Data::Visitor(3)

NAME
Data::Visitor - Visitor style traversal of Perl data structures

SYNOPSIS
        # NOTE
        # You probably want to use Data::Visitor::Callback for trivial things

        package FooCounter;
        use base qw/Data::Visitor/;

        BEGIN { __PACKAGE__->mk_accessors( "number_of_foos" ) };

        sub visit_value {
                my ( $self, $data ) = @_;

                if ( defined $data and $data eq "foo" ) {
                        $self->number_of_foos( ($self->number_of_foos || 0) + 1 );
                }

                return $data;
        }

        my $counter = FooCounter->new;

        $counter->visit( {
                this => "that",
                some_foos => [ qw/foo foo bar foo/ ],
                the_other => "foo",
        });

        $counter->number_of_foos; # this is now 4

DESCRIPTION
This module is a simple visitor implementation for Perl values.

It has a main dispatcher method, "visit", which takes a single perl value and then calls the methods appropriate for that value.

METHODS

visit $data
    This method takes any Perl value as it's only argument, and dispatches to the various other visiting methods, based on the data's type.

visit_object $object
    If the value is a blessed object, "visit" calls this method. The base implementation will just forward to "visit_value".

visit_array $array_ref
visit_hash $hash_ref
visit_glob $glob_ref
visit_scalar $scalar_ref
    These methods are called for the corresponding container type.

visit_value $value
    If the value is anything else, this method is called. The base implementation will return $value.

RETURN VALUE
This object can be used as an "fmap" of sorts - providing an ad-hoc functor interface for Perl data structures.

In void context this functionality is ignored, but in any other context the default methods will all try to return a value of similar structure, with it's children also fmapped.

SUBCLASSING
Create instance data using the Class::Accessor interface. Data::Visitor inherits Class::Accessor to get a sane "new".

Then override the callback methods in any way you like. To retain visitor behavior, make sure to retain the functionality of "visit_array" and "visit_hash".

TODO
Add support for "natural" visiting of trees.

SEE ALSO
Tree::Simple::VisitorFactory, Data::Traverse

<http://en.wikipedia.org/wiki/Visitor_pattern>, <http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#functors>, <http://en.wikipedia.org/wiki/Functor>

AUTHOR
Yuval Kogman <nothingmuch@woobling.org>

COPYRIGHT & LICENSE
        Copyright (c) 2006 Yuval Kogman. All rights reserved
        This program is free software; you can redistribute
        it and/or modify it under the same terms as Perl itself.