maybe.rb

require 'singleton'

class NothingClass
  include Singleton

  def bind
    self
  end

  def map
    self
  end

  def inspect
    "Nothing"
  end
end

Nothing = NothingClass.instance

class Just
  attr_reader :value

  def initialize(val)
    @value = val
  end

  def bind
    ret = yield value

    unless ret.is_a?(Just) || ret.is_a?(NothingClass)
      raise "The block passed to bind must return a Just or Nothing"
    end

    ret
  end

  def map
    bind { |v| Just(yield(v)) }
  end

  def inspect
    "Just(#{value})"
  end
end

def Just(value)
  Just.new(value)
end