NAME

grammar::peg::interp -
Interpreter for parsing expression grammars

SYNOPSIS

package require Tcl 8.4
package require grammar::mengine  ? 0.1 ? 
package require grammar::peg::interp  ? 0.1 ? 
::grammar::peg::interp::setup peg
::grammar::peg::interp::parse nextcmd errorvar astvar

DESCRIPTION

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.

It is built on top of the virtual machine provided by the package grammar::mengine and directly interprets the parsing expression grammar given to it. In other words, the grammar is not pre-compiled but used as is.

The grammar to be interpreted is taken from a container object following the interface specified by the package grammar::peg::container. Only the relevant parts are copied into the state of this package.

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.

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.

THE INTERPRETER API

The package exports the following API

::grammar::peg::interp::setup peg

This command (re)initializes the interpreter. It returns the empty string. This command has to be invoked first, before any matching run.
Its argument peg 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.

::grammar::peg::interp::parse nextcmd errorvar astvar

This command interprets the loaded grammar and tries to match it against the stream of characters represented by the command prefix nextcmd. Its result is a boolean value indicating whether the matching process was successful (true), or not (false). In case of a match failure error information will be stored into the variable referenced by errorvar. The variable referenced by astvar will always contain the generated abstract syntax tree, however in the case of an error it will be only partial and possibly malformed.
The abstract syntax tree is represented by a nested list, as described in section ABSTRACT SYNTAX TREE of package grammar::mengine.

BUGS, IDEAS, FEEDBACK

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category grammar_peg of the http://sourceforge.net/tracker/?group_id=12883. Please also report any ideas for enhancements you may have for either package and/or documentation.

KEYWORDS

grammar, expression, push down automaton, state, parsing expression, parsing expression grammar, context-free languages, parsing, transducer, LL(k), TDPL, top-down parsing languages, recursive descent, virtual machine, matching