1Data::Visitor(3) User Contributed Perl Documentation Data::Visitor(3)
2
6 Data::Visitor - Visitor style traversal of Perl data structures
7
9 version 0.30
10
12 # NOTE
13 # You probably want to use Data::Visitor::Callback for trivial things
14 package FooCounter;
16 use Moose;
17 extends qw(Data::Visitor);
19 has number_of_foos => (
21 isa => "Int",
22 is => "rw",
23 default => 0,
24 );
25 sub visit_value {
27 my ( $self, $data ) = @_;
28 if ( defined $data and $data eq "foo" ) {
30 $self->number_of_foos( $self->number_of_foos + 1 );
31 }
32 return $data;
34 }
35 my $counter = FooCounter->new;
37 $counter->visit( {
39 this => "that",
40 some_foos => [ qw/foo foo bar foo/ ],
41 the_other => "foo",
42 });
43 $counter->number_of_foos; # this is now 4
45
47 This module is a simple visitor implementation for Perl values.
48 It has a main dispatcher method, "visit", which takes a single perl
50 value and then calls the methods appropriate for that value.
51 It can recursively map (cloning as necessary) or just traverse most
53 structures, with support for per object behavior, circular structures,
54 visiting tied structures, and all ref types (hashes, arrays, scalars,
55 code, globs).
56 Data::Visitor is meant to be subclassed, but also ships with a callback
58 driven subclass, Data::Visitor::Callback.
59
61 visit $data
62 This method takes any Perl value as it's only argument, and
63 dispatches to the various other visiting methods using
64 "visit_no_rec_check", based on the data's type.
65 If the value is a reference and has already been seen then
67 "visit_seen" is called.
68 visit_seen $data, $first_result
70 When an already seen value is encountered again it's typically
71 replaced with the result of the first visitation of that value. The
72 value and the result of the first visitation are passed as
73 arguments.
74 Returns $first_result.
76 visit_no_rec_check $data
78 Called for any value that has not yet been seen. Does the actual
79 type based dispatch for "visit".
80 Should not be called directly unless forcing a circular structure
82 to be unfolded. Use with caution as this may cause infinite
83 recursion.
84 visit_object $object
86 If the value is a blessed object, "visit" calls this method. The
87 base implementation will just forward to "visit_value".
88 visit_ref $value
90 Generic recursive visitor. All non blessed values are given to
91 this.
92 "visit_object" can delegate to this method in order to visit the
94 object anyway.
95 This will check if the visitor can handle "visit_$reftype"
97 (lowercase), and if not delegate to "visit_value" instead.
98 visit_array $array_ref
100 visit_hash $hash_ref
101 visit_glob $glob_ref
102 visit_code $code_ref
103 visit_scalar $scalar_ref
104 These methods are called for the corresponding container type.
105 visit_value $value
107 If the value is anything else, this method is called. The base
108 implementation will return $value.
109 visit_hash_entries $hash
111 visit_hash_entry $key, $value, $hash
112 Delegates to "visit_hash_key" and "visit_hash_value". The value is
113 passed as $_[2] so that it is aliased.
114 visit_hash_key $key, $value, $hash
116 Calls "visit" on the key and returns it.
117 visit_hash_value $value, $key, $hash
119 The value will be aliased (passed as $_[1]).
120 visit_array_entries $array
122 visit_array_entry $value, $index, $array
123 Delegates to "visit" on value. The value is passed as $_[1] to
124 retain aliasing.
125 visit_tied $object, $var
127 When "tied_as_objects" is enabled and a tied variable (hash, array,
128 glob or scalar) is encountered this method will be called on the
129 tied object. If a valid mapped value is returned, the newly
130 constructed result container will be tied to the return value and
131 no iteration of the contents of the data will be made (since all
132 storage is delegated to the tied object).
133 If a non blessed value is returned from "visit_tied" then the
135 structure will be iterated normally, and the result container will
136 not be tied at all.
137 This is because tying to the same class and performing the tie
139 operations will not yield the same results in many cases.
140 retain_magic $orig, $copy
142 Copies over magic from $orig to $copy.
143 Currently only handles "bless". In the future this might be
145 expanded using Variable::Magic but it isn't clear what the correct
146 semantics for magic copying should be.
147 trace
149 Called if the "DEBUG" constant is set with a trace message.
150
152 This object can be used as an "fmap" of sorts - providing an ad-hoc
153 functor interface for Perl data structures.
154 In void context this functionality is ignored, but in any other context
156 the default methods will all try to return a value of similar
157 structure, with it's children also fmapped.
158
160 Data::Visitor is a Moose class, so it should be subclassed using Moose.
161 Then override the callback methods in any way you like. To retain
163 visitor behavior, make sure to retain the functionality of
164 "visit_array" and "visit_hash".
165
167 · Add support for "natural" visiting of trees.
168 · Expand "retain_magic" to support tying at the very least, or even
170 more with Variable::Magic if possible.
171
173 Data::Rmap, Tree::Simple::VisitorFactory, Data::Traverse
174 <http://en.wikipedia.org/wiki/Visitor_pattern>,
176 <http://www.ninebynine.org/Software/Learning-Haskell-Notes.html#functors>,
177 <http://en.wikipedia.org/wiki/Functor>
178
180 · Yuval Kogman <nothingmuch@woobling.org>
181 · Marcel Grünauer <marcel@cpan.org>
183
185 This software is copyright (c) 2013 by Yuval Kogman.
186 This is free software; you can redistribute it and/or modify it under
188 the same terms as the Perl 5 programming language system itself.
189perl v5.30.1 2020-01-29 Data::Visitor(3)