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# pt_parse_peg(i) 1 tcllib "Parser Tools"

## Name

pt_parse_peg - Parser Tools PEG Parser

## Synopsis

- package require Tcl 8.5
- package require pt::parse::peg 1

- pt::parse::peg ?objectName?
- 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 parsers for parsing expression grammars in textual form.

## Class API

pt::parse::peg ?objectName?

  The class command constructs parser instances, i.e. objects. The result of the command is the fully-qualified name of the instance command.

  If no objectName is specified the class will generate and use an automatic name. If the objectName was specified, but is not fully qualified the command will be created in the current namespace.

## Instances API

All parser instances provide at least the methods shown below:

objectName destroy

  This method destroys the parser instance, releasing all claimed memory and other resources, and deleting the instance command.

  The result of the command is the empty string.

objectName parse chan

  This method runs the parser using the contents of chan as input (starting at the current location in the channel), until parsing is not possible anymore, either because parsing has completed, or run into a syntax error.

  Note here that the Parser Tools are based on Tcl 8.5+. In other words, the channel argument is not restricted to files, sockets, etc. We have the full power of reflected channels available.

  It should also be noted that the parser pulls the characters from the input stream as it needs them. If a parser created by this package has to be operated in a push aka event-driven manner it will be necessary to go to Tcl 8.6+ and use the coroutine::auto to wrap it into a coroutine where read is properly changed for push-operation.

  Upon successful completion the command returns an abstract syntax tree as its result. This AST is in the form specified in section AST serialization format. As a plain nested Tcl-list it can then be processed with any Tcl commands the user likes, doing transformations, semantic checks, etc. To help in this the package pt::ast provides a set of convenience commands for validation of the tree's basic structure, printing it for debugging, and walking it either from the bottom up, or top down.

  When encountering a syntax error the command will throw an error instead. This error will be a 4-element Tcl-list, containing, in the order listed below:

  1. The string pt::rde identifying it as parser runtime error.

  2. The location of the parse error, as character offset from the beginning of the parsed input.

  3. The location of parse error, now as a 2-element list containing line-number and column in the line.

  4. A set of atomic parsing expressions indicating encoding the characters and/or nonterminal symbols the parser expected to see at the location of the parse error, but did not get. For the specification of atomic parsing expressions please see the section PE serialization format.

objectName parset text

  This method runs the parser using the string in text as input. In all other ways it behaves like the method parse, shown above.

## 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 <andreas_kupries@users.sourceforge.net>