Example of a Ruby enum

enum.rb

module Enum
  def self.included(mod)
    mod.extend(ClassMethods)
  end

  module ClassMethods

    def enum(*args)
      raise "Enum already defined!" if @enum_hash
      @enum_hash = {}

      if args.length == 1
        args.first.each { |k, v| init_enum(k, v) }
      else
        args.each_with_index { |k, v| init_enum(k, v) }
      end

      @enum_hash.freeze
    end

    def enum_keys
      @enum_hash.keys
    end

    def enum_values
      @enum_hash.values
    end

    private

    def init_enum(key, value)
      key = prep_enum_key(key)
      value.freeze
      @enum_hash[key] = value
      instance_variable_set("@#{key}".to_sym, value)
      eigen.send(:attr_reader, key)
    end

    def prep_enum_key(key)
      key.to_s.downcase.gsub(/\W/, "_").to_sym
    end

    def eigen
      @eigen ||= class << self; self; end
    end

  end
end




# Example --------------------------
module Colors
  include Enum
  enum :red, :blue, :green
end

Colors.red     # => 1
Colors.blue    # => 2
Colors.green   # => 3

module Colors
  enum :orange, :violet, :aqua # => RuntimeError: Enum already defined!
end

module Shapes
  include Enum
  enum :circle   => 0,
       :triangle => 3,
       :square   => 4
end

Shapes.circle   # => 0
Shapes.triangle # => 3
Shapes.square   # => 4