NAME¶
pt::peg::from::peg - PEG Conversion. Read PEG format
SYNOPSIS¶
package require
Tcl 8.5
package require
pt::peg::from::peg ?1?
pt::peg::from::peg convert text
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 PEG markup to parsing expression
grammars.
It resides in the Import 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 import manager provided by
pt::peg::import. The latter is
intented for use in untrusted environments and done through the corresponding
import plugin
pt::peg::import::peg sitting between converter and import
manager.
IMAGE: arch_core_iplugins
API¶
The API provided by this package satisfies the specification of the Converter
API found in the
Parser Tools Import API specification.
- pt::peg::from::peg convert text
- This command takes the PEG markup encoding a parsing
expression grammar and contained in text, and generates the
canonical serialization of said grammar, as specified in section PEG
serialization format. The created value is then returned as the result
of the command.
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 SEMICOLON WHITESPACE ;
# --------------------------------------------------------------------
# Lexing constructs
Identifier <- Ident WHITESPACE ;
leaf: Ident <- ('_' / ':' / <alpha>) ('_' / ':' / <alnum>)* ;
Char <- CharSpecial / CharOctalFull / CharOctalPart
/ CharUnicode / CharUnescaped
;
leaf: CharSpecial <- "\\" [nrt'"\[\]\\] ;
leaf: CharOctalFull <- "\\" [0-2][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" 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: END <- "END" WHITESPACE ;
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 <- '(' Expression ')' / (Factor (MulOp Factor)*) ;
MulOp <- [*/] ;
Factor <- Term (AddOp Term)* ;
AddOp <- [-+] ;
Term <- Number ;
END;
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}
}
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
SF Trackers [
http://sourceforge.net/tracker/?group_id=12883]. Please also
report any ideas for enhancements you may have for either package and/or
documentation.
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>