pt::peg(n) Parser Tools pt::peg(n) ______________________________________________________________________________
NAME
pt::peg - Parsing Expression Grammar Serialization
SYNOPSIS
package require Tcl 8.5
package require pt::peg ?1?
package require pt::pe
::pt::peg verify serial ?canonvar?
::pt::peg verify-as-canonical serial
::pt::peg canonicalize serial
::pt::peg print serial
::pt::peg merge seriala serialb
::pt::peg equal seriala serialb
_________________________________________________________________
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 provides commands to work with the serializations of pars-
ing expression grammars as managed by the Parser Tools, and specified
in section PEG serialization format.
This is a supporting package in the Core Layer of Parser Tools.
IMAGE: arch_core_support
API
::pt::peg verify serial ?canonvar?
This command verifies that the content of serial is a valid
serialization of a parsing expression and will throw an error if
that is not the case. The result of the command is the empty
string.
If the argument canonvar is specified it is interpreted as the
name of a variable in the calling context. This variable will be
written to if and only if serial is a valid regular serializa-
tion. Its value will be a boolean, with True indicating that the
serialization is not only valid, but also canonical. False will
be written for a valid, but non-canonical serialization.
For the specification of serializations see the section PE seri-
alization format.
::pt::peg verify-as-canonical serial
This command verifies that the content of serial is a valid
canonical serialization of a PEG and will throw an error if that
is not the case. The result of the command is the empty string.
For the specification of canonical serializations see the sec-
tion PEG serialization format.
::pt::peg canonicalize serial
This command assumes that the content of serial is a valid regu-
lar serialization of a PEG and will throw an error if that is
not the case.
It will then convert the input into the canonical serialization
of the contained PEG and return it as its result. If the input
is already canonical it will be returned unchanged.
For the specification of regular and canonical serializations
see the section PEG serialization format.
::pt::peg print serial
This command assumes that the argument serial contains a valid
serialization of a parsing expression and returns a string con-
taining that PE in a human readable form.
The exact format of this form is not specified and cannot be
relied on for parsing or other machine-based activities.
For the specification of serializations see the section PEG
serialization format.
::pt::peg merge seriala serialb
This command accepts the regular serializations of two grammars
and uses them to create their union. The result of the command
is the canonical serialization of this unified grammar.
A merge errors occurs if for any nonterminal symbol S occuring
in both input grammars the two input grammars specify different
semantic modes.
The semantic mode of each nonterminal symbol S is the semantic
mode of S in any of its input grammars. The previous rule made
sure that for symbols occuring in both grammars these values are
identical.
The right-hand side of each nonterminal symbol S occuring in
both input grammars is the choice between the right-hand sides
of S in the input grammars, with the parsing expression of S in
seriala coming first, except if both expressions are identical.
In that case the first expression is taken.
The right-hand side of each nonterminal symbol S occuring in
only one of the input grammars is the right-hand side of S in
its input grammar.
The start expression of the unified grammar is the choice
between the start expressions of the input grammars, with the
start expression of seriala coming first, except if both expres-
sions are identical. In that case the first expression is taken
::pt::peg equal seriala serialb
This command tests the two grammars seriala and serialb for
structural equality. The result of the command is a boolean
value. It will be set to true if the expressions are identical,
and false otherwise.
String equality is usable only if we can assume that the two
grammars are pure Tcl lists and dictionaries.
PEG SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Pars-
ing 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 expres-
sion. 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 dictio-
nary 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 serial-
ization format.
mode The value can be one of three values
specifying how a parser should han-
dle the semantic value produced by
the symbol.
value The semantic value of the
nonterminal symbol is an
abstract syntax tree consist-
ing of a single node node for
the nonterminal itself, which
has the ASTs of the symbol's
right hand side as its chil-
dren.
leaf The semantic value of the
nonterminal symbol is an
abstract syntax tree consist-
ing 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 seman-
tic value. Any ASTs generated
by the symbol's right hand
side are discarded (as well).
start The value is the serialization of the start pars-
ing expression of the grammar, as specified in the
section PE serialization format.
[4] The terminal symbols of the grammar are specified implic-
itly 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 spec-
ified 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 con-
tain 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 <- '(' Expression ')' / (Factor (MulOp Factor)*) ;
MulOp <- '*' / '/' ;
Factor <- Term (AddOp Term)* ;
AddOp <- '+'/'-' ;
Term <- 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 {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}} mode value}
Factor {is {x {n Term} {* {x {n AddOp} {n Term}}}} 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 {n Number} mode value}
}
start {n Expression}
}
PE SERIALIZATION FORMAT
Here we specify the format used by the Parser Tools to serialize Pars-
ing 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 expres-
sion. 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 charac-
ter. 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 expres-
sion. 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 charac-
ter. 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, includ-
ing 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 charac-
ter. This is a custom extension of PEs based on
Tcl's builtin command string is.
[14] The string wordchar is an atomic parsing expres-
sion. It matches any Unicode word character. This
is any alphanumeric character (see alnum), and any
connector punctuation characters (e.g. under-
score). 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 repeti-
tions.
[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 for-
mat 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 con-
tain 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 <- '(' Expression ')'
/ Factor (MulOp Factor)*
then its canonical serialization (except for whitespace) is
{/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}
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, expression, grammar,
matching, parser, parsing expression, parsing expression grammar, push
down automaton, recursive descent, state, top-down parsing languages,
transducer
CATEGORY
Parsing and Grammars
COPYRIGHT
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
pt 1 pt::peg(n)
Mac OS X 10.8 - Generated Tue Sep 11 09:48:05 CDT 2012