1asa(3) User Contributed Perl Documentation asa(3)
2
6 asa - Lets your class/object say it works like something else
7
9 version 1.04
10
12 #########################################
13 # Your Code
14 package My::WereDuck;
16 use base 'My::Lycanthrope';
18 use asa 'Duck';
19 sub quack {
21 return "Hi! errr... Quack!";
22 }
23 ################################################
25 # Their Code
26 sub strangle {
28 my $duck = shift;
29 unless ( $duck->isa('Duck') ) {
30 die "We only strangle ducks";
31 }
32 print "Farmer Joe wrings the duck's neck\n";
33 print "Said the duck, '" . $duck->quack . "'\n";
34 print "We ate well that night.\n";
35 }
36
38 Perl 5 doesn't natively support Java-style interfaces, and it doesn't
39 support Perl 6 style roles either.
40 You can get both of these things in half a dozen different ways via
42 various CPAN modules, but they usually require that you buy into "their
43 way" of implementing your code.
44 Other have turned to "duck typing".
46 This is, for the most part, a fairly naive check that says "can you do
48 this method", under the "if it looks like a duck, and quacks like a
49 duck, then it must be a duck".
50 It assumes that if you have a "->quack" method, then they will treat
52 you as a duck, because doing things like adding "Duck" to your @ISA
53 array means you are also forced to take their implementation.
54 There is, of course, a better way.
56 For better or worse, Perl's "->isa" functionality to determine if
58 something is or is not a particular class/object is defined as a
59 method, not a function, and so that means that as well as adding
60 something to you @ISA array, so that Perl's "UNIVERSAL::isa" method can
61 work with it, you are also allowed to simply overload your own "isa"
62 method and answer directly whether or not you are something.
63 The simplest form of the idiom looks like this.
65 sub isa {
67 return 1 if $_[1] eq 'Duck';
68 shift->SUPER::isa(@_);
69 }
70 This reads "Check my type as normal, but if anyone wants to know if I'm
72 a duck, then tell them yes".
73 Now, there are a few people that have argued that this is "lying" about
75 your class, but this argument is based on the idea that @ISA is somehow
76 more "real" than using the method directly.
77 It also assumes that what you advertise you implement needs to be in
79 sync with the method resolution for any given function. But in the best
80 and cleanest implementation of code, the API is orthogonal (although
81 most often related) to the implementation.
82 And although @ISA is about implementation and API, overloading "isa" to
84 let you change your API is not at all bad when seen in this light.
85 What does asa.pm do?
87 Much as base provides convenient syntactic sugar for loading your
88 parent class and setting @ISA, this pragma will provide convenient
89 syntactic sugar for creating your own custom overloaded isa functions.
90 Beyond just the idiom above, it implements various workarounds for some
92 edge cases, so you don't have to, and allows clear separation of
93 concerns.
94 You should just be able to use it, and if something ever goes wrong,
96 then it's my fault, and I'll fix it.
97 What doesn't asa.pm do?
99 In Perl, highly robust introspection is really hard. Which is why most
100 modules that provide some level of interface functionality require you
101 to explicitly define them in multiple classes, and start to tread on
102 your toes.
103 This class does not do any strict enforcement of interfaces. 90% of the
105 time, what you want to do, and the methods you need to implement, are
106 going to be pretty obvious, so it's your fault if you don't provide
107 them.
108 But at least this way, you can implement them however you like, and
110 "asa" will just take care of the details of safely telling everyone
111 else you are a duck :)
112 What if a Duck method clashes with a My::Package method?
114 Unlike Perl 6, which implements a concept called "multi-methods", Perl
115 5 does not have a native approach to solving the problem of "API
116 collision".
117 Originally from the Java/C++ world, the problem of overcoming language
119 API limitations can be done through the use of one of several "design
120 patterns".
121 For you, the victim of API collision, you might be interested in the
123 "Adapter" pattern.
124 For more information on implementing the Adapter pattern in Perl, see
126 Class::Adapter, which provides a veritable toolkit for creating an
127 implementation of the Adapter pattern which can solve your problem.
128
130 <http://ali.as/>
131
133 Bugs may be submitted through the RT bug tracker
134 <https://rt.cpan.org/Public/Dist/Display.html?Name=asa> (or
135 bug-asa@rt.cpan.org <mailto:bug-asa@rt.cpan.org>).
136
138 Adam Kennedy, <adamk@cpan.org>
139
141 · Adam Kennedy <adam@ali.as>
142 · Karen Etheridge <ether@cpan.org>
144
146 This software is copyright (c) 2006 by Adam Kennedy.
147 This is free software; you can redistribute it and/or modify it under
149 the same terms as the Perl 5 programming language system itself.
150perl v5.30.1 2020-01-30 asa(3)