Parse::RecDescent::Topiary - tree surgery for Parse::RecDescent autotrees


  use Parse::RecDescent::Topiary;
  my $parser = Parse::RecDescent->new($grammar);
  my $tree = topiary(
                tree => $parser->mainrule,
                namespace => 'MyModule::Foo',
                ucfirst => 1


Parse::RecDescent has a mechanism for automatically generating parse trees. What this does is to bless each resulting node into a package namespace corresponding to the rule. This might not be desirable, for a couple of reasons:

  • You probably don't want to pollute the top-level namespace with packages, and you probably don't want your grammar rules to be named according to CPAN naming conventions. Also, the namespaces could collide if an application has two different RecDescent grammars, that share some rule names.

  • Parse::RecDescent merely blesses the data structures. It does not call a constructor. Parse::RecDescent::Topiary calls new for each class. A base class, Parse::RecDescent::Topiary::Base is provided in the distribution, to construct hashref style objects. The user can always supply their own - inside out or whatever.


This is a function which recursively rebuilds an autotree returned by Parse::RecDescent, using constructors for each node.

This exported function takes a list of option / value pairs:


Pass in the resulting autotree returned by a Parse::RecDescent object.


If not specified, topiary will not use objects in the new parse tree. This can be specified either as a single prefix value, or a list of namespaces as an arrayref.

As the tree is walked, each blessed node is used to form a candidate class name, and if such a candidate class has a constructor, i.e. if Foo::Bar::Token->can('new') returns true, this will be used to construct the new node object (see delegation_class).

If a list of namespaces are given, each one is tried in turn, until a new method is found. If no constructor is found, the node is built as a data structure, i.e. it is not blessed or constructed.


Optional flag to upper case the first character of the rule when forming the class name.


Optional flag that causes topiary to reduce the nesting, unambiguously, of optionally quantified productions. The production foo(?) causes generation of the hash entry 'foo(?)' containing an arrayref of either 0 or 1 elements depending whether foo was present or not in the input string.

If consolidate is a true value, topiary processes this entry, and either generates a hash entry foo => foo_object if foo was present, or does not generate a hash entry if it was absent.


Optional user arguments passed in. These are available to the constructors, and the default constructor will put them into the new objects as $self->{__ARGS__}.


  @class_list = qw(Foo::Bar Foo::Baz);
  my $class = delegation_class( 'Dongle', \@class_list, 'wiggle' );

This subroutine is not exported by default, and is used internally by topiary. $class is set to Foo::Bar::Dongle if Foo::Bar::Dongle->can('wiggle') or set to Foo::Baz::Dongle if Foo::Baz::Dongle->can('wiggle') or return undef if no match is found.


Please report bugs to


    Ivor Williams



This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.