Improve test for selector replacement in actor

test/gleam/otp/actor_test.gleam

@@ -1,5 +1,5 @@
 import gleam/otp/actor.{Continue, Selecting}
-import gleam/erlang/process.{Pid, Selector}
+import gleam/erlang/process.{Pid, Subject}
 import gleam/erlang/atom.{Atom}
 import gleam/otp/system
 import gleam/dynamic.{Dynamic}
@@ -134,49 +134,58 @@ pub fn unexpected_message_handled_test() {
   |> should.equal(dynamic.from("Unexpected message 1"))
 }
 
-type ActorMessage(inner) {
-  UserMessage(inner)
-  ReplaceSelector(selector: Selector(ActorMessage(inner)))
+type ActorMessage {
+  UserMessage(String)
+  ReplaceSelector(
+    reply: Subject(Subject(String)),
+    mapper: fn(String) -> ActorMessage,
+  )
 }
 
 pub fn replace_selector_test() {
   let assert Ok(subject) =
     actor.start(
       "init",
-      fn(msg: ActorMessage(String), state) {
+      fn(msg: ActorMessage, state) {
         case msg {
           UserMessage(string) -> Continue("user message: " <> string)
-          ReplaceSelector(selector) -> Selecting(state, selector)
+          ReplaceSelector(reply, mapper) -> {
+            // Create a new subject and create a selector
+            // for it using the mapper function
+            let subject = process.new_subject()
+            let selector =
+              process.new_selector()
+              |> process.selecting(subject, mapper)
+
+            // Send new subject back to caller
+            process.send(reply, subject)
+
+            Selecting(state, selector)
+          }
         }
       },
     )
 
+  // Send initial user message to original subject
   process.send(subject, UserMessage("test 1"))
 
   subject
   |> process.subject_owner
   |> system.get_state()
   |> should.equal(dynamic.from("user message: test 1"))
 
-  process.send(
-    subject,
-    ReplaceSelector(
-      process.new_selector()
-      |> process.selecting_anything(fn(data) {
-        data
-        |> dynamic.unsafe_coerce
-        |> UserMessage
-      }),
-    ),
-  )
+  // Get a new subject with replaced selector
+  let replaced = process.call(subject, ReplaceSelector(_, UserMessage), 1000)
 
-  raw_send(process.subject_owner(subject), "test 2")
+  // Send to new subject
+  process.send(replaced, "test 2")
 
   subject
   |> process.subject_owner
   |> system.get_state()
   |> should.equal(dynamic.from("user message: test 2"))
 
+  // Send to original subject
   process.send(subject, UserMessage("test 3"))
 
   subject