GraphViz::Parse::RecDescent - Visualise grammars


  use GraphViz::Parse::RecDescent;

  # Either pass in the grammar
  my $graph = GraphViz::Parse::RecDescent->new($grammar);
  print $g->as_png;

  # or a Parse::RecDescent parser object
  my $graph = GraphViz::Parse::RecDescent->new($parser);
  print $g->as_ps;


This module makes it easy to visualise Parse::RecDescent grammars. Writing Parse::RecDescent grammars is tricky at the best of times, and grammars almost always evolve in ways unforseen at the start. This module aims to visualise a grammar as a graph in order to make the structure clear and aid in understanding the grammar.

Rules are represented as nodes, which have their name on the left of the node and their productions on the right of the node. The subrules present in the productions are represented by edges to the subrule nodes.

Thus, every node (rule) should be connected to the graph - otherwise a rule is not part of the grammar.

This uses the GraphViz module to draw the graph. Thanks to Damian Conway for the idea.

Note that the Parse::RecDescent module should be installed.



This is the constructor. It takes one mandatory argument, which can either be the grammar text or a Parse::RecDescent parser object of the grammar to be visualised. A GraphViz object is returned.

  # Either pass in the grammar
  my $graph = GraphViz::Parse::RecDescent->new($grammar);

  # or a Parse::RecDescent parser object
  my $graph = GraphViz::Parse::RecDescent->new($parser);


The grammar can be visualised in a number of different graphical formats. Methods include as_ps, as_hpgl, as_pcl, as_mif, as_pic, as_gd, as_gd2, as_gif, as_jpeg, as_png, as_wbmp, as_ismap, as_imap, as_vrml, as_vtx, as_mp, as_fig, as_svg. See the GraphViz documentation for more information. The two most common methods are:

  # Print out a PNG-format file
  print $g->as_png;

  # Print out a PostScript-format file
  print $g->as_ps;


Translating the grammar to a graph is accomplished by peeking inside the internals of a parser object, which is a tad scary. A new version of Parse::RecDescent with different internals may break this module.

At the moment, almost all Parse::RecDescent directives are supported. If you find one that has been missed - let me know!

Unfortunately, alternations (such as the following) do not produce very pretty graphs, due to the fact that they are implicit (unamed) rules and are implemented by new long-named subrules.

  character: 'the' ( good | bad | ugly ) /dude/

Hopefully Parse::FastDescent will make this all much easier.


Leon Brocard <>


Copyright (C) 2001, Leon Brocard

This module is free software; you can redistribute it or modify it under the Perl License, a copy of which is available at