Tcl Library Source Code

Documentation
Login


[ Main Table Of Contents | Table Of Contents | Keyword Index | Categories | Modules | Applications ]

NAME

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

Table Of Contents

SYNOPSIS

package require Tcl 8.5 9
package require pt::peg::interp ?1.0.2?
package require pt::rde ?1?
package require snit

::pt::peg::interp objectName grammar
objectName use grammar
objectName destroy
objectName parse chan
objectName parset text

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 class whose instances are Packrat parsers configurable with a parsing expression grammar. The grammar is executed directly, i.e. interpreted, with the underlying runtime provided by the package pt::rde, basing everything on the PARAM.

Like the supporting runtime this package resides in the Execution section of the Core Layer of Parser Tools.

The interpreted grammar is copied from an instance of pt::peg::container, or anything providing the same API, like the container classes created by pt::peg::to::container or the associated export plugin pt::peg::export::container.

Class API

The package exports the API described here.

Object API

All objects created by this package provide the following methods.

AST serialization format

Here we specify the format used by the Parser Tools to serialize Abstract Syntax Trees (ASTs) as immutable values for transport, comparison, etc.

Each node in an AST represents a nonterminal symbol of a grammar, and the range of tokens/characters in the input covered by it. ASTs do not contain terminal symbols, i.e. tokens/characters. These can be recovered from the input given a symbol's location.

We distinguish between regular and canonical serializations. While a tree may have more than one regular serialization only exactly one of them will be canonical.

Example

Assuming the parsing expression grammar below

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;

and the input string

120+5

then a parser should deliver the abstract syntax tree below (except for whitespace)

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}
            }
        }
    }
}

Or, more graphical

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.

Example

Assuming the parsing expression shown on the right-hand side of the rule

Expression <- Term (AddOp Term)*

then its canonical serialization (except for whitespace) is

{x {n Term} {* {x {n AddOp} {n Term}}}}

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. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

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 © 2009 Andreas Kupries