I recently learned that it’s possible to delegate common implementations of protocols using defdelegate in the implementations, but I’m curious to know if this is possible using a macro that provides overridable functions.

The following code works, but shows a warning (see comment below) :

defprotocol MyProtocol do
  def foo(data)
  def bar(data)
end

defmodule MyProtocol.Defaults do
  defmacro __before_compile__(_env) do
    quote do
      def bar(data) do
        IO.puts "Default implementation of bar, data: #{data}"
      end
      defoverridable [bar: 1]
    end
  end
end

defimpl MyProtocol, for: Integer do
  @before_compile MyProtocol.Defaults

  def foo(data) do
    IO.puts "int implementation of foo, data: #{data}"
  end

  # Is it possible to override `bar` from before_compile without this warning:
  #
  # warning: this clause cannot match because a previous clause at line 28 always matches
  # my_protocol.exs:17
  def bar(data) do
    IO.puts "int implementation of bar, data: #{data}"
  end
end

Do I get this warning because before_compile’s return value is injected at the end of the module? Is there any way to get around this?