Data::Visitor - Visitor style traversal of Perl data structures
version 0.31
# 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
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).
the Data::Visitor manpage is meant to be subclassed, but also ships with a callback
driven subclass, the Data::Visitor::Callback manpage.
- 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
the Variable::Magic manpage 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.
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.
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
the Variable::Magic manpage if possible.
the Data::Rmap manpage, the Tree::Simple::VisitorFactory manpage, the Data::Traverse manpage
http://en.wikipedia.org/wiki/Visitor_pattern,
http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#functors,
http://en.wikipedia.org/wiki/Functor
Bugs may be submitted through the RT bug tracker
(or bug-Data-Visitor@rt.cpan.org).
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.
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