From: Kedar Mhaswade <kedar.mhaswade@...>
Date: 2011-05-20T23:35:09+09:00
Subject: Reasonable attempt to do type-safe enum for Ruby?

Perhaps this has been tried before. But I didn't find it. Google
searches showed many different ways to implement an "enum" data type for
Ruby. Searching on forum suggested using hashes, arrays and Struct. But
none of them seemed type-safe to me. So, I went ahead and did something
like below, for an enum named GenericState which represents a "state" of
some kind.

Please comment on this approach.


# An enum kind of model. No persistent equivalent.
# I still don't know the correct way of doing enums in Ruby!
class GenericState
  attr_reader :state

  def initialize(s)
    @state = s
  end

  def to_s
    @state.to_s
  end

ACCEPTED = GenericState.new(:accepted)
EXPIRED  = GenericState.new(:expired)
IGNORED  = GenericState.new(:ignored)
REJECTED = GenericState.new(:rejected)
SENT     = GenericState.new(:sent)
UNSURE   = GenericState.new(:unsure)
STATES   = [ACCEPTED, EXPIRED, IGNORED, REJECTED, SENT, UNSURE]

  class << self
    include Enumerable
    def each(&block)
      STATES.each(&block)
    end
    alias all entries
  end

# convenience methods
  def one_of_my_predicates?(meth)
    # meth is a symbol
    $1 if meth =~ /(.+)\?$/ && STATES.any?{|ss| ss.to_s == $1}
  end

  def run_predicate(method_name)
    method_name == to_s
  end

  def method_missing(meth, *args, &block)
    matched = one_of_my_predicates?(meth)
    if matched
      run_predicate(matched)
    else
      super
    end
  end

end

The method_missing protocol does not have to be implemented, but it
facilitates more readable usage as indicated below.

#usage
puts GenericState.any? {|x| x == GenericState::SENT} # => true
puts GenericState.all # => An array of all GenericState objects!
s1 = GenericState::SENT
s2 = GenericState::ACCEPTED
s3 = GenericState::ACCEPTED
puts s1 == s2 # => false
puts s3 == s2 # => true
puts s2.object_id; puts s3.object_id #=> they are the same objects
puts "Does s1 imply sent? : #{s1.sent?}" # => true
puts "Does s3 imply accepted? : #{s3.accepted?}" # => true
puts s1.to_s #=> "sent"

Best Regards,
Kedar

-- 
Posted via http://www.ruby-forum.com/.