How to properly implement dynamic dispatch?

Questions & Help Questions
8

^ What does adding that extra callback do? IIUC, it’s making three part of the contract of the behavior (so any code that relies on an X can expect a three even if the implementation doesn’t use X?

defmodule X do
  @callback one(any) :: integer
  @callback two(any) :: integer
  @callback three(integer, integer) :: integer

  defmacro __using__(_options) do
    quote do
      @behaviour X
      defoverridable: three: 2
      def three(o, t) do
        one(o) + two(t)
      end
    end
  end
end

Is that right?

And say I want X to use a behavior as well, like GenServer. So all implementers of X should transitively use GenServer. Does that go under __using__ or in the main module definition?

so is it

defmodule X 
use GenServer
@callback ... 
  
defmacro __using__ do 
...
end
end

or

defmodule X 
@callback ... 

defmacro __using__ do quote do 
use GenServer
end 
end 
end