構造体へのFunctor / fmapのオリジナル定義

functor_for_struct.ex

defmodule Product do
  defstruct name: "", price: 0
  def fmap(func, %Product{ name: name, price: price }) do
    { n_name, n_price } = func.(name, price)
    %Product{ name: n_name, price: n_price }
  end
end

stones = %Product{ name: "大人気!その辺に落ちていた石の詰め合わせ", price: 2000 }
summer_sale = Product.fmap(fn name, price -> { "[SummerSale]: #{name}", price - 200 } end, stones)
# %Product{
#   name: "[SummerSale]: 大人気!その辺に落ちていた石の詰め合わせ",
#   price: 1800
# }

price_hike = Product.fmap(fn name, price -> { "[市場価格高騰]: #{name}", price + 2000 } end, stones)
# %Product{
#   name: "[市場価格高騰]: 大人気!その辺に落ちていた石の詰め合わせ",
#   price: 4000
# }