Provided by: tcllib_2.0+dfsg-4_all 
NAME
pt::peg - Parsing Expression Grammar Serialization
SYNOPSIS
package require Tcl 8.5 9
package require pt::peg ?1.1?
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 parsing 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 serialization.
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 serialization 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 section PEG serialization format.
::pt::peg canonicalize serial
This command assumes that the content of serial is a valid regular 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 containing 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 expressions 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 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, 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>
tcllib 1.1.1 pt::peg(3tcl)