pt::peg::export::container(n) Parser Tools pt::peg::export::container(n)
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NAME
pt::peg::export::container - PEG Export Plugin. Write CONTAINER format
SYNOPSIS
package require Tcl 8.5
package require pt::peg::export::container ?1?
package require pt::peg::to::container
export serial configuration
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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 parsing expression grammar export plugin for the generation of CONTAINER markup.
It resides in the Export section of the Core Layer of Parser Tools and is intended to be used by pt::peg::export, the export manager, sit-
ting between it and the corresponding core conversion functionality provided by pt::peg::to::container.
IMAGE: arch_core_eplugins
While the direct use of this package with a regular interpreter is possible, this is strongly disrecommended and requires a number of con-
tortions to provide the expected environment. The proper way to use this functionality depends on the situation:
[1] In an untrusted environment the proper access is through the package pt::peg::export and the export manager objects it provides.
[2] In a trusted environment however simply use the package pt::peg::to::container and access the core conversion functionality
directly.
API
The API provided by this package satisfies the specification of the Plugin API found in the Parser Tools Export API specification.
export serial configuration
This command takes the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format,
and contained in serial, the configuration, a dictionary, and generates CONTAINER markup encoding the grammar. The created string
is then returned as the result of the command.
CONFIGURATION
The CONTAINER export plugin recognizes the following configuration variables and changes its behaviour as they specify.
enum mode
The value of this configuration variable controls which methods of pt::peg instances the plugin will use to specify the grammar.
There are two legal values
bulk In this mode the methods start, add, modes, and rules are used to specify the grammar in a bulk manner, i.e. as a set of non-
terminal symbols, and two dictionaries mapping from the symbols to their semantic modes and parsing expressions.
This mode is the default.
incremental
In this mode the methods start, add, mode, and rule are used to specify the grammar piecemal, with each nonterminal having
its own block of defining commands.
string template
If this configuration variable is set it is assumed to contain a string into which to put the generated code and other configuration
data. The various locations are expected to be specified with the following placeholders:
@user@ To be replaced with the value of the configuration variable user.
@format@
To be replaced with the the constant CONTAINER.
@file@ To be replaced with the value of the configuration variable file.
@name@ To be replaced with the value of the configuration variable name.
@mode@ To be replaced with the value of the configuration variable mode.
@code@ To be replaced with the generated code.
If this configuration variable is not set, or empty, then the plugin falls back to a standard template, which is defined as "@code@".
Note that this plugin may ignore the standard configuration variables user, format, file, and their values, depending on the chosen tem-
plate.
The content of the standard configuration variable name, if set, is used as name of the grammar in the output. Otherwise the plugin falls
back to the default name a_pe_grammar.
GRAMMAR CONTAINER
The container format is another form of describing parsing expression grammars. While data in this format is executable it does not consti-
tute a parser for the grammar. It always has to be used in conjunction with the package pt::peg::interp, a grammar interpreter.
The format represents grammars by a snit::type, i.e. class, whose instances are API-compatible to the instances of the pt::peg::container
package, and which are preloaded with the grammar in question.
It has no direct formal specification beyond what was said above.
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;
one possible CONTAINER serialization for it is
snit::type a_pe_grammar {
constructor {} {
install myg using pt::peg::container ${selfns}::G
$myg start {n Expression}
$myg add AddOp Digit Expression Factor MulOp Number Sign Term
$myg modes {
AddOp value
Digit value
Expression value
Factor value
MulOp value
Number value
Sign value
Term value
}
$myg rules {
AddOp {/ {t -} {t +}}
Digit {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}
Expression {/ {x {t 50} {n Expression} {t 51}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}
Factor {x {n Term} {* {x {n AddOp} {n Term}}}}
MulOp {/ {t *} {t /}}
Number {x {? {n Sign}} {+ {n Digit}}}
Sign {/ {t -} {t +}}
Term {n Number}
}
return
}
component myg
delegate method * to myg
}
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 rel-
evant 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 serial-
ization 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 con-
straints 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 super-
fluous 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 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 exten-
sion 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 exten-
sion 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 cus-
tom 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 cus-
tom 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 exten-
sion 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 clo-
sure, 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 predi-
cate.
[6] For a parsing expression e the result of [list ! e] is a parsing expression as well. This is the not lookahead predi-
cate.
[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 satis-
fies 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 superflu-
ous 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
CONTAINER, EBNF, LL(k), PEG, TDPL, context-free languages, export, expression, grammar, matching, parser, parsing expression, parsing
expression grammar, plugin, 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>
pt 1 pt::peg::export::container(n)