Data::Visitor(3) User Contributed Perl Documentation Data::Visitor(3)NAMEData::Visitor - Visitor style traversal of Perl data structures
SYNOPSIS
# NOTE
# 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
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.
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.
METHODS
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.
trace
Called if the "DEBUG" constant is set with a trace message.
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.
· Expand "retain_magic" to support tying at the very least, or even
more with Variable::Magic if possible.
SEE ALSO
Data::Rmap, Tree::Simple::VisitorFactory, Data::Traverse
<http://en.wikipedia.org/wiki/Visitor_pattern>,
http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#functors
<http://www.ninebynine.org/Software/Learning-Haskell-
Notes.html#functors>, <http://en.wikipedia.org/wiki/Functor>
AUTHOR
Yuval Kogman "<nothingmuch@woobling.org>"
Marcel Gruenauer, "<marcel@cpan.org>"
COPYRIGHT & LICENSE
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.
perl v5.14.1 2010-02-03 Data::Visitor(3)
