Monoids in Ruby

monoids.rb

class Monoid
  attr_reader :mempty

  def initialize(mempty, mappend)
    @mempty = mempty
    @mappend = mappend
  end

  def mappend(x, y)
    @mappend.call(x, y)
  end

  def mconcat(enumerable)
    enumerable.reduce(mempty &@mconcat)
  end
end

sum = Monoid.new(0) { |x, y| x + y }

def hashmerge(monoid)
  Monoid.new({}) do |h1, h2|
    h2.merge(h2) do |key, old, new|
      monoid.mappend(old, new)
    end
  end
end

def function(monoid)
  Monoid.new(lambda { |x| monoid.mempty }) do |f, g|
    lambda do |x|
      monoid.mappend(f.call(x), g.call(x))
    end
  end
end