Provided by: libdata-visitor-perl_0.31-1_all 
NAME
Data::Visitor - Visitor style traversal of Perl data structures
VERSION
version 0.31
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 its 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 is 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 its children also
fmapped.
SUBCLASSING
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".
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://en.wikipedia.org/wiki/Functor>
SUPPORT
Bugs may be submitted through the RT bug tracker
<https://rt.cpan.org/Public/Dist/Display.html?Name=Data-Visitor> (or
bug-Data-Visitor@rt.cpan.org <mailto:bug-Data-Visitor@rt.cpan.org>).
AUTHORS
• Yuval Kogman <nothingmuch@woobling.org>
• Marcel Grünauer <marcel@cpan.org>
CONTRIBUTORS
• Jesse Luehrs <doy@tozt.net>
• Florian Ragwitz <rafl@debian.org>
• Karen Etheridge <ether@cpan.org>
• David Steinbrunner <dsteinbrunner@pobox.com>
• Robin Smidsrød <robin@smidsrod.no>
COPYRIGHT AND LICENCE
This software is copyright (c) 2020 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.