How to define Enums in Ruby?

Back | ruby | 3/16/2011 |

This is going to heavily quote http://www.rubyfleebie.com/enumerations-and-ruby/, please read that first. The proposed implementation lets you iterate over enumerated values, which is quite awesome. But it offers little in terms of reuse. Lets improve upon it and split the methods in a way that lets us include an Enum implementation with all its class methods along the way. Full Enum.rb at the end.

module Enum

  ... instance methods ...



  def self.included(base)

    base.extend(ClassMethods)

  end

  module ClassMethods

    ... class methods ...

  end

end

The trick here is that when a class includes a module the module self.included method is invoked. The base parameter is the class object for the class that includes the module, so we can extend it with the ClassMethods implementation. Magical.

To define a gender enumeration we can now write the following.

class Gender

  include Enum



  Gender.define :MALE, "male"

  Gender.define :FEMALE, "female"

end

You can call Gender.all and Gender::MALE.

What’s next?

I want to be able to write define :MALE = “male” inside Gender class and I want to prevent instances of Gender outside of within Enum. First one to accomplish both gets a beer.

Full Enum.rb

 module Enum
 
  def initialize(key, value)
    @key = key
    @value = value
  end
 
  def key
    @key
  end
 
  def value
    @value
  end
  
  def self.included(base)
    base.extend(ClassMethods)    
  end
 
  module ClassMethods
    def define(key, value)
      @hash ||= {}
      @hash[key] = self.new(key, value)
    end
 
    def const_missing(key)
      @hash[key].value
    end    
 
    def each
      @hash.each do |key, value|
        yield key, value
      end
    end
 
    def all
      @hash.values
    end
 
    def all_to_hash
      hash = {}
      each do |key, value|
        hash[key] = value.value
      end
      hash
    end
  end
end
 