Easy enumerated types

enum_type.rb

# Provide support for defining enumerated types.
#
# Example
#
#   class Colour
#
#     extend EnumType
#
#     define :red do
#       # ...
#     end
#
#     define :green do
#       # ...
#     end
#
#     define :blue do
#       # ...
#     end
#
#   end
#
module EnumType

  def self.extended(target)
    class << target
      protected :new # prevent arbitrary new instances
    end
  end

  # Public: Declare a enumerated value.
  #
  # code - a unique (String) code that identifies this value
  # block - optional block defines behaviour specific to this value
  #
  # Example
  #
  #   class Colour; extend EnumType; end
  #
  #   Colour.define :red do
  #
  #     def rgb
  #       "#ff0000"
  #     end
  #
  #   end
  #
  def define(code, &block)
    value_class = generate_value_subclass(code.to_s, &block)
    register_value(value_class.new)
  end

  # Public: Succintly declare methods to return hardcoded values.
  #
  # attributes - a Hash; a method will be generated for each key, returning
  #              the corresponding value
  #
  # Example
  #
  #   Colour.define :red do
  #
  #     hardcode :rgb => "#ff0000"
  #
  #   end
  #
  def hardcode(attributes)
    attributes.each do |name, value|
      define_method(name) { value }
    end
  end

  def get(name)
    @all_by_code[name.to_s]
  end

  alias :[] :get

  def fetch(name)
    get(name) || raise(ArgumentError, "no such #{self}: #{name.inspect}")
  end

  def all
    @all_by_code.values
  end

  def codes
    @all_by_code.keys
  end

  private

  def generate_value_subclass(code, &block)
    const_name = "#{self}::#{code.upcase}"
    Class.new(self) do
      define_method(:code) { code }
      define_method(:to_s) { code }
      define_method("#{code}?") { true }
      define_method(:inspect) { const_name }
      class_eval(&block) if block
    end
  end

  def register_value(value)
    code = value.code
    const_set(code.upcase, value)
    singleton_class.send(:define_method, code) { value }
    class_eval "def #{code}?; false; end"
    @all_by_code ||= {}
    @all_by_code[code] = value
  end

end