gistfile1.rb

class Additive
  attr :value
  def initialize(value)
    @value = value
  end
  def mappend(m) Additive.new(@value + m.value) end
end

class Multiplicative
  attr :value
  def initialize(value)
    @value = value
  end
  def mappend(m) Multiplicative.new(@value * m.value) end
end

def mconcat(ms)
  ms.inject { |a,b| a.mappend(b) }
end

def sum(xs)
  mconcat( xs.map { |x| Additive.new(x) } ).value
end

def prod(xs)
  mconcat( xs.map { |x| Multiplicative.new(x) } ).value
end