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pt(n) Parser Tools pt(n)
____________________________________________________________________________________________________________
NAME
pt - Parser Tools Application
SYNOPSIS
package require Tcl 8.5
pt generate resultformat ?options...? resultfile inputformat inputfile
____________________________________________________________________________________________________________
DESCRIPTION
Are you lost ? Do you have trouble understanding this document ? In that case please read the over-view overview
view provided by the Introduction to Parser Tools. This document is the entrypoint to the whole sys-tem system
tem the current package is a part of.
This document describes pt, the main application of the module, a parser generator. Its intended
audience are people who wish to create a parser for some language of theirs. Should you wish to mod-ify modify
ify the application instead, please see the section about the application's Internals for the basic
references.
It resides in the User Application Layer of Parser Tools.
IMAGE: arch_user_app
COMMAND LINE
pt generate resultformat ?options...? resultfile inputformat inputfile
This sub-command of the application reads the parsing expression grammar stored in the input-file inputfile
file in the format inputformat, converts it to the resultformat under the direction of the
(format-specific) set of options specified by the user and stores the result in the result-file. resultfile.
file.
The inputfile has to exist, while the resultfile may be created, overwriting any pre-existing
content of the file. Any missing directory in the path to the resultfile will be created as
well.
The exact form of the result for, and the set of options supported by the known result-for-mats, result-formats,
mats, are explained in the upcoming sections of this document, with the list below providing
an index mapping between format name and its associated section. In alphabetical order:
c A resultformat. See section C Parser.
container
A resultformat. See section Grammar Container.
critcl A resultformat. See section C Parser Embedded In Tcl.
json A input- and resultformat. See section JSON Grammar Exchange.
oo A resultformat. See section TclOO Parser.
peg A input- and resultformat. See section PEG Specification Language.
snit A resultformat. See section Snit Parser.
Of the seven possible results four are parsers outright (c, critcl, oo, and snit), one (container)
provides code which can be used in conjunction with a generic parser (also known as a grammar inter-preter), interpreter),
preter), and the last two (json and peg) are doing double-duty as input formats, allowing the trans-formation transformation
formation of grammars for exchange, reformatting, and the like.
The created parsers fall into three categories:
IMAGE: gen_options
Specialized parsers implemented in C
The fastest parsers are created when using the result formats c and critcl. The first returns
the raw C code for the parser, while the latter wraps it into a Tcl package using CriTcl.
This makes the latter much easier to use than the former. On the other hand, the former can be
adapted to the users' requirements through a multitude of options, allowing for things like
usage of the parser outside of a Tcl environment, something the critcl format doesn't support.
As such the c format is meant for more advanced users, or users with special needs.
A disadvantage of all the parsers in this section is the need to run them through a C compiler
to make them actually executable. This is not something everyone has the necessary tools for.
The parsers in the next section are for people under such restrictions.
Specialized parsers implemented in Tcl
As the parsers in this section are implemented in Tcl they are quite a bit slower than any-thing anything
thing from the previous section. On the other hand this allows them to be used in pure-Tcl
environments, or in environments which allow only a limited set of binary packages. In the
latter case it will be advantageous to lobby for the inclusion of the C-based runtime support
(notes below) into the environment to reduce the impact of Tcl's on the speed of these
parsers.
The relevant formats are snit and oo. Both place their result into a Tcl package containing a
snit::type, or TclOO class respectively.
Of the supporting runtime, which is the package pt::rde, the user has to know nothing but that
it does exist and that the parsers are dependent on it. Knowledge of the API exported by the
runtime for the parsers' consumption is not required by the parsers' users.
Interpreted parsing implemented in Tcl
The last category, grammar interpretation. This means that an interpreter for parsing expres-sion expression
sion grammars takes the description of the grammar to parse input for, and uses it guide the
parsing process. This is the slowest of the available options, as the interpreter has to con-tinually continually
tinually run through the configured grammar, whereas the specialized parsers of the previous
sections have the relevant knowledge about the grammar baked into them.
The only places where using interpretation make sense is where the grammar for some input may
be changed interactively by the user, as the interpretation allows for quick turnaround after
each change, whereas the previous methods require the generation of a whole new parser, which
is not as fast. On the other hand, wherever the grammar to use is fixed, the previous methods
are much more advantageous as the time to generate the parser is minuscule compared to the
time the parser code is in use.
The relevant result format is container. It (quickly) generates grammar descriptions (instead
of a full parser) which match the API expected by ParserTools' grammar interpreter. The lat-ter latter
ter is provided by the package pt::peg::interp.
All the parsers generated by critcl, snit, and oo, and the grammar interpreter share a common API for
access to the actual parsing functionality, making them all plug-compatible. It is described in the
Parser API specification document.
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 lan-guage. language.
guage. It was defined to make writing the specification of a grammar easy, something the other for-mats formats
mats found in the Parser Tools do not lend themselves too.
For either an introduction to or the formal specification of the language, please go and read the PEG
Language Tutorial.
When used as a result-format this format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-template string
The value of this option is a string into which to put the generated text and the values of
the other options. The various locations for user-data are expected to be specified with the
placeholders listed below. The default value is "@code@".
@user@ To be replaced with the value of the option -user.
@format@
To be replaced with the the constant PEG.
@file@ To be replaced with the value of the option -file.
@name@ To be replaced with the value of the option -name.
@code@ To be replaced with the generated text.
JSON GRAMMAR EXCHANGE
The json format for parsing expression grammars was written as a data exchange format not bound to
Tcl. It was defined to allow the exchange of grammars with PackRat/PEG based parser generators for
other languages.
For the formal specification of the JSON grammar exchange format, please go and read The JSON Grammar
Exchange Format.
When used as a result-format this format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-indented boolean
If this option is set the system will break the generated JSON across lines and indent it
according to its inner structure, with each key of a dictionary on a separate line.
If the option is not set (the default), the whole JSON object will be written on a single
line, with minimum spacing between all elements.
-aligned boolean
If this option is set the system will ensure that the values for the keys in a dictionary are
vertically aligned with each other, for a nice table effect. To make this work this also
implies that -indented is set.
If the option is not set (the default), the output is formatted as per the value of indented,
without trying to align the values for dictionary keys.
C PARSER EMBEDDED IN TCL
The critcl format is executable code, a parser for the grammar. It is a Tcl package with the actual
parser implementation written in C and embedded in Tcl via the critcl package.
This result-format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-class string
The value of this option is the name of the class to generate, without leading colons. The
default value is CLASS.
For a simple value X without colons, like CLASS, the parser command will be X::X. Whereas for
a namespaced value X::Y the parser command will be X::Y.
-package string
The value of this option is the name of the package to generate. The default value is PACK-AGE. PACKAGE.
AGE.
C PARSER
The c format is executable code, a parser for the grammar. The parser implementation is written in C
and can be tweaked to the users' needs through a multitude of options.
The critcl format, for example, is implemented as a canned configuration of these options on top of
the generator for c.
This result-format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-template string
The value of this option is a string into which to put the generated text and the other con-figuration configuration
figuration settings. The various locations for user-data are expected to be specified with the
placeholders listed below. The default value is "@code@".
@user@ To be replaced with the value of the option -user.
@format@
To be replaced with the the constant C/PARAM.
@file@ To be replaced with the value of the option -file.
@name@ To be replaced with the value of the option -name.
@code@ To be replaced with the generated Tcl code.
The following options are special, in that they will occur within the generated code, and are
replaced there as well.
@statedecl@
To be replaced with the value of the option state-decl.
@stateref@
To be replaced with the value of the option state-ref.
@strings@
To be replaced with the value of the option string-varname.
@self@ To be replaced with the value of the option self-command.
@def@ To be replaced with the value of the option fun-qualifier.
@ns@ To be replaced with the value of the option namespace.
@main@ To be replaced with the value of the option main.
@prelude@
To be replaced with the value of the option prelude.
-state-decl string
A C string representing the argument declaration to use in the generated parsing functions to
refer to the parsing state. In essence type and argument name. The default value is the
string RDE_PARAM p.
-state-ref string
A C string representing the argument named used in the generated parsing functions to refer to
the parsing state. The default value is the string p.
-self-command string
A C string representing the reference needed to call the generated parser function (methods
...) from another parser fonction, per the chosen framework (template). The default value is
the empty string.
-fun-qualifier string
A C string containing the attributes to give to the generated functions (methods ...), per the
chosen framework (template). The default value is static.
-namespace string
The name of the C namespace the parser functions (methods, ...) shall reside in, or a general
prefix to add to the function names. The default value is the empty string.
-main string
The name of the main function (method, ...) to be called by the chosen framework (template) to
start parsing input. The default value is __main.
-string-varname string
The name of the variable used for the table of strings used by the generated parser, i.e.
error messages, symbol names, etc. The default value is p_string.
-prelude string
A snippet of code to be inserted at the head of each generated parsing function. The default
value is the empty string.
-indent integer
The number of characters to indent each line of the generated code by. The default value is
0.
SNIT PARSER
The snit format is executable code, a parser for the grammar. It is a Tcl package holding a
snit::type, i.e. a class, whose instances are parsers for the input grammar.
This result-format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-class string
The value of this option is the name of the class to generate, without leading colons. Note,
it serves double-duty as the name of the package to generate too. The default value is CLASS.
TCLOO PARSER
The oo format is executable code, a parser for the grammar. It is a Tcl package holding a TclOO
class, whose instances are parsers for the input grammar.
This result-format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-class string
The value of this option is the name of the class to generate, without leading colons. Note,
it serves double-duty as the name of the package to generate too. The default value is CLASS.
GRAMMAR CONTAINER
The container format is another form of describing parsing expression grammars. While data in this
format is executable it does not constitute 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.
This result-format supports the following options:
-file string
The value of this option is the name of the file or other entity from which the grammar came,
for which the command is run. The default value is unknown.
-name string
The value of this option is the name of the grammar we are processing. The default value is
a_pe_grammar.
-user string
The value of this option is the name of the user for which the command is run. The default
value is unknown.
-mode bulk|incremental
The value of this option controls which methods of pt::peg::container instances are used to
specify the grammar, i.e. preload it into the container. There are two legal values, as listed
below. The default is bulk.
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 nonterminal 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.
-template string
The value of this option is a string into which to put the generated code and the other con-figuration configuration
figuration settings. The various locations for user-data are expected to be specified with the
placeholders listed below. The default value is "@code@".
@user@ To be replaced with the value of the option -user.
@format@
To be replaced with the the constant CONTAINER.
@file@ To be replaced with the value of the option -file.
@name@ To be replaced with the value of the option -name.
@mode@ To be replaced with the value of the option -mode.
@code@ To be replaced with the generated code.
EXAMPLE
In this section we are working a complete example, starting with a PEG grammar and ending with run-ning running
ning 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 <- '(' Expression ')' / (Factor (MulOp Factor)*) ;
MulOp <- '*' / '/' ;
Factor <- Term (AddOp Term)* ;
AddOp <- '+'/'-' ;
Term <- Number ;
END;
From this we create a snit-based parser via
pt generate snit calculator.tcl -class calculator -name calculator peg calculator.peg
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
120+5
A more graphical representation of the tree would be
IMAGE: expr_ast
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 pars-
ing 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.
INTERNALS
This section is intended for users of the application which wish to modify or extend it. Users only
interested in the generation of parsers can ignore it.
The main functionality of the application is encapsulated in the package pt::pgen. Please read it for
more information.
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://source -
forge.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 pars-ing parsing
ing languages, transducer
CATEGORY
Parsing and Grammars
COPYRIGHT
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
pt 1 pt(n)
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