Implementing Enumerable to simulate Monads

maybe.rb

class Maybe
  include Enumerable

  def initialize(value)
    @value = value
  end

  def each
    yield value unless value.nil?
  end

  attr_reader :value
end

a = Maybe.new('foo')
a.map { |x| "Alice says '#{x}'" }
#=> ["Alice says 'foo'"]

b = Maybe.new(nil)
b.map { |x| "Alice says '#{x}'" }
#=> []

c = Maybe.new(4)
c.select(&:even?)
#=> [4]
c.select(&:odd?)
#=> []