At my work, we exten­sive­ly use the Moose object sys­tem to take care of what would ordi­nar­i­ly be very tedious boil­er­plate object-​oriented Perl code. In one part of the code­base, we have a fam­i­ly of class­es that, among oth­er things, map Perl meth­ods to the names of var­i­ous calls in a third-​party API with­in our larg­er orga­ni­za­tion. Those pri­vate Perl meth­ods are in turn called from pub­lic meth­ods pro­vid­ed by roles con­sumed by these class­es so that oth­er areas aren’t con­cerned with said API’s details.

Without going into too many specifics, I had a bunch of class­es all with sec­tions that looked like this:

sub _create_method    { return 'api_add'     }
sub _retrieve_method  { return 'api_info'    }
sub _search_method    { return 'api_list'    }
sub _update_method    { return 'api_update'  }
sub _cancel_method    { return 'api_remove'  }
sub _suspend_method   { return 'api_disable' }
sub _unsuspend_method { return 'api_restore' }

... # etc.

The val­ues returned by these very sim­ple meth­ods might dif­fer from class to class depend­ing on the API call need­ed, and dif­fer­ent class­es might have a dif­fer­ent mix of these meth­ods depend­ing on what roles they consume.

These meth­ods had built up over time as devel­op­ers had expand­ed the class­es’ func­tion­al­i­ty, and this week it was my turn. I decid­ed to apply the DRY (don’t repeat your­self) prin­ci­ple and cre­ate them from a sim­ple hash table like so:

my %METHOD_MAP = (
  _create_method    => 'api_add',
  _retrieve_method  => 'api_info',
  _search_method    => 'api_list',
  _update_method    => 'api_update',
  _cancel_method    => 'api_remove',
  _suspend_method   => 'api_disable',
  _unsuspend_method => 'api_restore',
);

At first, I thought to myself, These look like pri­vate read-​only attrib­ut­es!” So I wrote:

use Moose;

...

has $_ => (
  is       => 'ro',
  init_arg => undef,
  default  => $METHOD_MAP{$_},
) for keys %METHOD_MAP;

Of course, I’d have to move the class­es’ with state­ments after these def­i­n­i­tions so the roles they con­sume could see” these runtime-​defined attrib­ut­es. But some of the meth­ods used to read these are class meth­ods (e.g., called as ClassName->foo() rather than $object->foo()), and Moose attrib­ut­es are only set after the con­struc­tion of a class instance.

Then I thought, Hey, Moose has a MOP (meta-​object pro­to­col)! I’ll use that to gen­er­ate these meth­ods at runtime!”

my $meta = __PACKAGE__->meta;

while (my ($method, $api_call) = each %METHOD_MAP) {
    $meta->add_method( $method => sub {$api_call} );
}

The add_method doc­u­men­ta­tion strong­ly encourage[s]” you to pass a metamethod object rather than a code ref­er­ence, though, so that would look like:

use Moose::Meta::Method;

my $meta = __PACKAGE__->meta;

while (my ($method, $api_call) = each %METHOD_MAP) {
    $meta->add_method( $method = Moose::Meta::Method->wrap(
      sub {$api_call}, __PACKAGE__, $meta,
    );
}

This was get­ting ugly. There had to be a bet­ter way, and for­tu­nate­ly there was in the form of Dave Rolskys MooseX::ClassAttribute mod­ule. It sim­pli­fies the above to:

use MooseX::ClassAttribute;

class_has $_ => (
  is      => 'ro',
  default => $METHOD_MAP{$_},
) for keys %METHOD_MAP;

Note there’s no need for init_arg => undef to pre­vent set­ting the attribute in the con­struc­tor. Although they’re still Moose attrib­ut­es, they act like class meth­ods so long as the class con­sumes the roles that require them after the attribute definitions.

Lastly, if we were using Moo as a light­weight alter­na­tive to Moose, I could have instead select­ed Toby Inksters MooX::ClassAttribute. Although it has some caveats, it’s pret­ty much the only alter­na­tive to our ini­tial class method def­i­n­i­tions as Moo lacks a meta-​object pro­to­col.

The les­son as always is to check CPAN (or the appro­pri­ate mix of your language’s soft­ware repos­i­to­ry, forums like Stack Overflow, etc.) for any­thing that could con­ceiv­ably have appli­ca­tion out­side of your par­tic­u­lar cir­cum­stances. Twenty-​five years into my career and I’m still leap­ing into code with­out first con­sid­er­ing that some­one smarter than me has already done the work.

One thought on “Taming the Moose: Classing up Perl attributes

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