Duck Typing 〜 Elixirの多態

# 鶩
defmodule Duck do
  defstruct name: nil
end

# 犬
defmodule Dog do
  defstruct name: nil
end

# 猫
defmodule Cat do
  defstruct name: nil
end

# 鳴くプロトタイプ
defprotocol Sound do
  def sound(_)
end

# 鶩が鳴く実装
defimpl Sound, for: Duck do
  def sound(animal) do
    IO.puts "#{animal.name} the Duck sounds quack quack."
  end
end

# 犬が鳴く実装
defimpl Sound, for: Dog do
  def sound(animal) do
    IO.puts "#{animal.name} the Dog sounds bow-wow."
  end
end

# 猫が鳴く実装
defimpl Sound, for: Cat do
  def sound(animal) do
    IO.puts "#{animal.name} the Cat sounds miaow miaow."
  end
end

defmodule Test do
  duck = %Duck{name: "Donald"} # ドナルド
  dog = %Dog{name: "Shiro"}    # しろ
  cat = %Cat{name: "Tama"}     # たま

  Sound.sound(duck)
  Sound.sound(dog)
  Sound.sound(cat)
end

実行。

$ elixir sound.ex 
Donald the Duck sounds quack quack.
Shiro the Dog sounds bow-wow.
Tama the Cat sounds miaow miaow.