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


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


        # 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


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.


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_ref $value
Generic recursive visitor. All non blessed values are given to this.

visit_object can delegate to this method in order to visit the object anyway.

This will check if the visitor can handle visit_$reftype (lowercase), and if not delegate to visit_value instead.

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.

visit_hash_entry $key, $value, $hash
Delegates to visit_hash_key and visit_hash_value. The value is passed as $_[2] so that it is aliased.

visit_hash_key $key, $value, $hash
Calls visit on the key and returns it.

visit_hash_value $value, $key, $hash
The value will be aliased (passed as $_[1]).

visit_array_entry $value, $index, $array
Delegates to visit on value. The value is passed as $_[1] to retain aliasing.

visit_tied $object, $var
When tied_as_objects is enabled and a tied variable (hash, array, glob or scalar) is encountered this method will be called on the tied object. If a valid mapped value is returned, the newly constructed result container will be tied to the return value and no iteration of the contents of the data will be made (since all storage is delegated to the tied object).

If a non blessed value is returned from visit_tied then the structure will be iterated normally, and the result container will not be tied at all.

This is because tying to the same class and performing the tie operations will not yield the same results in many cases.


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.


Create instance data using the the Class::Accessor manpage interface. the Data::Visitor manpage inherits the Class::Accessor manpage 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.



the Data::Rmap manpage, the Tree::Simple::VisitorFactory manpage, the Data::Traverse manpage,,


Yuval Kogman <>


        Copyright (c) 2006-2008 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.
 Data::Visitor - Visitor style traversal of Perl data structures