Provided by: tcllib_2.0+dfsg-2_all 
NAME
pt::peg::to::peg - PEG Conversion. Write PEG format
SYNOPSIS
package require Tcl 8.5 9
package require pt::peg::to::peg ?1.0.3?
package require pt::peg
package require pt::pe
package require text::write
pt::peg::to::peg reset
pt::peg::to::peg configure
pt::peg::to::peg configure option
pt::peg::to::peg configure option value...
pt::peg::to::peg convert serial
________________________________________________________________________________________________________________
DESCRIPTION
Are you lost ? Do you have trouble understanding this document ? In that case please read the overview
provided by the Introduction to Parser Tools. This document is the entrypoint to the whole system the
current package is a part of.
This package implements the converter from parsing expression grammars to PEG markup.
It resides in the Export section of the Core Layer of Parser Tools, and can be used either directly with
the other packages of this layer, or indirectly through the export manager provided by pt::peg::export.
The latter is intented for use in untrusted environments and done through the corresponding export plugin
pt::peg::export::peg sitting between converter and export manager.
IMAGE: arch_core_eplugins
API
The API provided by this package satisfies the specification of the Converter API found in the Parser
Tools Export API specification.
pt::peg::to::peg reset
This command resets the configuration of the package to its default settings.
pt::peg::to::peg configure
This command returns a dictionary containing the current configuration of the package.
pt::peg::to::peg configure option
This command returns the current value of the specified configuration option of the package. For
the set of legal options, please read the section Options.
pt::peg::to::peg configure option value...
This command sets the given configuration options of the package, to the specified values. For the
set of legal options, please read the section Options.
pt::peg::to::peg convert serial
This command takes the canonical serialization of a parsing expression grammar, as specified in
section PEG serialization format, and contained in serial, and generates PEG markup encoding the
grammar, per the current package configuration. The created string is then returned as the result
of the command.
OPTIONS
The converter to the PEG language recognizes the following options and changes its behaviour as they
specify.
-file string
The value of this option is the name of the file or other entity from which the grammar came, for
which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default value
is unknown.
-template string
The value of this option is a string into which to put the generated text and the values of the
other options. The various locations for user-data are expected to be specified with the
placeholders listed below. The default value is "@code@".
@user@ To be replaced with the value of the option -user.
@format@
To be replaced with the the constant PEG.
@file@ To be replaced with the value of the option -file.
@name@ To be replaced with the value of the option -name.
@code@ To be replaced with the generated text.
PEG SPECIFICATION LANGUAGE
peg, a language for the specification of parsing expression grammars is meant to be human readable, and
writable as well, yet strict enough to allow its processing by machine. Like any computer language. It
was defined to make writing the specification of a grammar easy, something the other formats found in the
Parser Tools do not lend themselves too.
It is formally specified by the grammar shown below, written in itself. For a tutorial / introduction to
the language please go and read the PEG Language Tutorial.
PEG pe_grammar_for_peg (Grammar)
# --------------------------------------------------------------------
# Syntactical constructs
Grammar <- WHITESPACE Header Definition* Final EOF ;
Header <- PEG Identifier StartExpr ;
Definition <- Attribute? Identifier IS Expression SEMICOLON ;
Attribute <- (VOID / LEAF) COLON ;
Expression <- Sequence (SLASH Sequence)* ;
Sequence <- Prefix+ ;
Prefix <- (AND / NOT)? Suffix ;
Suffix <- Primary (QUESTION / STAR / PLUS)? ;
Primary <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
/ GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
/ WORDCHAR / XDIGIT
/ Identifier
/ OPEN Expression CLOSE
/ Literal
/ Class
/ DOT
;
Literal <- APOSTROPH (!APOSTROPH Char)* APOSTROPH WHITESPACE
/ DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
Class <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
Range <- Char TO Char / Char ;
StartExpr <- OPEN Expression CLOSE ;
void: Final <- "END" WHITESPACE SEMICOLON WHITESPACE ;
# --------------------------------------------------------------------
# Lexing constructs
Identifier <- Ident WHITESPACE ;
leaf: Ident <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
Char <- CharSpecial / CharOctalFull / CharOctalPart
/ CharUnicode / CharUnescaped
;
leaf: CharSpecial <- "\\" [nrt'"\[\]\\] ;
leaf: CharOctalFull <- "\\" [0-3][0-7][0-7] ;
leaf: CharOctalPart <- "\\" [0-7][0-7]? ;
leaf: CharUnicode <- "\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
leaf: CharUnescaped <- !"\\" . ;
void: HexDigit <- [0-9a-fA-F] ;
void: TO <- '-' ;
void: OPENB <- "[" ;
void: CLOSEB <- "]" ;
void: APOSTROPH <- "'" ;
void: DAPOSTROPH <- '"' ;
void: PEG <- "PEG" !([_:] / <alnum>) WHITESPACE ;
void: IS <- "<-" WHITESPACE ;
leaf: VOID <- "void" WHITESPACE ; # Implies that definition has no semantic value.
leaf: LEAF <- "leaf" WHITESPACE ; # Implies that definition has no terminals.
void: SEMICOLON <- ";" WHITESPACE ;
void: COLON <- ":" WHITESPACE ;
void: SLASH <- "/" WHITESPACE ;
leaf: AND <- "&" WHITESPACE ;
leaf: NOT <- "!" WHITESPACE ;
leaf: QUESTION <- "?" WHITESPACE ;
leaf: STAR <- "*" WHITESPACE ;
leaf: PLUS <- "+" WHITESPACE ;
void: OPEN <- "(" WHITESPACE ;
void: CLOSE <- ")" WHITESPACE ;
leaf: DOT <- "." WHITESPACE ;
leaf: ALNUM <- "<alnum>" WHITESPACE ;
leaf: ALPHA <- "<alpha>" WHITESPACE ;
leaf: ASCII <- "<ascii>" WHITESPACE ;
leaf: CONTROL <- "<control>" WHITESPACE ;
leaf: DDIGIT <- "<ddigit>" WHITESPACE ;
leaf: DIGIT <- "<digit>" WHITESPACE ;
leaf: GRAPH <- "<graph>" WHITESPACE ;
leaf: LOWER <- "<lower>" WHITESPACE ;
leaf: PRINTABLE <- "<print>" WHITESPACE ;
leaf: PUNCT <- "<punct>" WHITESPACE ;
leaf: SPACE <- "<space>" WHITESPACE ;
leaf: UPPER <- "<upper>" WHITESPACE ;
leaf: WORDCHAR <- "<wordchar>" WHITESPACE ;
leaf: XDIGIT <- "<xdigit>" WHITESPACE ;
void: WHITESPACE <- (" " / "\t" / EOL / COMMENT)* ;
void: COMMENT <- '#' (!EOL .)* EOL ;
void: EOL <- "\n\r" / "\n" / "\r" ;
void: EOF <- !. ;
# --------------------------------------------------------------------
END;
EXAMPLE
Our example specifies the grammar for a basic 4-operation calculator.
PEG calculator (Expression)
Digit <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9' ;
Sign <- '-' / '+' ;
Number <- Sign? Digit+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- '*' / '/' ;
Term <- Factor (MulOp Factor)* ;
AddOp <- '+'/'-' ;
Factor <- '(' Expression ')' / Number ;
END;
Using higher-level features of the notation, i.e. the character classes (predefined and custom), this
example can be rewritten as
PEG calculator (Expression)
Sign <- [-+] ;
Number <- Sign? <ddigit>+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- [*/];
Term <- Factor (MulOp Factor)* ;
AddOp <- [-+];
Factor <- '(' Expression ')' / Number ;
END;
PEG SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Parsing Expression Grammars as immutable
values for transport, comparison, etc.
We distinguish between regular and canonical serializations. While a PEG may have more than one regular
serialization only exactly one of them will be canonical.
regular serialization
[1] The serialization of any PEG is a nested Tcl dictionary.
[2] This dictionary holds a single key, pt::grammar::peg, and its value. This value holds the
contents of the grammar.
[3] The contents of the grammar are a Tcl dictionary holding the set of nonterminal symbols and
the starting expression. The relevant keys and their values are
rules The value is a Tcl dictionary whose keys are the names of the nonterminal symbols
known to the grammar.
[1] Each nonterminal symbol may occur only once.
[2] The empty string is not a legal nonterminal symbol.
[3] The value for each symbol is a Tcl dictionary itself. The relevant keys and
their values in this dictionary are
is The value is the serialization of the parsing expression describing
the symbols sentennial structure, as specified in the section PE
serialization format.
mode The value can be one of three values specifying how a parser should
handle the semantic value produced by the symbol.
value The semantic value of the nonterminal symbol is an abstract
syntax tree consisting of a single node node for the
nonterminal itself, which has the ASTs of the symbol's right
hand side as its children.
leaf The semantic value of the nonterminal symbol is an abstract
syntax tree consisting of a single node node for the
nonterminal, without any children. Any ASTs generated by the
symbol's right hand side are discarded.
void The nonterminal has no semantic value. Any ASTs generated by
the symbol's right hand side are discarded (as well).
start The value is the serialization of the start parsing expression of the grammar, as
specified in the section PE serialization format.
[4] The terminal symbols of the grammar are specified implicitly as the set of all terminal
symbols used in the start expression and on the RHS of the grammar rules.
canonical serialization
The canonical serialization of a grammar has the format as specified in the previous item, and
then additionally satisfies the constraints below, which make it unique among all the possible
serializations of this grammar.
[1] The keys found in all the nested Tcl dictionaries are sorted in ascending dictionary order,
as generated by Tcl's builtin command lsort -increasing -dict.
[2] The string representation of the value is the canonical representation of a Tcl dictionary.
I.e. it does not contain superfluous whitespace.
EXAMPLE
Assuming the following PEG for simple mathematical expressions
PEG calculator (Expression)
Digit <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9' ;
Sign <- '-' / '+' ;
Number <- Sign? Digit+ ;
Expression <- Term (AddOp Term)* ;
MulOp <- '*' / '/' ;
Term <- Factor (MulOp Factor)* ;
AddOp <- '+'/'-' ;
Factor <- '(' Expression ')' / Number ;
END;
then its canonical serialization (except for whitespace) is
pt::grammar::peg {
rules {
AddOp {is {/ {t -} {t +}} mode value}
Digit {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}} mode value}
Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}} mode value}
Factor {is {/ {x {t (} {n Expression} {t )}} {n Number}} mode value}
MulOp {is {/ {t *} {t /}} mode value}
Number {is {x {? {n Sign}} {+ {n Digit}}} mode value}
Sign {is {/ {t -} {t +}} mode value}
Term {is {x {n Factor} {* {x {n MulOp} {n Factor}}}} mode value}
}
start {n Expression}
}
PE SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Parsing Expressions as immutable values
for transport, comparison, etc.
We distinguish between regular and canonical serializations. While a parsing expression may have more
than one regular serialization only exactly one of them will be canonical.
Regular serialization
Atomic Parsing Expressions
[1] The string epsilon is an atomic parsing expression. It matches the empty string.
[2] The string dot is an atomic parsing expression. It matches any character.
[3] The string alnum is an atomic parsing expression. It matches any Unicode alphabet or
digit character. This is a custom extension of PEs based on Tcl's builtin command
string is.
[4] The string alpha is an atomic parsing expression. It matches any Unicode alphabet
character. This is a custom extension of PEs based on Tcl's builtin command string
is.
[5] The string ascii is an atomic parsing expression. It matches any Unicode character
below U0080. This is a custom extension of PEs based on Tcl's builtin command string
is.
[6] The string control is an atomic parsing expression. It matches any Unicode control
character. This is a custom extension of PEs based on Tcl's builtin command string
is.
[7] The string digit is an atomic parsing expression. It matches any Unicode digit
character. Note that this includes characters outside of the [0..9] range. This is a
custom extension of PEs based on Tcl's builtin command string is.
[8] The string graph is an atomic parsing expression. It matches any Unicode printing
character, except for space. This is a custom extension of PEs based on Tcl's
builtin command string is.
[9] The string lower is an atomic parsing expression. It matches any Unicode lower-case
alphabet character. This is a custom extension of PEs based on Tcl's builtin command
string is.
[10] The string print is an atomic parsing expression. It matches any Unicode printing
character, including space. This is a custom extension of PEs based on Tcl's builtin
command string is.
[11] The string punct is an atomic parsing expression. It matches any Unicode punctuation
character. This is a custom extension of PEs based on Tcl's builtin command string
is.
[12] The string space is an atomic parsing expression. It matches any Unicode space
character. This is a custom extension of PEs based on Tcl's builtin command string
is.
[13] The string upper is an atomic parsing expression. It matches any Unicode upper-case
alphabet character. This is a custom extension of PEs based on Tcl's builtin command
string is.
[14] The string wordchar is an atomic parsing expression. It matches any Unicode word
character. This is any alphanumeric character (see alnum), and any connector
punctuation characters (e.g. underscore). This is a custom extension of PEs based
on Tcl's builtin command string is.
[15] The string xdigit is an atomic parsing expression. It matches any hexadecimal digit
character. This is a custom extension of PEs based on Tcl's builtin command string
is.
[16] The string ddigit is an atomic parsing expression. It matches any decimal digit
character. This is a custom extension of PEs based on Tcl's builtin command regexp.
[17] The expression [list t x] is an atomic parsing expression. It matches the terminal
string x.
[18] The expression [list n A] is an atomic parsing expression. It matches the
nonterminal A.
Combined Parsing Expressions
[1] For parsing expressions e1, e2, ... the result of [list / e1 e2 ... ] is a parsing
expression as well. This is the ordered choice, aka prioritized choice.
[2] For parsing expressions e1, e2, ... the result of [list x e1 e2 ... ] is a parsing
expression as well. This is the sequence.
[3] For a parsing expression e the result of [list * e] is a parsing expression as well.
This is the kleene closure, describing zero or more repetitions.
[4] For a parsing expression e the result of [list + e] is a parsing expression as well.
This is the positive kleene closure, describing one or more repetitions.
[5] For a parsing expression e the result of [list & e] is a parsing expression as well.
This is the and lookahead predicate.
[6] For a parsing expression e the result of [list ! e] is a parsing expression as well.
This is the not lookahead predicate.
[7] For a parsing expression e the result of [list ? e] is a parsing expression as well.
This is the optional input.
Canonical serialization
The canonical serialization of a parsing expression has the format as specified in the previous
item, and then additionally satisfies the constraints below, which make it unique among all the
possible serializations of this parsing expression.
[1] The string representation of the value is the canonical representation of a pure Tcl list.
I.e. it does not contain superfluous whitespace.
[2] Terminals are not encoded as ranges (where start and end of the range are identical).
EXAMPLE
Assuming the parsing expression shown on the right-hand side of the rule
Expression <- Term (AddOp Term)*
then its canonical serialization (except for whitespace) is
{x {n Term} {* {x {n AddOp} {n Term}}}}
BUGS, IDEAS, FEEDBACK
This document, and the package it describes, will undoubtedly contain bugs and other problems. Please
report such in the category pt of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please
also report any ideas for enhancements you may have for either package and/or documentation.
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by
going to the Edit form of the ticket immediately after its creation, and then using the left-most button
in the secondary navigation bar.
KEYWORDS
EBNF, LL(k), PEG, TDPL, context-free languages, conversion, expression, format conversion, grammar,
matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent,
serialization, state, top-down parsing languages, transducer
CATEGORY
Parsing and Grammars
COPYRIGHT
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
tcllib 1.0.3 pt::peg::to::peg(3tcl)