example.ex

defmodule T.Behaviour do
  defmacro __using__(_) do
    quote do
      @behaviour T.Behaviour

      import T.Macros, only: [deftest: 2]

      deftest :a do
        IO.puts z
      end
    end
  end
end

defmodule T.Macros do
  defmacro deftest(name, do: body) do
    IO.puts inspect body
    quote do
      def unquote(name)(var!(z)) do
        unquote(body)
      end
    end
  end
end

defmodule T.Impl do
  use T.Behaviour
end

defmodule T.OK do
  import T.Macros, only: [deftest: 2]
  deftest :a do
    IO.puts z
  end
end

## Trying to compile this:
# > mix
# lib/example.ex:25: warning: behaviour T.Behaviour undefined
# lib/example.ex:26: warning: variable z is unused

# == Compilation error on file lib/example.ex ==
# ** (CompileError) lib/example.ex:26: function z/0 undefined
#     (stdlib) lists.erl:1337: :lists.foreach/2
#     (stdlib) erl_eval.erl:669: :erl_eval.do_apply/6

## If the T.Impl module is commented:
# > mix
# Compiled lib/example.ex
# Generated  app

# > iex -S mix
# Erlang/OTP 18 [erts-7.2.1] [source] [64-bit] [smp:4:4] [async-threads:10]

# Interactive Elixir (1.1.1) - press Ctrl+C to exit (type h() ENTER for help)
# iex(1)> T.OK.a 5
# 5
# :ok