format, file, and their values, depending on the chosen template.
90
91       The  content  of  the  standard configuration variable name, if set, is
92       used as name of the grammar in the output. Otherwise the  plugin  falls
93       back to the default name a_pe_grammar.
94

PEG SPECIFICATION LANGUAGE

96       peg, a language for the specification of parsing expression grammars is
97       meant to be human readable, and writable as well, yet strict enough  to
98       allow  its  processing  by  machine. Like any computer language. It was
99       defined to make writing the specification of a grammar easy,  something
100       the other formats found in the Parser Tools do not lend themselves too.
101
102       It is formally specified by the grammar shown below, written in itself.
103       For a tutorial / introduction to the language please go  and  read  the
104       PEG Language Tutorial.
105
106              PEG pe-grammar-for-peg (Grammar)
107
108                # --------------------------------------------------------------------
109                      # Syntactical constructs
110
111                      Grammar         <- WHITESPACE Header Definition* Final EOF ;
112
113                      Header          <- PEG Identifier StartExpr ;
114                      Definition      <- Attribute? Identifier IS Expression SEMICOLON ;
115                      Attribute       <- (VOID / LEAF) COLON ;
116                      Expression      <- Sequence (SLASH Sequence)* ;
117                      Sequence        <- Prefix+ ;
118                      Prefix          <- (AND / NOT)? Suffix ;
119                      Suffix          <- Primary (QUESTION / STAR / PLUS)? ;
120                      Primary         <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
121                                      /  GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
122                                      /  WORDCHAR / XDIGIT
123                                      / Identifier
124                                      /  OPEN Expression CLOSE
125                                      /  Literal
126                                      /  Class
127                                      /  DOT
128                                      ;
129                      Literal         <- APOSTROPH  (!APOSTROPH  Char)* APOSTROPH  WHITESPACE
130                                      /  DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
131                      Class           <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
132                      Range           <- Char TO Char / Char ;
133
134                      StartExpr       <- OPEN Expression CLOSE ;
135              void:   Final           <- "END" WHITESPACE SEMICOLON WHITESPACE ;
136
137                      # --------------------------------------------------------------------
138                      # Lexing constructs
139
140                      Identifier      <- Ident WHITESPACE ;
141              leaf:   Ident           <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
142                      Char            <- CharSpecial / CharOctalFull / CharOctalPart
143                                      /  CharUnicode / CharUnescaped
144                                      ;
145
146              leaf:   CharSpecial     <- "\\" [nrt'"\[\]\\] ;
147              leaf:   CharOctalFull   <- "\\" [0-2][0-7][0-7] ;
148              leaf:   CharOctalPart   <- "\\" [0-7][0-7]? ;
149              leaf:   CharUnicode     <- "\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
150              leaf:   CharUnescaped   <- !"\\" . ;
151
152              void:   HexDigit        <- [0-9a-fA-F] ;
153
154              void:   TO              <- '-'           ;
155              void:   OPENB           <- "["           ;
156              void:   CLOSEB          <- "]"           ;
157              void:   APOSTROPH       <- "'"           ;
158              void:   DAPOSTROPH      <- '"'           ;
159              void:   PEG             <- "PEG" !([_:] / <alnum>) WHITESPACE ;
160              void:   IS              <- "<-"    WHITESPACE ;
161              leaf:   VOID            <- "void"  WHITESPACE ; # Implies that definition has no semantic value.
162              leaf:   LEAF            <- "leaf"  WHITESPACE ; # Implies that definition has no terminals.
163              void:   SEMICOLON       <- ";"     WHITESPACE ;
164              void:   COLON           <- ":"     WHITESPACE ;
165              void:   SLASH           <- "/"     WHITESPACE ;
166              leaf:   AND             <- "&"     WHITESPACE ;
167              leaf:   NOT             <- "!"     WHITESPACE ;
168              leaf:   QUESTION        <- "?"     WHITESPACE ;
169              leaf:   STAR            <- "*"     WHITESPACE ;
170              leaf:   PLUS            <- "+"     WHITESPACE ;
171              void:   OPEN            <- "("     WHITESPACE ;
172              void:   CLOSE           <- ")"     WHITESPACE ;
173              leaf:   DOT             <- "."     WHITESPACE ;
174
175              leaf:   ALNUM           <- "<alnum>"    WHITESPACE ;
176              leaf:   ALPHA           <- "<alpha>"    WHITESPACE ;
177              leaf:   ASCII           <- "<ascii>"    WHITESPACE ;
178              leaf:   CONTROL         <- "<control>"  WHITESPACE ;
179              leaf:   DDIGIT          <- "<ddigit>"   WHITESPACE ;
180              leaf:   DIGIT           <- "<digit>"    WHITESPACE ;
181              leaf:   GRAPH           <- "<graph>"    WHITESPACE ;
182              leaf:   LOWER           <- "<lower>"    WHITESPACE ;
183              leaf:   PRINTABLE       <- "<print>"    WHITESPACE ;
184              leaf:   PUNCT           <- "<punct>"    WHITESPACE ;
185              leaf:   SPACE           <- "<space>"    WHITESPACE ;
186              leaf:   UPPER           <- "<upper>"    WHITESPACE ;
187              leaf:   WORDCHAR        <- "<wordchar>" WHITESPACE ;
188              leaf:   XDIGIT          <- "<xdigit>"   WHITESPACE ;
189
190              void:   WHITESPACE      <- (" " / "\t" / EOL / COMMENT)* ;
191              void:   COMMENT         <- '#' (!EOL .)* EOL ;
192              void:   EOL             <- "\n\r" / "\n" / "\r" ;
193              void:   EOF             <- !. ;
194
195                      # --------------------------------------------------------------------
196              END;
197
198
199   EXAMPLE
200       Our example specifies the grammar for a basic 4-operation calculator.
201
202              PEG calculator (Expression)
203                  Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'       ;
204                  Sign       <- '-' / '+'                                     ;
205                  Number     <- Sign? Digit+                                  ;
206                  Expression <- Term (AddOp Term)*                            ;
207                  MulOp      <- '*' / '/'                                     ;
208                  Term       <- Factor (MulOp Factor)*                        ;
209                  AddOp      <- '+'/'-'                                       ;
210                  Factor     <- '(' Expression ')' / Number                   ;
211              END;
212
213
214       Using higher-level features of the notation, i.e. the character classes
215       (predefined and custom), this example can be rewritten as
216
217              PEG calculator (Expression)
218                  Sign       <- [-+] ;
219                  Number     <- Sign? <ddigit>+;
220                  Expression <- '(' Expression ')' / (Factor (MulOp Factor)*);
221                  MulOp      <- [*/];
222                  Factor     <- Term (AddOp Term)*;
223                  AddOp      <- [-+];
224                  Term       <- Number;
225              END;
226
227

PEG SERIALIZATION FORMAT

229       Here we specify the format used by the Parser Tools to serialize  Pars‐
230       ing  Expression Grammars as immutable values for transport, comparison,
231       etc.
232
233       We distinguish between regular and canonical serializations.   While  a
234       PEG  may  have  more than one regular serialization only exactly one of
235       them will be canonical.
236
237       regular serialization
238
239              [1]    The serialization of any PEG is a nested Tcl dictionary.
240
241              [2]    This dictionary holds a single key, pt::grammar::peg, and
242                     its value. This value holds the contents of the grammar.
243
244              [3]    The  contents of the grammar are a Tcl dictionary holding
245                     the set of nonterminal symbols and the  starting  expres‐
246                     sion. The relevant keys and their values are
247
248                     rules  The  value  is a Tcl dictionary whose keys are the
249                            names of the  nonterminal  symbols  known  to  the
250                            grammar.
251
252                            [1]    Each  nonterminal  symbol  may  occur  only
253                                   once.
254
255                            [2]    The empty string is not a legal nonterminal
256                                   symbol.
257
258                            [3]    The  value for each symbol is a Tcl dictio‐
259                                   nary itself. The relevant  keys  and  their
260                                   values in this dictionary are
261
262                                   is     The  value  is  the serialization of
263                                          the  parsing  expression  describing
264                                          the symbols sentennial structure, as
265                                          specified in the section PE  serial‐
266                                          ization format.
267
268                                   mode   The value can be one of three values
269                                          specifying how a parser should  han‐
270                                          dle  the  semantic value produced by
271                                          the symbol.
272
273                                          value  The  semantic  value  of  the
274                                                 nonterminal   symbol   is  an
275                                                 abstract syntax tree consist‐
276                                                 ing of a single node node for
277                                                 the nonterminal itself, which
278                                                 has  the ASTs of the symbol's
279                                                 right hand side as its  chil‐
280                                                 dren.
281
282                                          leaf   The  semantic  value  of  the
283                                                 nonterminal  symbol   is   an
284                                                 abstract syntax tree consist‐
285                                                 ing of a single node node for
286                                                 the  nonterminal, without any
287                                                 children. Any ASTs  generated
288                                                 by  the  symbol's  right hand
289                                                 side are discarded.
290
291                                          void   The nonterminal has no seman‐
292                                                 tic value. Any ASTs generated
293                                                 by the  symbol's  right  hand
294                                                 side are discarded (as well).
295
296                     start  The  value is the serialization of the start pars‐
297                            ing expression of the grammar, as specified in the
298                            section PE serialization format.
299
300              [4]    The terminal symbols of the grammar are specified implic‐
301                     itly as the set of all terminal symbols used in the start
302                     expression and on the RHS of the grammar rules.
303
304       canonical serialization
305              The canonical serialization of a grammar has the format as spec‐
306              ified in the previous item, and then additionally satisfies  the
307              constraints  below,  which make it unique among all the possible
308              serializations of this grammar.
309
310              [1]    The keys found in all the  nested  Tcl  dictionaries  are
311                     sorted  in  ascending  dictionary  order, as generated by
312                     Tcl's builtin command lsort -increasing -dict.
313
314              [2]    The string representation of the value is  the  canonical
315                     representation of a Tcl dictionary. I.e. it does not con‐
316                     tain superfluous whitespace.
317
318   EXAMPLE
319       Assuming the following PEG for simple mathematical expressions
320
321              PEG calculator (Expression)
322                  Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'       ;
323                  Sign       <- '-' / '+'                                     ;
324                  Number     <- Sign? Digit+                                  ;
325                  Expression <- Term (AddOp Term)*                            ;
326                  MulOp      <- '*' / '/'                                     ;
327                  Term       <- Factor (MulOp Factor)*                        ;
328                  AddOp      <- '+'/'-'                                       ;
329                  Factor     <- '(' Expression ')' / Number                   ;
330              END;
331
332
333       then its canonical serialization (except for whitespace) is
334
335              pt::grammar::peg {
336                  rules {
337                      AddOp      {is {/ {t -} {t +}}                                                                mode value}
338                      Digit      {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}                mode value}
339                      Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}}                                        mode value}
340                      Factor     {is {/ {x {t (} {n Expression} {t )}} {n Number}}                                  mode value}
341                      MulOp      {is {/ {t *} {t /}}                                                                mode value}
342                      Number     {is {x {? {n Sign}} {+ {n Digit}}}                                                 mode value}
343                      Sign       {is {/ {t -} {t +}}                                                                mode value}
344                      Term       {is {x {n Factor} {* {x {n MulOp} {n Factor}}}}                                    mode value}
345                  }
346                  start {n Expression}
347              }
348
349

PE SERIALIZATION FORMAT

351       Here we specify the format used by the Parser Tools to serialize  Pars‐
352       ing Expressions as immutable values for transport, comparison, etc.
353
354       We  distinguish  between regular and canonical serializations.  While a
355       parsing expression may have more than one  regular  serialization  only
356       exactly one of them will be canonical.
357
358       Regular serialization
359
360              Atomic Parsing Expressions
361
362                     [1]    The  string  epsilon  is an atomic parsing expres‐
363                            sion. It matches the empty string.
364
365                     [2]    The string dot is an atomic parsing expression. It
366                            matches any character.
367
368                     [3]    The  string alnum is an atomic parsing expression.
369                            It matches any Unicode alphabet or  digit  charac‐
370                            ter.  This  is  a custom extension of PEs based on
371                            Tcl's builtin command string is.
372
373                     [4]    The string alpha is an atomic parsing  expression.
374                            It matches any Unicode alphabet character. This is
375                            a custom extension of PEs based on  Tcl's  builtin
376                            command string is.
377
378                     [5]    The  string ascii is an atomic parsing expression.
379                            It matches any Unicode character below U0080. This
380                            is  a  custom  extension  of  PEs  based  on Tcl's
381                            builtin command string is.
382
383                     [6]    The string control is an  atomic  parsing  expres‐
384                            sion.  It  matches  any Unicode control character.
385                            This is a custom extension of PEs based  on  Tcl's
386                            builtin command string is.
387
388                     [7]    The  string digit is an atomic parsing expression.
389                            It matches any Unicode digit character. Note  that
390                            this  includes  characters  outside  of the [0..9]
391                            range. This is a custom extension of PEs based  on
392                            Tcl's builtin command string is.
393
394                     [8]    The  string graph is an atomic parsing expression.
395                            It matches any Unicode printing character,  except
396                            for space. This is a custom extension of PEs based
397                            on Tcl's builtin command string is.
398
399                     [9]    The string lower is an atomic parsing  expression.
400                            It matches any Unicode lower-case alphabet charac‐
401                            ter. This is a custom extension of  PEs  based  on
402                            Tcl's builtin command string is.
403
404                     [10]   The  string print is an atomic parsing expression.
405                            It matches any Unicode printing character, includ‐
406                            ing space. This is a custom extension of PEs based
407                            on Tcl's builtin command string is.
408
409                     [11]   The string punct is an atomic parsing  expression.
410                            It matches any Unicode punctuation character. This
411                            is a  custom  extension  of  PEs  based  on  Tcl's
412                            builtin command string is.
413
414                     [12]   The  string space is an atomic parsing expression.
415                            It matches any Unicode space character. This is  a
416                            custom  extension  of  PEs  based on Tcl's builtin
417                            command string is.
418
419                     [13]   The string upper is an atomic parsing  expression.
420                            It matches any Unicode upper-case alphabet charac‐
421                            ter. This is a custom extension of  PEs  based  on
422                            Tcl's builtin command string is.
423
424                     [14]   The  string  wordchar is an atomic parsing expres‐
425                            sion. It matches any Unicode word character.  This
426                            is any alphanumeric character (see alnum), and any
427                            connector  punctuation  characters  (e.g.   under‐
428                            score). This is a custom extension of PEs based on
429                            Tcl's builtin command string is.
430
431                     [15]   The string xdigit is an atomic parsing expression.
432                            It  matches  any hexadecimal digit character. This
433                            is a  custom  extension  of  PEs  based  on  Tcl's
434                            builtin command string is.
435
436                     [16]   The string ddigit is an atomic parsing expression.
437                            It matches any decimal digit character. This is  a
438                            custom  extension  of  PEs  based on Tcl's builtin
439                            command regexp.
440
441                     [17]   The expression [list t x]  is  an  atomic  parsing
442                            expression. It matches the terminal string x.
443
444                     [18]   The  expression  [list  n  A] is an atomic parsing
445                            expression. It matches the nonterminal A.
446
447              Combined Parsing Expressions
448
449                     [1]    For parsing expressions e1, e2, ... the result  of
450                            [list  /  e1  e2  ... ] is a parsing expression as
451                            well.  This is the ordered choice, aka prioritized
452                            choice.
453
454                     [2]    For  parsing expressions e1, e2, ... the result of
455                            [list x e1 e2 ... ] is  a  parsing  expression  as
456                            well.  This is the sequence.
457
458                     [3]    For  a  parsing expression e the result of [list *
459                            e] is a parsing expression as well.  This  is  the
460                            kleene  closure,  describing  zero or more repeti‐
461                            tions.
462
463                     [4]    For a parsing expression e the result of  [list  +
464                            e]  is  a parsing expression as well.  This is the
465                            positive kleene closure, describing  one  or  more
466                            repetitions.
467
468                     [5]    For  a  parsing expression e the result of [list &
469                            e] is a parsing expression as well.  This  is  the
470                            and lookahead predicate.
471
472                     [6]    For  a  parsing expression e the result of [list !
473                            e] is a parsing expression as well.  This  is  the
474                            not lookahead predicate.
475
476                     [7]    For  a  parsing expression e the result of [list ?
477                            e] is a parsing expression as well.  This  is  the
478                            optional input.
479
480       Canonical serialization
481              The canonical serialization of a parsing expression has the for‐
482              mat as specified in the previous  item,  and  then  additionally
483              satisfies  the constraints below, which make it unique among all
484              the possible serializations of this parsing expression.
485
486              [1]    The string representation of the value is  the  canonical
487                     representation  of a pure Tcl list. I.e. it does not con‐
488                     tain superfluous whitespace.
489
490              [2]    Terminals are not encoded as ranges (where start and  end
491                     of the range are identical).
492
493   EXAMPLE
494       Assuming  the  parsing  expression  shown on the right-hand side of the
495       rule
496
497                  Expression <- Term (AddOp Term)*
498
499
500       then its canonical serialization (except for whitespace) is
501
502                  {x {n Term} {* {x {n AddOp} {n Term}}}}
503
504

BUGS, IDEAS, FEEDBACK

506       This document, and the package it describes, will  undoubtedly  contain
507       bugs  and other problems.  Please report such in the category pt of the
508       Tcllib Trackers  [http://core.tcl.tk/tcllib/reportlist].   Please  also
509       report  any  ideas  for  enhancements  you  may have for either package
510       and/or documentation.
511
512       When proposing code changes, please provide unified diffs, i.e the out‐
513       put of diff -u.
514
515       Note  further  that  attachments  are  strongly  preferred over inlined
516       patches. Attachments can be made by going  to  the  Edit  form  of  the
517       ticket  immediately  after  its  creation, and then using the left-most
518       button in the secondary navigation bar.
519

KEYWORDS

521       EBNF, LL(k), PEG, TDPL,  context-free  languages,  export,  expression,
522       grammar, matching, parser, parsing expression, parsing expression gram‐
523       mar, plugin, push down  automaton,  recursive  descent,  serialization,
524       state, top-down parsing languages, transducer
525

CATEGORY

527       Parsing and Grammars
528
530       Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
531
532
533
534
535tcllib                                 1               pt::peg::export::peg(n)
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