'\"
'\" Generated from file 'peg_interp\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2005-2011 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
'\"
.TH "grammar::peg::interp" n 0\&.1\&.1 tcllib "Grammar operations and usage"
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.SH NAME
grammar::peg::interp \- Interpreter for parsing expression grammars
.SH SYNOPSIS
package require \fBTcl  8\&.4\fR
.sp
package require \fBgrammar::mengine  ?0\&.1?\fR
.sp
package require \fBgrammar::peg::interp  ?0\&.1\&.1?\fR
.sp
\fB::grammar::peg::interp::setup\fR \fIpeg\fR
.sp
\fB::grammar::peg::interp::parse\fR \fInextcmd\fR \fIerrorvar\fR \fIastvar\fR
.sp
.BE
.SH DESCRIPTION
.PP
This package provides commands for the controlled matching of a
character stream via a parsing expression grammar and the creation
of an abstract syntax tree for the stream and partials\&.
.PP
It is built on top of the virtual machine provided by the package
\fBgrammar::me::tcl\fR and directly interprets the parsing
expression grammar given to it\&.
In other words, the grammar is \fInot\fR pre-compiled but used as is\&.
.PP
The grammar to be interpreted is taken from a container object
following the interface specified by the package
\fBgrammar::peg::container\fR\&. Only the relevant parts
are copied into the state of this package\&.
.PP
It should be noted that the package provides exactly one instance
of the interpreter, and interpreting a second grammar requires
the user to either abort or complete a running interpretation, or
to put them into different Tcl interpreters\&.
.PP
Also of note is that the implementation assumes a pull-type
handling of the input\&. In other words, the interpreter pulls
characters from the input stream as it needs them\&. For usage
in a push environment, i\&.e\&. where the environment pushes new
characters as they come we have to put the engine into its
own thread\&.
.SH "THE INTERPRETER API"
The package exports the following API
.TP
\fB::grammar::peg::interp::setup\fR \fIpeg\fR
This command (re)initializes the interpreter\&. It returns the
empty string\&. This command has to be invoked first, before any
matching run\&.
.sp
Its argument \fIpeg\fR is the handle of an object containing the
parsing expression grammar to interpret\&. This grammar has to be
valid, or an error will be thrown\&.
.TP
\fB::grammar::peg::interp::parse\fR \fInextcmd\fR \fIerrorvar\fR \fIastvar\fR
This command interprets the loaded grammar and tries to match it
against the stream of characters represented by the command prefix
\fInextcmd\fR\&.
.sp
The command prefix \fInextcmd\fR represents the input stream of
characters and is invoked by the interpreter whenever the a new
character from the stream is required\&.
The callback has to return either the empty list, or a list of 4
elements containing the token, its lexeme attribute, and its location
as line number and column index, in this order\&.
The empty list is the signal that the end of the input stream has been
reached\&. The lexeme attribute is stored in the terminal cache, but
otherwise not used by the machine\&.
.sp
The result of the command is a boolean value indicating whether the
matching process was successful (\fBtrue\fR), or not
(\fBfalse\fR)\&. In the case of a match failure error information will
be stored into the variable referenced by \fIerrorvar\fR\&. The variable
referenced by \fIastvar\fR will always contain the generated abstract
syntax tree, however in the case of an error it will be only partial
and possibly malformed\&.
.sp
The abstract syntax tree is represented by a nested list, as
described in section \fBAST VALUES\fR of
document \fIgrammar::me_ast\fR\&.
.PP
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIgrammar_peg\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.SH KEYWORDS
LL(k), TDPL, context-free languages, expression, grammar, matching, parsing, parsing expression, parsing expression grammar, push down automaton, recursive descent, state, top-down parsing languages, transducer, virtual machine
.SH CATEGORY
Grammars and finite automata
.SH COPYRIGHT
.nf
Copyright (c) 2005-2011 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>

.fi