Maybe Monad

maybe.rb

require 'singleton'
 
class Maybe
  def self.unit(value)
    if value.nil? || (value.respond_to?(:empty?) && value.empty?)
      Nothing.instance
    else
      Just.new(value)
    end
  end
end
 
class Just < Maybe
  attr_reader :value
 
  def initialize(value)
    @value = value
  end
 
  def bind
    yield @value
  end
 
  def empty?
    false
  end
 
  def inspect
    "Just(#{@value.inspect})"
  end
end
 
class Nothing < Maybe
  include Singleton
 
  def bind
    self
  end
 
  def empty?
    true
  end
 
  def inspect
    "Nothing"
  end
end