Protocols are powered by behaviours behind the scenes

lib/elixir/lib/protocol.ex

@@ -59,11 +59,10 @@ defmodule Protocol do
       Protocol.assert_protocol(protocol)
 
       defmodule name do
-        def __impl__, do: unquote(protocol)
+        @behaviour unquote(protocol)
         unquote(block)
+        def __impl__, do: unquote(protocol)
       end
-
-      Protocol.assert_impl(name, protocol)
     end
   end
 
@@ -85,28 +84,14 @@ defmodule Protocol do
     end
   end
 
-  @doc """
-  Check if the given `impl` is a valid impl for `protocol`.
-  Raises an error if not.
-
-  """
-  def assert_impl(impl, protocol) do
-    remaining = protocol.__protocol__(:functions) -- impl.__info__(:functions)
-
-    if remaining != [] do
-      pp = Enum.map_join remaining, ", ", fn {x,y} -> "#{x}/#{y}" end
-      raise ArgumentError,
-        message: "#{inspect impl} did not implement #{inspect protocol}, missing: #{pp}"
-    end
-  end
-
   @doc """
   Defines meta information about the protocol and internal callbacks.
   """
   def meta(env, functions, fallback) do
     contents = quote do
-      def __protocol__(:name),      do: __MODULE__
-      def __protocol__(:functions), do: unquote(:lists.sort(functions))
+      def __protocol__(:name),        do: __MODULE__
+      def __protocol__(:functions),   do: unquote(:lists.sort(functions))
+      def behaviour_info(:callbacks), do: [{ :__impl__, 0 }|__protocol__(:functions)]
 
       def __impl_for__(arg) do
         case __raw_impl__(arg) do

lib/elixir/src/elixir_errors.erl

@@ -89,8 +89,9 @@ handle_file_warning(_, _File, { _Line, v3_kernel, bad_call }) -> [];
 
 %% Rewrite
 handle_file_warning(_, File, {Line,erl_lint,{undefined_behaviour_func,{Fun,Arity},Module}}) ->
-  Raw = "undefined callback function ~s/~B (behaviour ~s)",
-  Message = io_lib:format(Raw, [Fun,Arity,inspect(Module)]),
+  Kind    = protocol_or_behaviour(Module),
+  Raw     = "undefined ~s function ~s/~B (for ~s ~s)",
+  Message = io_lib:format(Raw, [Kind, Fun, Arity, Kind, inspect(Module)]),
   io:format(file_format(Line, File, Message));
 
 handle_file_warning(_, File, {Line,erl_lint,{undefined_behaviour,Module}}) ->
@@ -153,4 +154,21 @@ format_error([], Desc) ->
   io_lib:format("~p", [Desc]);
 
 format_error(Module, Desc) ->
-  Module:format_error(Desc).
+  Module:format_error(Desc).
+
+protocol_or_behaviour(Module) ->
+  case is_protocol(Module) of
+    true  -> protocol;
+    false -> behaviour
+  end.
+
+is_protocol(Module) ->
+  case code:ensure_loaded(Module) of
+    { ok, _ } ->
+      case erlang:function_exported(Module, '__protocol__', 1) of
+        true  -> Module:'__protocol__'(name) == Module;
+        false -> false
+      end;
+    { error, _ } ->
+      false
+  end.