maybe_for.rb

class Maybe
  def initialize(value)
    @value = value
  end
  
  def map(&block)
    Maybe.new(bind &block)
  end
  
  def bind
    if @value.nil?
      Maybe.new(nil)
    else
      yield @value
    end
  end
end

class Array
  def for(*m_acc, &block)
    if self.count > 1
      first.bind {|v| self[1..-1].for(*(m_acc + [v]), &block) }
    else
      first.map {|v| block.call(*(m_acc + [v])) }
    end
  end
end

module Kernel
  def Maybe(v)
    Maybe.new(v)
  end
end

mx, my, mz = Maybe(1), Maybe(2), Maybe(3)

[mx, my, mz].for {|x, y, z| x + y + z }
#=> #<Maybe @value=6>
[mx, Maybe(nil), mz].for {|x, y, z| x + y + z }
#=> #<Maybe @value=nil>