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Toby Inkster

NAME

MooseX::NiftyDelegation - extra sugar for method delegation

SYNOPSIS

   use 5.014;
   use strict;
   use warnings;
   
   package My::Process {
      use Moose;
      use MooseX::NiftyDelegation -all;
      
      has status => (
         is       => 'rw',
         isa      => 'Str',
         traits   => [ Nifty ],
         required => 1,
         handles  => {
            is_in_progress  => value_is 'in progress',
            is_failed       => value_is 'failed',
            is_complete     => value_like qr/^complete/,
            completion_date => sub { /^completed (.+)$/ and $1 },
         },
      );
   }
   
   package main {
      use Test::More;
      
      my $process = My::Process->new(
         status  => 'completed 2012-11-19',
      );
      
      ok( not $process->is_in_progress );
      ok( not $process->is_failed );
      ok(     $process->is_complete );
      
      is( $process->completion_date, '2012-11-19' );
      
      done_testing;
   }

DESCRIPTION

Moose has an undocumented feature whereby you can delegate methods to coderefs like this:

   has status => (
      is       => 'rw',
      isa      => 'Str',
      handles  => {
         is_in_progress  => sub {
            my $self = shift;
            $self->status eq 'in progress';
         },
      },
   );

Kinda ugly though. The MooseX::NiftyDelegation::Trait::Attribute trait pretties it up a little by automatically wrapping the coderef with a little gubbin that sets $_ to $self->$attribute. Thus:

   has status => (
      is       => 'rw',
      isa      => 'Str',
      traits   => ['MooseX::NiftyDelegation::Trait::Attribute'],
      handles  => {
         is_in_progress  => sub { $_ eq 'in progress' },
      },
   );

A little prettier. The rest of MooseX::NiftyDelegation gives you some handy functions to make these coderefs a cuter still...

Nifty

This is a constant which returns the string 'MooseX::NiftyDelegation::Trait::Attribute' so you don't have to type that out every time. It is exported by default.

value_is $number

Returns a coderef that evaluates $_ for numeric equality with the given number. This function is not exported by default.

value_is $string

Returns a coderef that evaluates $_ for string equality with the given string. This function is not exported by default.

value_like $regexp

Returns a coderef that evaluates $_ for matching the given regular expression. This function is not exported by default.

Now, why would you want to stuff these "delegted" methods into attributes? Why not just write them as regular methods?

   sub is_in_progress {
      my $self = shift;
      $self->status eq 'in progress';
   }

A good question. Writing methods which are closely associated with a single attribute as delegated methods just seems to me to be a nice way of grouping related methods. You can even use it for builders:

   has user_agent => (
      is         => 'ro',
      isa        => 'Object',
      lazy_build => 1,
      handles    => {
         get               => 'get',
         _build_user_agent => sub { LWP::UserAgent->new },
      },
   );

EXPORT

This module uses Sub::Exporter so it's possible to rename exported functions:

   use MooseX::NiftyDelegation
      Nifty       => {},
      value_is    => { -as => 'value_is_exactly' },
      value_like  => { -as => 'value_matches' },
   ;

See Sub::Exporter for further details.

CAVEATS

  • Using a coderef in the delegation hashref is not documented, it's not tested for, and Jesse Luehrs says he doesn't like it. So the feature could get removed at any point.

    In that case, I'll need to update this module with a bunch of extra metahackery. I'm 95% sure it would still be doable - just a lot more code.

  • This module doesn't work in conjunction with attribute native traits. This is native traits insists that the delegated method is either a string or arrayref.

    Patches to get this working with native traits are welcome.

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=MooseX-NiftyDelegation.

SEE ALSO

Moose::Manual::Delegation.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.