Monoid examples in elixir

monoids.ex

defmodule Monoid do
  defmacro __using__(_opts) do
    quote do
      def concat([head|tail]) do
        append(head, concat(tail))
      end

      def concat([]) do
        empty
      end
    end
  end
end

defmodule Sum do
  use Monoid

  def empty do
    0
  end

  def append(a,b) do
    a + b
  end
end

defmodule Product do
  use Monoid

  def empty do
    1
  end

  def append(a,b) do
    a * b
  end
end

defmodule Any do
  use Monoid

  def empty do
    false
  end

  def append(a,b) do
    a || b
  end
end

defmodule All do
  use Monoid

  def empty do
    true
  end

  def append(a,b) do
    a && b
  end
end

defmodule StringMonoid do
  use Monoid

  def empty do
    ""
  end

  def append(a,b) do
    a <> b
  end
end

defmodule ListMonoid do
  use Monoid

  def empty do
    []
  end

  def append(a,b) do
    Enum.concat([a,b])
  end
end

defmodule MapSetMonoid do
  use Monoid

  def empty do
    MapSet.new
  end

  def append(a,b) do
    MapSet.union(a,b)
  end
end

defmodule MapMonoid do
  use Monoid

  def empty do
    %{}
  end

  def append(a,b) do
    Map.merge(a,b)
  end
end