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Re: Delegating to a role in Moose

by Arunbear (Prior)
on Jan 12, 2018 at 11:50 UTC ( #1207152=note: print w/replies, xml ) Need Help??

in reply to Delegating to a role in Moose

Hi nysus, the intention of
handles => 'Some::Role',
is that Some::Role simply acts as an interface i.e. it describes required methods but does not implement them.

There is an example of this in the Moose test suite:

{ package Foo::Bar; use Moose::Role; requires 'foo'; requires 'bar'; package Foo::Baz; use Moose; sub foo { 'Foo::Baz::FOO' } sub bar { 'Foo::Baz::BAR' } sub baz { 'Foo::Baz::BAZ' } package Foo::Thing; use Moose; has 'thing' => ( is => 'rw', isa => 'Foo::Baz', handles => 'Foo::Bar', ); package Foo::OtherThing; use Moose; use Moose::Util::TypeConstraints; has 'other_thing' => ( is => 'rw', isa => 'Foo::Baz', handles => Moose::Util::TypeConstraints::find_type_constraint( +'Foo::Bar'), ); }
Here you can see that the Foo::Bar role is used to specify delegation, but the actual methods are implemented in Foo::Baz.

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Re^2: Delegating to a role in Moose
by nysus (Vicar) on Jan 12, 2018 at 15:11 UTC

    Thanks, but I'm not sure I follow. The documentation doesn't mention using Moose::Util::TypeConstraints. What, exactly, does the code in the documentation do?</code>

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      Moose::Util::TypeConstraints isn't really important here and I should have edited it out of the snippet. The important thing to understand is
      package Foo::Bar; use Moose::Role; requires 'foo'; requires 'bar';
      i.e. required methods. This is how a role indicates that a class consuming it should have some extra behaviour that the role depends on. Taking this to an extreme you can even have a role that only has required method specifiers - this gives you something similar to the interface construct found in Java/C#/PHP.

      And this latter type of role is what is intended to be used when you specify a role to the 'handles' clause.

      Does that make sense?

        In the documentation, it's made to look like you can extend or override methods provided by URI but from what I can tell it does nothing of the sort. Take this, for example, which emulates what is in the documentation:

        { package MyURIRole 0.000001; use Moose::Role; requires 'host'; requires 'path'; sub print_hi { my $s = shift; print "hi\n"; } } { package MyURI 0.000001; use Carp; use Moose; use URI; has 'uri' => ( is => 'ro', isa => 'URI', handles => 'MyURIRole', ); } #! /usr/bin/env perl use MyURI; use strict; use warnings; my $uri = MyURI->new( uri => URI->new('')) +; print $uri->host; print "\n"; print $uri->path; print "\n"; $uri->print_hi;

        From the documentation's explanation, I would expect the $uri->print_hi to print out "hi". Instead, I get an error:

        Can't locate object method "print_hi" via package "URI::http" at inline delegation in MyURI for uri->print_hi (attribute declared in at line 7) line 18.

        So I'm not sure why the documentation is suggesting that using a role name with handles is a good way to delegate when using another package like URI. What is to be gained from it? Am I missing something?

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