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# pt_peg_op(i) 1.2.0 tcllib "Parser Tools"

## Name

pt_peg_op - Parser Tools PE Grammar Utility Operations

## Synopsis

- package require Tcl 8.5 9
- package require pt::peg::op ?1.2.0?

- ::peg::peg::op called container
- ::peg::peg::op dechain container
- ::peg::peg::op drop unreachable container
- ::peg::peg::op drop unrealizable container
- ::peg::peg::op flatten container
- ::peg::peg::op minimize container
- ::peg::peg::op modeopt container
- ::peg::peg::op reachable container
- ::peg::peg::op realizable container

## 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 number of utility commands manipulating a PE grammar (container) in various ways.

## API

::peg::peg::op called container

  This command determines the static call structure for the nonterminal symbols of the grammar stored in the container.

  The result of the command is a dictionary mapping from each symbol to the symbols it calls. The empty string is the key used to represent the start expression of the grammar.

  The grammar in the container is not modified.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op dechain container

  This command simplifies all symbols which just chain to a different symbol by inlining the right hand side of the called symbol in its callers. This works if and only the modes match properly, per the decision table below.

      caller called | dechain | notes
      --------------+---------+-----------------------
      value  value  |  yes    |  value is passed
      value  leaf   |  yes    |  value is passed
      value  void   |  yes    |  caller is implied void
      leaf   value  |  no     |  generated value was discarded, inlined would not. called may be implied void.
      leaf   leaf   |  no     |  s.a.
      leaf   void   |  no     |  s.a.
      void   value  |  no     |  caller drops value, inlined would not.
      void   leaf   |  no     |  s.a.
      void   void   |  yes    |

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op drop unreachable container

  This command removes all symbols from the grammar which are not reachable.

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op drop unrealizable container

  This command removes all symbols from the grammar which are not realizable.

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op flatten container

  This command flattens (see pt::pe::op) all expressions in the grammar, i.e. the start expression and the right hand sides of all nonterminal symbols.

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op minimize container

  This command reduces the provided grammar by applying most of the other methods of this package.

  After flattening the expressions it removes unreachable and unrealizable symbols, flattens the expressions again, then optimizes the symbol modes before collapsing symbol chains as much as possible.

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op modeopt container

  This command optimizes the semantic modes of non-terminal symbols according to the two rules below.

  1. If a symbol X with mode value calls no other symbols, i.e. uses only terminal symbols in whatever combination, then this can be represented simpler by using mode leaf.

  2. If a symbol X is only called from symbols with modes leaf or void then this symbol should have mode void also, as any AST it could generate will be discarded anyway.

  The result of the command is the empty string.

  The grammar in the container is directly modified. If that is not wanted, a copy of the original container has to be used.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op reachable container

  This command computes the set of all nonterminal symbols which are reachable from the start expression of the grammar. This is essentially the transitive closure over called and the symbol's right hand sides, beginning with the start expression.

  The result of the command is the list of reachable symbols.

  The grammar in the container is not modified.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

::peg::peg::op realizable container

  This command computes the set of all nonterminal symbols which are realizable, i.e. can derive pure terminal phrases. This is done iteratively, starting with state unrealizable for all and any, and then updating all symbols which are realizable, propagating changes, until nothing changes any more.

  The result of the command is the list of realizable symbols.

  The grammar in the container is not modified.

  The container instance has to expose a method API as is provided by the package pt::peg::container.

## 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>