Moose and enum
At $work, we are using Moose a lot. So I spend a lot of time declaring attributes, and some of them are Strings that should be picked from a fixed list of values.
For instance, I need to be able to define a task that can be only one of 'profit', 'world_domination'. And use it like so:
# works fine
$attribute->task('profit');
# fails
$attribute->task('failure');So basically, the attribute is an enum.
The painful way
Until now, I was defining a new subtype for each enum I needed. Something like:
# type definition
use Moose::Util::TypeConstraints;
subtype 'My:Type:Task' => as 'Str' =>
where { my $r = join( '|', __PACKAGE__->get_possible_tasks ); /^(?:$r)$/ };
coerce 'My:Type:Task' => from 'Str' => via {lc};
sub get_possible_tasks {qw(profit world_domination)}
# attribute declaration
has task => ( isa => 'My:Type:Task' );It’s very verbose, painful and not readable, isn’t it ?
The right way
Moose::Util::TypeConstraints provides the enum method, that serves exactly
our purpose. It’s basically a shortcut to create a subtype based on ‘Str’,
limited to a list of possible values. You can use it to build a named enum
subtype, or an anonymous subtype. See the usage in our case :
# using a named enum subtype:
use Moose::Util::TypeConstraints;
enum 'My:Enum:Task', [qw(profit world_domination)];
has task => ( isa => 'My:Enum:Task' );
# using an anonymous enum subtype:
use Moose::Util::TypeConstraints;
has task => ( isa => enum([qw(profit world_domination)]) );So that’s all, I guess this kind of feature is nothing new for seasoned Moose
developers, but it may help beginners, as there is no mention of enum in
Moose::Manual::Attributes.
