asa


SYNOPSIS
         #########################################
         # Your Code

         package My::WereDuck;

         use base 'My::Lycanthrope';
         use asa  'Duck';

         sub quack {
             return "Hi! errr... Quack!";
         }

         ################################################
         # Their Code

         sub strangle {
             my $duck = shift;
             unless ( $duck->isa('Duck') ) {
                 die "We only strangle ducks";
             }
             print "Farmer Joe wrings the duck's neck\n";
             print "Said the duck, '" . $duck->quack . "'\n";
             print "We ate well that night.\n";
         }

DESCRIPTION
       Perl 5 doesn't natively support Java-style interfaces, and it doesn't
       support Perl 6 style roles either.

       You can get both of these things in half a dozen different ways via
       various CPAN modules, but they usually require that you buy into "their
       way" of implementing your code.

       Other have turned to "duck typing".

       This is, for the most part, a fairly naive check that says "can you do
       this method", under the "if it looks like a duck, and quacks like a
       duck, then it must be a duck".

       It assumes that if you have a "->quack" method, then they will treat
       you as a duck, because doing things like adding "Duck" to your @ISA
       array means you are also forced to take their implementation.

       There is, of course, a better way.

       For better or worse, Perl's "->isa" functionality to determine if
       something is or is not a particular class/object is defined as a
       method, not a function, and so that means that as well as adding
       something to you @ISA array, so that Perl's "UNIVERSAL::isa" method can
       work with it, you are also allowed to simply overload your own "isa"
       method and answer directly whether or not you are something.

       more "real" than using the method directly.

       It also assumes that what you advertise you implement needs to be in
       sync with the method resolution for any given function. But in the best
       and cleanest implementation of code, the API is orthogonal (although
       most often related) to the implementation.

       And although @ISA is about implementation and API, overloading "isa" to
       let you change your API is not at all bad when seen in this light.

   What does asa.pm do?
       Much as base provides convenient syntactic sugar for loading your
       parent class and setting @ISA, this pragma will provide convenient
       syntactic sugar for creating your own custom overloaded isa functions.

       Beyond just the idiom above, it implements various workarounds for some
       edge cases, so you don't have to, and allows clear seperation of
       concerns.

       You should just be able to use it, and if something ever goes wrong,
       then it's my fault, and I'll fix it.

   What doesn't asa.pm do?
       In Perl, highly robust introspection is really hard. Which is why most
       modules that provide some level of interface functionality require you
       to explicitly define them in multiple classes, and start to tread on
       your toes.

       This class does not do any strict enforcement of interfaces. 90% of the
       time, what you want to do, and the methods you need to implement, are
       going to be pretty obvious, so it's your fault if you don't provide
       them.

       But at least this way, you can implement them however you like, and
       "asa" will just take care of the details of safely telling everyone
       else you are a duck :)

   What if a Duck method clashes with a My::Package method?
       Unlike Perl 6, which implements a concept called "multi-methods", Perl
       5 does not have a native approach to solving the problem of "API
       collision".

       Originally from the Java/C++ world, the problem of overcoming language
       API limitations can be done through the use of one of several "design
       patterns".

       For you, the victim of API collision, you might be interested in the
       "Adapter" pattern.

       For more information on implementing the Adapter pattern in Perl, see
       Class::Adapter, which provides a veritable toolkit for creating an
       implementation of the Adapter pattern which can solve your problem.

SUPPORT

COPYRIGHT
       Copyright (c) 2006 Adam Kennedy. All rights reserved.

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       The full text of the license can be found in the LICENSE file included
       with this module.



perl v5.10.0                      2006-05-10                          asa(3pm)
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