zip_blog_post_6.ex

defprotocol Subtract do
  def calculate(a, b)
end

# And implement it
defimpl Subtract, for: Integer do
  def calculate(a, b) when is_integer(b), do: a - b
end

defimpl Subtract, for: Decimal do
  def calculate(a, b = %Decimal{}), do: Decimal.sub(a, b)
end