Data::Visitor(3) Visitor style traversal of Perl data structures


version 0.30


# You probably want to use Data::Visitor::Callback for trivial things
package FooCounter;
use Moose;
extends qw(Data::Visitor);
has number_of_foos => (
isa => "Int",
is => "rw",
default => 0,
sub visit_value {
my ( $self, $data ) = @_;
if ( defined $data and $data eq "foo" ) {
$self->number_of_foos( $self->number_of_foos + 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.

It can recursively map (cloning as necessary) or just traverse most structures, with support for per object behavior, circular structures, visiting tied structures, and all ref types (hashes, arrays, scalars, code, globs).

Data::Visitor is meant to be subclassed, but also ships with a callback driven subclass, Data::Visitor::Callback.


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

If the value is a reference and has already been seen then "visit_seen" is called.

visit_seen $data, $first_result
When an already seen value is encountered again it's typically replaced with the result of the first visitation of that value. The value and the result of the first visitation are passed as arguments.

Returns $first_result.

visit_no_rec_check $data
Called for any value that has not yet been seen. Does the actual type based dispatch for "visit".

Should not be called directly unless forcing a circular structure to be unfolded. Use with caution as this may cause infinite recursion.

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_code $code_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_entries $hash
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_entries $array
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.

retain_magic $orig, $copy
Copies over magic from $orig to $copy.

Currently only handles "bless". In the future this might be expanded using Variable::Magic but it isn't clear what the correct semantics for magic copying should be.

Called if the "DEBUG" constant is set with a trace message.


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.


Data::Visitor is a Moose class, so it should be subclassed using Moose.

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".


  • Add support for ``natural'' visiting of trees.
  • Expand "retain_magic" to support tying at the very least, or even more with Variable::Magic if possible.



This software is copyright (c) 2013 by Yuval Kogman.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.