2017 twenty four merry days of Perl Feed

For Elves, Shorter is Better

MooseX::AttributeShortcuts - 2017-12-16

One of the problems in being the Wise Old Elf was, well, the old part. The Wise Old Elf had been typing for so many years - rushing every Christmas - that he'd developed a case of RSI that flared up at the most inconvenient times.

As such, he was always looking for ways to avoid typing so much, and this Christmas, he thought he'd found one.

Verbose. Verbosity. So long winded. On and On.

One of the problems with Moose in its default configuration is that it can be, well, a little wordy...


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has lead_reindeer => (
  is => 'ro',
  isa => class_type('SantasReindeer'),
  lazy => 1,
  builder => '_build_lead_reindeer',
);

has second_reindeer => (
  is => 'ro',
  isa => class_type('SantasReindeer'),
  lazy => 1,
  builder => '_build_second_reindeer',
);

has third_reindeer => (
  is => 'ro',
  isa => class_type('SantasReindeer'),
  lazy => 1,
  builder => '_build_third_reindeer',
);

has forth_reindeer => (
  is => 'ro',
  isa => class_type('SantasReindeer'),
  lazy => 1,
  builder => '_build_forth_reindeer',
);

...; # and on and on for another five reindeer!

 

Putting aside the fact that this is probably better written using some sort of array data structure (which is literally an article for another day) what can be done to make this less verbose?

Enter MooseX::AttributeShortcuts

Why, yes, we can make this much shorter, with the help of the MooseX::AttributeShortcuts CPAN module:


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use MooseX::AttributeShortcuts;

has [qw(
  lead_reindeer second_reindeer third_reindeer forth_reindeer
  fifth_reindeer sixth_reindeer seveth_reindeer eighth_reindeer
)
] => (
  is => 'ro',
  isa_instance_of => 'SantasReindeer',
  lazy => 1,
  builder => 1,
);

 

Loading MooseX::AttributeShortcuts into your Moose class enables a whole bunch of extra parameter handing for has that provides shortcuts to make writing attribute code simpler. In the above example we see two such shortcuts: The first shortcut, writing isa_instance_of => 'SantasReindeer' instead of isa => class_type('SantasReindeer') provides us a little win, the biggest reduction in code comes from builder => 1. By setting the builder to a plain old 1 we're letting MooseX::AttributeShortcuts tell Moose that we want to use the default name for our builder - whatever the accessor is called with _build_ prepended to it. And because now we don't need a custom builder parameter for each attribute, we can declare all nine in a single has declaration!

Can we make this shorter still? Yep! We can just say, like with Moo, that this is lazy.


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has [qw(
  lead_reindeer second_reindeer third_reindeer forth_reindeer
  fifth_reindeer sixth_reindeer seveth_reindeer eighth_reindeer
)
] => (
  is => 'lazy',
  isa_instance_of => 'SantasReindeer',
);

 

That shortcut create a read only accessor that is lazy and uses the default Moose accessor.

In addition to lazy, MooseX::AttributeShortcuts also accepts is => 'rwp'. rwp creates a public reader accessor of the form foo and a private writer accessor of the form _set_foo (since by convention in Perl methods starting with an underscore are private and shouldn't be called from outside of the class.)

Custom Anonymous Types

It would be nice to make sure that the lead reindeer has a glowing nose, but that would probably involve a custom type class to check the value's glowing_nose attribute:


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package NorthPole::Types;

use MooseX::Types -declare => [
    qw(
        SantasReindeerWithAGlowingNose
    )

];

subtype(
    SantasReindeerWithAGlowingNose,
    as class_type('SantasReindeer'),
    where {},
    inline_as {
        $_[0]->parent()->_inline_check( $_[1] ) . " && $_[1]->glowing_nose";
    }
);

 

And then use that new type:


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use NorthPole::Types qw( SantasReindeerWithAGlowingNose );

has lead_reindeer => (
  is => 'lazy',
  isa => SantasReindeerWithAGlowingNose,
);

 

That is a lot of typing if you're only going to use it in one place. How about we use another shortcut?


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has lead_reindeer => (
  is => 'lazy',
  isa => sub {
     die unless blessed && $_->isa('SantasReindeer') && $_->glowing_nose;
  },
);

 

Here we're quickly defining a new one off type inline! It's not the most readable thing in the world though, is it? MooseX::AttributeShortcuts allows us to specify this in a more straight forward manner by allowing an inline constraint, which is an additional condition that will be checked for truthfulness after the isa / isa_instance_of check has occurred.


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has lead_reindeer => (
  is => 'lazy',
  isa_instance_of => 'SantasReindeer',
  constraint => sub { $_->glowing_nose },
);

 

As an added benefit writing a constraint / isa pair is that this subtypes the original type, meaning that coercions associated with the original type will just work!

Much much more

The Wise Old Elf continued to read the documentation for MooseX::AttributeShortcuts finding out about all sorts of things, from easy trigger, clearer and predicate declaration through quickly defining inline coercions and builder methods. He, like you should, was going to spend a good afternoon playing with this module - not only to avoid so much typing, but also to avoid writing so much boilerplate code and reduce the potential for bugs.

Gravatar Image This article contributed by: Mark Fowler <mark@twoshortplanks.com>