1. Please note that @callback attribute is set in the module which defines the interface. While @behaviour is set in the module which implements the interface.

2. So you’ll actually need something like this

defmodule MishkaPub.ActivityStream.Validator do
  @type action() :: :build | :validate
  @type t :: any() # Or what kind of argument Validator generally wants

  @callback build(t()) :: {:ok, action(), t()} | {:error, action(), any()}
  @callback build(t(), list(String.t())) ::
            {:ok, action(), t()} | {:error, action(), any()}

  @callback validate(t()) :: {:ok, action(), t()} | {:error, action(), any()}
  @callback validate(t(), list(String.t())) ::
            {:ok, action(), t()} | {:error, action(), any()}
end

defmodule MishkaPub.ActivityStream.Type.Object do
  alias MishkaPub.ActivityStream.Validator

  @type t :: %__MODULE__{
          id: String.t(),
          type: String.t(),
          name: String.t(),
          replies: list(String.t())
        }

  defstruct [
    :id,
    :type,
    ..
  ]

  @behaviour Validator

  @impl true
  @spec build(t()) :: {:ok, Validator.action(), t()} | {:error, Validator.action(), any()}
  def build(%__MODULE__{} = params) do
    {:ok, :build, Map.merge(%__MODULE__{}, params)}
  rescue
    _e ->
      {:ok, :build, :unexpected}
  end

  ...
end

+1 to what @hst337 said, the behaviour and the implementation need to be separate modules.

Some other thoughts:

• passing __MODULE__ isn’t required, the module that said use is available as __MODULE__ inside the quote block.

  defmodule MacroDemo do
    defmacro __using__(opts) do
      IO.inspect(__MODULE__, label: "outside")
  
      quote(bind_quoted: [opts: opts]) do
        IO.inspect(__MODULE__, label: "inside")
      end
    end
  end
  
  defmodule Foo do
    use MacroDemo
  end
  
  # prints
  outside: MacroDemo
  inside: Foo
  

• apart from failing to compile, the only thing that type in Validator does is define t. What about just expecting the user to write @type t :: etc etc etc outside of the use and then using it in the @callbacks?

Thank you it is very good to know how to do.