support operationId that specifies a module name and function

lib/apical/path.ex

@@ -75,7 +75,6 @@ defmodule Apical.Path do
 
     version = Keyword.fetch!(opts, :version)
     root = Keyword.fetch!(opts, :root)
-    function = Keyword.get(opts, :alias, String.to_atom(operation_id))
 
     plug_opts =
       opts
@@ -84,10 +83,22 @@ defmodule Apical.Path do
       )
       |> Keyword.merge(path_parameters: path_parameters, path: path)
 
-    controller =
+    {controller, function} =
       case Keyword.fetch(opts, :controller) do
         {:ok, controller} when is_atom(controller) ->
-          controller
+          case Keyword.get(opts, :alias) do
+            nil -> 
+              case String.split(operation_id, ".", trim: true) do
+                [operation_id] -> {controller, String.to_atom(operation_id)}
+                parts -> 
+                  {alias_fun, module_parts} = List.pop_at(parts, -1)
+
+                  {Module.concat([controller] ++ module_parts), String.to_atom(alias_fun)}
+                end
+
+            alias_fun -> 
+              {controller, alias_fun}
+          end
 
         {:ok, controller} ->
           raise CompileError,

test/plug/operation_mf_test.exs

@@ -0,0 +1,43 @@
+defmodule ApicalTest.Plug.Operation.Module do
+  use ApicalTest.EndpointCase, with: Plug
+  alias Plug.Conn
+
+  use Apical.Plug.Controller
+
+  def fun(conn, _params) do
+    Conn.send_resp(conn, 200, "OK")
+  end
+
+  test "GET /" do
+    assert %{
+             status: 200,
+             body: "OK"
+           } = Req.get!("http://localhost:#{@port}/")
+  end
+end
+
+defmodule ApicalTest.Plug.Operation.Module.Router do
+  use Apical.Plug.Router
+
+  require Apical
+
+  Apical.router_from_string(
+    """
+    openapi: 3.1.0
+    info:
+      title: TestGet
+      version: 1.0.0
+    paths:
+      "/":
+        get:
+          operationId: Module.fun
+          responses:
+            "200":
+              description: OK
+    """,
+    for: Plug,
+    root: "/",
+    controller: ApicalTest.Plug.Operation,
+    encoding: "application/yaml"
+  )
+end