1asa(3)                User Contributed Perl Documentation               asa(3)
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NAME

6       asa - Lets your class/object say it works like something else
7

SYNOPSIS

9         #########################################
10         # Your Code
11
12         package My::WereDuck;
13
14         use base 'My::Lycanthrope';
15         use asa  'Duck';
16
17         sub quack {
18             return "Hi! errr... Quack!";
19         }
20
21         ################################################
22         # Their Code
23
24         sub strangle {
25             my $duck = shift;
26             unless ( $duck->isa('Duck') ) {
27                 die "We only strangle ducks";
28             }
29             print "Farmer Joe wrings the duck's neck\n";
30             print "Said the duck, '" . $duck->quack . "'\n";
31             print "We ate well that night.\n";
32         }
33

DESCRIPTION

35       Perl 5 doesn't natively support Java-style interfaces, and it doesn't
36       support Perl 6 style roles either.
37
38       You can get both of these things in half a dozen different ways via
39       various CPAN modules, but they usually require that you buy into "their
40       way" of implementing your code.
41
42       Other have turned to "duck typing".
43
44       This is, for the most part, a fairly naive check that says "can you do
45       this method", under the "if it looks like a duck, and quacks like a
46       duck, then it must be a duck".
47
48       It assumes that if you have a "->quack" method, then they will treat
49       you as a duck, because doing things like adding "Duck" to your @ISA
50       array means you are also forced to take their implementation.
51
52       There is, of course, a better way.
53
54       For better or worse, Perl's "->isa" functionality to determine if
55       something is or is not a particular class/object is defined as a
56       method, not a function, and so that means that as well as adding
57       something to you @ISA array, so that Perl's "UNIVERSAL::isa" method can
58       work with it, you are also allowed to simply overload your own "isa"
59       method and answer directly whether or not you are something.
60
61       The simplest form of the idiom looks like this.
62
63         sub isa {
64             return 1 if $_[1] eq 'Duck';
65             shift->SUPER::isa(@_);
66         }
67
68       This reads "Check my type as normal, but if anyone wants to know if I'm
69       a duck, then tell them yes".
70
71       Now, there are a few people that have argued that this is "lying" about
72       your class, but this argument is based on the idea that @ISA is somehow
73       more "real" than using the method directly.
74
75       It also assumes that what you advertise you implement needs to be in
76       sync with the method resolution for any given function. But in the best
77       and cleanest implementation of code, the API is orthogonal (although
78       most often related) to the implementation.
79
80       And although @ISA is about implementation and API, overloading "isa" to
81       let you change your API is not at all bad when seen in this light.
82
83   What does asa.pm do?
84       Much as base provides convenient syntactic sugar for loading your
85       parent class and setting @ISA, this pragma will provide convenient
86       syntactic sugar for creating your own custom overloaded isa functions.
87
88       Beyond just the idiom above, it implements various workarounds for some
89       edge cases, so you don't have to, and allows clear seperation of
90       concerns.
91
92       You should just be able to use it, and if something ever goes wrong,
93       then it's my fault, and I'll fix it.
94
95   What doesn't asa.pm do?
96       In Perl, highly robust introspection is really hard. Which is why most
97       modules that provide some level of interface functionality require you
98       to explicitly define them in multiple classes, and start to tread on
99       your toes.
100
101       This class does not do any strict enforcement of interfaces. 90% of the
102       time, what you want to do, and the methods you need to implement, are
103       going to be pretty obvious, so it's your fault if you don't provide
104       them.
105
106       But at least this way, you can implement them however you like, and
107       "asa" will just take care of the details of safely telling everyone
108       else you are a duck :)
109
110   What if a Duck method clashes with a My::Package method?
111       Unlike Perl 6, which implements a concept called "multi-methods", Perl
112       5 does not have a native approach to solving the problem of "API
113       collision".
114
115       Originally from the Java/C++ world, the problem of overcoming language
116       API limitations can be done through the use of one of several "design
117       patterns".
118
119       For you, the victim of API collision, you might be interested in the
120       "Adapter" pattern.
121
122       For more information on implementing the Adapter pattern in Perl, see
123       Class::Adapter, which provides a veritable toolkit for creating an
124       implementation of the Adapter pattern which can solve your problem.
125

SUPPORT

127       Bugs should be always be reported via the CPAN bug tracker at
128
129       <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=asa>
130
131       For other issues, or commercial enhancement or support, contact the
132       author.
133

AUTHORS

135       Adam Kennedy <cpan@ali.as<
136

SEE ALSO

138       base, <http://ali.as/>
139
141       Copyright (c) 2006 Adam Kennedy. All rights reserved.
142
143       This program is free software; you can redistribute it and/or modify it
144       under the same terms as Perl itself.
145
146       The full text of the license can be found in the LICENSE file included
147       with this module.
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151perl v5.12.0                      2006-05-10                            asa(3)
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