Add logging module

src/musicplayer/logging/control.gleam

@@ -0,0 +1,7 @@
+import gleam/erlang/process.{type Subject}
+
+pub type Control {
+  Write(String)
+
+  Exit(reply_to: Subject(Nil))
+}

src/musicplayer/logging/logging.gleam

@@ -0,0 +1,58 @@
+import gleam/erlang/process.{type Subject}
+import gleam/otp/actor
+import gleam/result
+import gleam/string
+import gleam/time/calendar
+import gleam/time/timestamp
+import simplifile
+
+import musicplayer/logging/control.{type Control}
+
+type State {
+  State(filepath: String)
+}
+
+pub fn new(filepath: String) -> Result(Subject(Control), String) {
+  use _ <- result.try(
+    case simplifile.is_file(filepath) {
+      Ok(True) -> Ok(Nil)
+      _ -> simplifile.create_file(filepath)
+    }
+    |> result.map_error(fn(e) {
+      "Could not access or create log file: " <> string.inspect(e)
+    }),
+  )
+
+  actor.new(State(filepath:))
+  |> actor.on_message(handle_message)
+  |> actor.start
+  |> result.map_error(fn(start_error) {
+    "Could not start logger: " <> string.inspect(start_error)
+  })
+  |> result.map(fn(started) { started.data })
+}
+
+fn handle_message(state: State, control: Control) -> actor.Next(State, Control) {
+  case control {
+    control.Write(content) -> {
+      let log_line =
+        timestamp.system_time()
+        |> timestamp.to_rfc3339(calendar.utc_offset)
+        <> ": "
+        <> content
+        <> "\n"
+
+      // Ignore any logging errors
+      let _ = simplifile.append(state.filepath, log_line)
+      actor.continue(state)
+    }
+    control.Exit(reply_to) -> {
+      process.send(reply_to, Nil)
+      actor.stop()
+    }
+  }
+}
+
+pub fn log(logger: Subject(Control), content: String) -> Nil {
+  process.send(logger, control.Write(content))
+}