Hey @JKWA,

First of all, thanks for your book.

I bought it recently and it’s been an enjoyable journey reading it.
I read your article and ran the LiveBook, and I liked your idea so much that I decided to create my own version of your implementation.
The main difference is that I created a separate module for the Protocol and used defdelegate to route the function depending on the arity.

I would like to your opinion about it.

Thanks!

defmodule Book do 
  defstruct [:id, :title, :author]
end

defmodule DVD do
  defstruct [:id, :title, :director]
end

defprotocol Eq.Protocol do
  def eq?(a, b)
end

defmodule Eq do
  defdelegate eq?(a, b), to: Eq.Protocol

  def eq?(a, b, comparator) when is_function(comparator, 2) do
    comparator.(a, b)
  end
end

defimpl Eq.Protocol, for: Book do
  def eq?(%Book{id: id1}, %Book{id: id2}), do: id1 == id2
  def eq?(_, _), do: false
end

defimpl Eq.Protocol, for: DVD do
  def eq?(%DVD{id: id1}, %DVD{id: id2}), do: id1 == id2
  def eq?(_, _), do: false
end

hobbit_book1 = %Book{id: 1, title: "Hobbit", author: "Tolkien"}
hobbit_book2 = %Book{id: 1, title: "Hobbits", author: "Tolkien"}

title_comparator = fn a, b -> 
  a.title == b.title
end

Eq.eq?(hobbit_book1, hobbit_book2, title_comparator)