The issue with defoverridable are two:

1. It leads to broad interfaces because you can define default functions that are “inherited” by everyone. While this is also possible with callbacks, the fact a callback is optional means you cannot rely on it as part of your API. This is part of an overall principle were you generally want to keep your interfaces narrow.

2. Overridable functions come from meta-programming, which is harder to debug, and an overriden implementation can call their previous definition using super, which is not something you can turn on or off and creates additional coupling, regardless if it is intentional or not. super can also cascade, which leads to more issues during trouble-shooting.

There are situations were defoverridable is preferred and I think @slouchpie gave a good example, where you don’t want to invoke Code.ensure_loaded? on every time. But for process-based behaviours or behaviours with clear entry-points, I’d recommend optional callbacks. I will improve the documentation.

As I said at the time, saying there are several instances does not move the needle, because the Elixir team doesn’t know where they are. My biggest concern is that, without quantifying them, we may be simply repeating information which is no longer true. Especially because, if there are examples such as this discussion, then I am certain they will be addressed.

EDIT: Improve docs for defoverridable and behaviours · elixir-lang/elixir@38f460e · GitHub

Normally, the implementations are called through the dependency injection and they don’t need any use clause. Like this

defmodule TestBeh do
  @callback ok :: :ok
  @callback ko :: :ko

  @optional_callbacks ko: 0

  @default_impl Application.compile_env(:foo, :test_beh, TestImpl)

  # default implementation
  def ko, do: :ko

  def ok(impl \\ @default_impl), do: impl.ok()
  def ko(impl \\ @default_impl) do
    impl = if function_exported(impl, :ko, 0), do: impl, else: __MODULE__
    impl.ko()
  end 
end

Imagine we want to use a behaviour like this:

defmodule NameBehaviour do
  @callback name :: String.t()
  @optional_callbacks [name: 0]
end

APPROACH 1
With defoverridable it would be:

defmodule Greeter do
  defmacro __using__(_opts) do
    quote do
      @behaviour NameBehaviour

      defdelegate name, to: unquote(__MODULE__)
      defoverridable name: 0
    end
  end

  def name do
    "stranger"
  end

  def greet(module) do
    "Hello " <> module.name()
  end
end

APPROACH 2
Without defoverridable (as docs suggest) it would be:

defmodule Greeter do
  defmacro __using__(_opts) do
    quote do
      @behaviour NameBehaviour
    end
  end

  def greet(module) do
    if Code.ensure_loaded?(module) and function_exported?(module, :name, 0) do
      "Hello " <> module.name()
    else
      "Hello stranger"
    end
  end
end

Personally, I think “Approach 1” looks much better.

Surely it is not good to have this Code.ensure_loaded? and function_exported? runtime evaluation every time I call this function?

Update: function_exported? is inlined by the compiler but I don’t know if Code.ensure_loaded? is.