1Data::Visitor(3) User Contributed Perl Documentation Data::Visitor(3)
2
6 Data::Visitor - Visitor style traversal of Perl data structures
7
9 # NOTE
10 # You probably want to use Data::Visitor::Callback for trivial things
11 package FooCounter;
13 use Moose;
14 extends qw(Data::Visitor);
16 has number_of_foos => (
18 isa => "Int",
19 is => "rw",
20 default => 0,
21 );
22 sub visit_value {
24 my ( $self, $data ) = @_;
25 if ( defined $data and $data eq "foo" ) {
27 $self->number_of_foos( $self->number_of_foos + 1 );
28 }
29 return $data;
31 }
32 my $counter = FooCounter->new;
34 $counter->visit( {
36 this => "that",
37 some_foos => [ qw/foo foo bar foo/ ],
38 the_other => "foo",
39 });
40 $counter->number_of_foos; # this is now 4
42
44 This module is a simple visitor implementation for Perl values.
45 It has a main dispatcher method, "visit", which takes a single perl
47 value and then calls the methods appropriate for that value.
48 It can recursively map (cloning as necessary) or just traverse most
50 structures, with support for per object behavior, circular structures,
51 visiting tied structures, and all ref types (hashes, arrays, scalars,
52 code, globs).
53 Data::Visitor is meant to be subclassed, but also ships with a callback
55 driven subclass, Data::Visitor::Callback.
56
58 visit $data
59 This method takes any Perl value as it's only argument, and
60 dispatches to the various other visiting methods using
61 "visit_no_rec_check", based on the data's type.
62 If the value is a reference and has already been seen then
64 "visit_seen" is called.
65 visit_seen $data, $first_result
67 When an already seen value is encountered again it's typically
68 replaced with the result of the first visitation of that value. The
69 value and the result of the first visitation are passed as
70 arguments.
71 Returns $first_result.
73 visit_no_rec_check $data
75 Called for any value that has not yet been seen. Does the actual
76 type based dispatch for "visit".
77 Should not be called directly unless forcing a circular structure
79 to be unfolded. Use with caution as this may cause infinite
80 recursion.
81 visit_object $object
83 If the value is a blessed object, "visit" calls this method. The
84 base implementation will just forward to "visit_value".
85 visit_ref $value
87 Generic recursive visitor. All non blessed values are given to
88 this.
89 "visit_object" can delegate to this method in order to visit the
91 object anyway.
92 This will check if the visitor can handle "visit_$reftype"
94 (lowercase), and if not delegate to "visit_value" instead.
95 visit_array $array_ref
97 visit_hash $hash_ref
98 visit_glob $glob_ref
99 visit_code $code_ref
100 visit_scalar $scalar_ref
101 These methods are called for the corresponding container type.
102 visit_value $value
104 If the value is anything else, this method is called. The base
105 implementation will return $value.
106 visit_hash_entries $hash
108 visit_hash_entry $key, $value, $hash
109 Delegates to "visit_hash_key" and "visit_hash_value". The value is
110 passed as $_[2] so that it is aliased.
111 visit_hash_key $key, $value, $hash
113 Calls "visit" on the key and returns it.
114 visit_hash_value $value, $key, $hash
116 The value will be aliased (passed as $_[1]).
117 visit_array_entries $array
119 visit_array_entry $value, $index, $array
120 Delegates to "visit" on value. The value is passed as $_[1] to
121 retain aliasing.
122 visit_tied $object, $var
124 When "tied_as_objects" is enabled and a tied variable (hash, array,
125 glob or scalar) is encountered this method will be called on the
126 tied object. If a valid mapped value is returned, the newly
127 constructed result container will be tied to the return value and
128 no iteration of the contents of the data will be made (since all
129 storage is delegated to the tied object).
130 If a non blessed value is returned from "visit_tied" then the
132 structure will be iterated normally, and the result container will
133 not be tied at all.
134 This is because tying to the same class and performing the tie
136 operations will not yield the same results in many cases.
137 retain_magic $orig, $copy
139 Copies over magic from $orig to $copy.
140 Currently only handles "bless". In the future this might be
142 expanded using Variable::Magic but it isn't clear what the correct
143 semantics for magic copying should be.
144 trace
146 Called if the "DEBUG" constant is set with a trace message.
147
149 This object can be used as an "fmap" of sorts - providing an ad-hoc
150 functor interface for Perl data structures.
151 In void context this functionality is ignored, but in any other context
153 the default methods will all try to return a value of similar
154 structure, with it's children also fmapped.
155
157 Create instance data using the Class::Accessor interface. Data::Visitor
158 inherits Class::Accessor to get a sane "new".
159 Then override the callback methods in any way you like. To retain
161 visitor behavior, make sure to retain the functionality of
162 "visit_array" and "visit_hash".
163
165 · Add support for "natural" visiting of trees.
166 · Expand "retain_magic" to support tying at the very least, or even
168 more with Variable::Magic if possible.
169
171 Data::Rmap, Tree::Simple::VisitorFactory, Data::Traverse
172 <http://en.wikipedia.org/wiki/Visitor_pattern>,
174 http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#functors
175 <http://www.ninebynine.org/Software/Learning-Haskell-
176 Notes.html#functors>, <http://en.wikipedia.org/wiki/Functor>
177
179 Yuval Kogman "<nothingmuch@woobling.org>"
180 Marcel Gruenauer, "<marcel@cpan.org>"
182
184 Copyright (c) 2006-2008 Yuval Kogman. All rights reserved
185 This program is free software; you can redistribute
186 it and/or modify it under the same terms as Perl itself.
187perl v5.12.0 2010-02-03 Data::Visitor(3)