package mixin;

use strict;
no strict 'refs';
use vars qw($VERSION);
$VERSION = '0.06';


=head1 NAME

mixin - Mix-in inheritance, an alternative to multiple inheritance

=head1 SYNOPSIS

  package Dog;
  sub speak { print "Bark!\n" }
  sub new { my $class = shift;  bless {}, $class }

  package Dog::Small;
  use base 'Dog';
  sub speak { print "Yip!\n"; }

  package Dog::Retriever;
  use mixin::with 'Dog';
  sub fetch { print "Get your own stinking $_[1]\n" }

  package Dog::Small::Retriever;
  use base 'Dog::Small';
  use mixin 'Dog::Retriever';

  my $small_retriever = Dog::Small::Retriever->new;
  $small_retriever->speak;          # Yip!
  $small_retriever->fetch('ball');  # Get your own stinking ball

=head1 DESCRIPTION

Mixin inheritance is an alternative to the usual multiple-inheritance
and solves the problem of knowing which parent will be called.
It also solves a number of tricky problems like diamond inheritence.

The idea is to solve the same sets of problems which MI solves without
the problems of MI.  For all practical purposes you can think of a
mixin as multiple inheritance without the actual inheritance.

Mixins are a band-aid for the problems of MI.  A better solution is to use traits (called "Roles" in Perl 6), which are like mixins on steroids.  Class::Trait implements this.


=head2 Using a mixin class

There are two steps to using a mixin-class.

First, make sure you are inherited from the class with which the
mixin-class is to be mixed.

  package Dog::Small::Retriever;
  use base 'Dog::Small';

Since Dog::Small isa Dog, that does it.  Then simply mixin the new
functionality

  use mixin 'Dog::Retriever';

and now you can use fetch().


=head2 Writing a mixin class

See L<mixin::with>.


=head2 Mixins, Inheritance and SUPER

A class which uses a mixin I<does not> inherit from it.  However, through some clever trickery, C<SUPER> continues to work.  Here's an example.

    {
        package Parent;
        sub foo { "Parent" }
    }

    {
        package Middle;
        use mixin::with "Parent";

        sub foo {
            my $self = shift;
            return $self->SUPER::foo(), "Middle";
        }
    }

    {
        package Child;
        use base "Parent";
        use mixin "Middle";

        sub foo {
            my $self = shift;
            return $self->SUPER::foo(), "Child";
        }
    }

    print join " ", Child->foo;  # Parent Middle Child

This will print C<Parent Middle Child>.  You'll note that this is the same result if Child inherited from Middle and Middle from Parent.  Its also the same result if Child multiply inherited from Middle and Parent but I<NOT> if it inherited from Parent then Middle.  The advantage of mixins vs multiple inheritance is such ambiguities do not exist.

Note that even though both the Child and Middle define foo() the Middle mixin does not overwrite Child's foo().  A mixin does not simply export its methods into the mixer and thus does not blow over existing methods.

=cut

sub import {
    my($class, @mixins) = @_;
    my $caller = caller;

    foreach my $mixin (@mixins) {
        # XXX This is lousy, but it will do for now.
        unless( defined ${$mixin.'::VERSION'} ) {
            eval qq{ require $mixin; };
            _croak($@) if $@ and $@ !~ /^Can't locate .*? at /;
            unless( %{$mixin."::"} ) {
                _croak(<<ERROR);
Mixin class package "$mixin" is empty.
    (Perhaps you need to 'use' the module which defines that package first?)
ERROR
            }
        }
        _mixup($mixin, $caller);
    }
}

sub _mixup {
    my($mixin, $caller) = @_;

    require mixin::with;
    my($with, $pkg) = mixin::with->__mixers($mixin);

    _croak("$mixin is not a mixin") unless $with;
    _croak("$caller must be a subclass of $with")
      unless $caller->isa($with);

    # This has to happen here and not in mixin::with because "use
    # mixin::with" typically runs *before* the rest of the mixin's
    # subroutines are declared.
    _thieve_public_methods( $mixin, $pkg );
    _thieve_isa( $mixin, $pkg, $with );

    unshift @{$caller.'::ISA'}, $pkg;
}


my %Thieved = ();
sub _thieve_public_methods {
    my($mixin, $pkg) = @_;

    return if $Thieved{$mixin}++;

    local *glob;
    while( my($sym, $glob) = each %{$mixin.'::'}) {
        next if $sym =~ /^_/;
        next unless defined $glob;
        *glob = *$glob;
        *{$pkg.'::'.$sym} = *glob{CODE} if *glob{CODE};
    }

    return 1;
}

sub _thieve_isa {
    my($mixin, $pkg, $with) = @_;

    @{$pkg.'::ISA'} = grep $_ ne $with, @{$mixin.'::ISA'};

    return 1;
}


sub _croak {
    require Carp;
    goto &Carp::croak;
}

sub _carp {
    require Carp;
    goto &Carp::carp;
}


=head1 NOTES

A mixin will not warn if the mixin and the user define the same method.

=head1 AUTHOR

Michael G Schwern E<lt>schwern@pobox.comE<gt>

=head1 SEE ALSO

L<Class::Trait> - mixin.pm is a gateway drug to traits
L<Class::C3> - another band-aid on multiple inheritance

=cut

1;
