Ruby Maybe

maybe.rb

module Monad
  def lift(&blk)
    v = bind { |value| blk.call(value) }
    self.class.unit(v)
  end
  
  def liftM2(m, &blk)
    lift do |val1|
      m.bind do |val2|
        blk.call(val1, val2)
      end
    end
  end
  
  def method_missing(method, *args, &block)
    lift { |value| value.public_send(method, *args, &block) }
  end
  
  def respond_to?(msg_id, priv = false)
    lift { |value| value.respond_to?(msg_id, priv) }
  end
end

class Maybe
  include Monad
  attr_reader :value

  def self.unit(value = nil)
    new(value)
  end

  def initialize(value)
    @value = value
  end

  def bind(&blk)
    blk.call(@value) unless @value.nil?
  end

end

def Maybe(value = nil)
  Maybe.unit(value)
end