NAME¶
pt::pgen - Parser Generator
SYNOPSIS¶
package require
Tcl 8.5
package require
pt::pgen ?1.0.2?
::pt::pgen inputformat text resultformat
?
options...?
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 a command implementing a
parser generator taking
parsing expression grammars as input.
It is the implementation of method
generate of
pt, the
Parser
Tools Application.
As such the intended audience of this document are people wishing to modify
and/or extend this part of
pt's functionality. Users of
pt on
the other hand are hereby refered to the applications' manpage, i.e.
Parser
Tools Application.
It resides in the User Package Layer of Parser Tools.
IMAGE: arch_user_pkg
API¶
- ::pt::pgen inputformat text resultformat
?options...?
- This command takes the parsing expression grammar in text (in the
format specified by inputformat), and returns the same grammar in
the format resultformat as the result of the command.
The two known input formats are peg and json. Introductions to
them, including their formal specifications, can be found in the PEG
Language Tutorial and The JSON Grammar Exchange Format. The
packages used to parse these formats are
- peg
- pt::peg::from::peg
- json
- pt::peg::from::json
On the output side the known formats, and the packages used to generate them are
- c
- pt::peg::to::cparam
- container
- pt::peg::to::container
- critcl
- pt::peg::to::cparam + pt::cparam::configuration::critcl
- json
- pt::peg::to::json
- oo
- pt::peg::to::tclparam +
pt::tclparam::configuration::tcloo
- peg
- pt::peg::to::peg
- snit
- pt::peg::to::tclparam +
pt::tclparam::configuration::snit
- The options supported by each of these formats are documented with their
respective packages.
EXAMPLE¶
In this section we are working a complete example, starting with a PEG grammar
and ending with running the parser generated from it over some input,
following the outline shown in the figure below:
IMAGE: flow
Our grammar, assumed to the stored in the file "
calculator.peg" is
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;
From this we create a snit-based parser using the script "
gen"
package require Tcl 8.5
package require fileutil
package require pt::pgen
lassign $argv name
set grammar [fileutil::cat $name.peg]
set pclass [pt::pgen peg $gr snit -class $name -file $name.peg -name $name]
fileutil::writeFile $name.tcl $pclass
exit 0
calling it like
which leaves us with the parser package and class written to the file "
calculator.tcl". Assuming that this package is then properly
installed in a place where Tcl can find it we can now use this class via a
script like
package require calculator
lassign $argv input
set channel [open $input r]
set parser [calculator]
set ast [$parser parse $channel]
$parser destroy
close $channel
... now process the returned abstract syntax tree ...
where the abstract syntax tree stored in the variable will look like
set ast {Expression 0 4
{Factor 0 4
{Term 0 2
{Number 0 2
{Digit 0 0}
{Digit 1 1}
{Digit 2 2}
}
}
{AddOp 3 3}
{Term 4 4
{Number 4 4
{Digit 4 4}
}
}
}
}
assuming that the input file and channel contained the text
A more graphical representation of the tree would be
.nf +- Digit 0 0 | 1 | | +- Term 0 2 --- Number 0 2 -+- Digit 1 1 | 2 | | | | +-
Digit 2 2 | 0 | | Expression 0 4 --- Factor 0 4
-+----------------------------- AddOp 3 3 | + | | +- Term 4 4 --- Number 4 4
--- Digit 4 4 | 5 .fi
Regardless, at this point it is the user's responsibility to work with the tree
to reach whatever goal she desires. I.e. analyze it, transform it, etc. The
package
pt::ast should be of help here, providing commands to walk such
ASTs structures in various ways.
One important thing to note is that the parsers used here return a data
structure representing the structure of the input per the grammar underlying
the parser. There are
no callbacks during the parsing process, i.e. no
parsing actions, as most other parsers will have.
Going back to the last snippet of code, the execution of the parser for some
input, note how the parser instance follows the specified
Parser API.
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.
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>