zip_blog_post_7.ex

defimpl Zip, for: Map do
  def apply(a, b, operation) do
    # We take the all the keys from b that are also in a
    intersection = Map.take(b, Map.keys(a))

    Enum.reduce(a, %{}, fn {key, value}, acc ->
      Map.put(acc, key, operation.calculate(value, Map.fetch!(intersection, key)))
    end)
  end
end