Test if GPIO is alive using Process.alive?/1 instead of using

GenServer's state.
Fix signal sending twice because of copy-paste error.

morse/lib/morse_server.ex

@@ -8,7 +8,7 @@ defmodule MorseServer do
   end
 
   def start_link do
-    GenServer.start_link(__MODULE__, :off, name: __MODULE__)
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
   end
 
   def start_morse do
@@ -21,17 +21,14 @@ defmodule MorseServer do
   end
 
   @impl true
-  def handle_call(:start, _from, :on) do
-    {:reply, {:error, :already_started}, :on}
-  end
+  def handle_call(:start, _from, pid) do
+    cond do
+      pid == nil or not Process.alive?(pid) ->
+        pid = spawn(&MorseSignaler.signal/0)
+        {:reply, :ok, pid}
 
-  def handle_call(:start, _from, :off) do
-    spawn(MorseSignaler, :signal, [self()])
-    {:reply, :ok, :on}
+      true ->
+        {:reply, {:error, :already_started}, pid}
     end
-
-  @impl true
-  def handle_cast(:done, :on) do
-    {:noreply, :off}
   end
 end

morse/lib/morse_signaler.ex

@@ -19,14 +19,11 @@ defmodule MorseSignaler do
   Notifies the parent when the signalling is done.
   """
 
-  def signal(server_pid) do
+  def signal() do
     {:ok, gpio} = GPIO.open(relay_pin(), :output)
     GPIO.write(gpio, @off)
     Process.sleep(@sleep_start)
     signal_sentence(gpio, String.graphemes(secret_code()))
-
-    GenServer.cast(server_pid, :done)
-    :ok
   end
 
   # Signal a whole sentence of symbols with GPIO.
@@ -65,6 +62,6 @@ defmodule MorseSignaler do
   end
 
   defp secret_code do
-    signal(Application.fetch_env!(:morse, :morse_message))
+    Application.fetch_env!(:morse, :morse_message)
   end
 end