MouseX::Types::Enum - A module for writing Java-style enum (enumerated type) classes in Perl

2018-05-20

MouseX::Types::Enum

I wanted to use enumerated types with fields and methods in Perl, just like Java's enum type, so I created a module called MouseX::Types::Enum.

Dependencies

I built it as an extension of Mouse, so it depends on Mouse.

Usage example

With MouseX::Types::Enum, each enum constant can have member variables and methods, just like Java's enumerated types. Here is an example of how to use it. In this example:

I defined three enum constants: APPLE, ORANGE, and BANANA. When calling MouseX::Types::Enum in the use statement, you pass it the array of enum constants you want to define.
I defined member variables name, color, and has_seed on the enum constants. You can use Mouse's has syntax to define member variables.
I defined a method called make_sentence. It builds and returns a string based on the arguments and member variables.

package Fruits;

use Mouse;
use MouseX::Types::Enum (
    APPLE  => { name => 'Apple', color => 'red' },
    ORANGE => { name => 'Cherry', color => 'red' },
    BANANA => { name => 'Banana', color => 'yellow', has_seed => 0 }
);

has name => (is => 'ro', isa => 'Str');
has color => (is => 'ro', isa => 'Str');
has has_seed => (is => 'ro', isa => 'Int', default => 1);

sub make_sentence {
    my ($self, $suffix) = @_;
    $suffix ||= "";
    return sprintf("%s is %s%s", $self->name, $self->color, $suffix);
}

__PACKAGE__->meta->make_immutable;

use Fruits;

Fruits->APPLE == Fruits->APPLE; # 1
Fruits->APPLE == Fruits->ORANGE; # ''
Fruits->APPLE->to_string; # 'APPLE'

Fruits->APPLE->name; # 'Apple';
Fruits->APPLE->color; # 'red'
Fruits->APPLE->has_seed; # 1

Fruits->APPLE->make_sentence('!!!'); # 'Apple is red!!!'

Fruits->enums; # { APPLE  => Fruits->APPLE, ORANGE => Fruits->ORANGE, BANANA => Fruits->BANANA }

How to declare when member variables aren't needed

If you don't need member variables, you can also declare them like this:

package Day;

use MouseX::Types::Enum qw/
    Sun
    Mon
    Tue
    Wed
    Thu
    Fri
    Sat
/;

__PACKAGE__->meta->make_immutable;

use Day;

Day->Sun == Day->Sun; # 1
Day->Sun == Day->Mon; # ''
Day->Sun->to_string;  # 'APPLE'
Day->enums; # { Sun => Day->Sun, Mon => Day->Mon, ... }

Notes

Java's enum type has an instance method called ordinal(). When called on an enum constant, it returns that constant's ordinal (a unique number). However, because the ordinal is determined by the declaration order of the enum constants, it has the property that changing the declaration order also changes the corresponding ordinals. For this reason, if you persist an ordinal obtained via ordinal() somewhere and then later reorder the enum constants, or insert a new declaration between existing ones, there's a risk that the ordinals will become inconsistent.

Since this behavior can lead to unexpected bugs, I decided not to define an ordinal() method or the concept of ordinals in MouseX::Types::Enum. It's a trade-off against having to write more code, but I think it's better to write the number ⇔ enum constant mapping explicitly somewhere in your code.