diff --git a/src/main.rs b/src/main.rs
index 0b3f624..81e8f37 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -39,7 +39,7 @@ fn main() {
       flags.threads,
       flags.stream_head,
       flags.stream_tail,
-      |result| {
+      |result: Result<Whispered, String>| {
          match result {
             Ok(whispered) => {
                eprintln!("{}: {:?}", chrono::Local::now(), whispered);
@@ -104,10 +104,10 @@ struct WhisperService {
    jobs: std::sync::mpsc::SyncSender<AWhisper>,
 }
 
-fn new_whisper_service(model_path: String, threads: i32, stream_head: f32, stream_tail: f32, handler_fn: fn(Result<Whispered, String>)) -> Result<WhisperService, String> {
+fn new_whisper_service<F>(model_path: String, threads: i32, stream_head: f32, stream_tail: f32, handler_fn: F) -> Result<WhisperService, String> where F: FnMut(Result<Whispered, String>) + Send + 'static {
    match new_whisper_engine(model_path, threads) {
       Ok(engine) => {
-         let whisper = new_whisper_impl(engine, stream_head, stream_tail, handler_fn);
+         let mut whisper = new_whisper_impl(engine, stream_head, stream_tail, handler_fn);
          let (send, recv) = std::sync::mpsc::sync_channel(100);
          thread::spawn(move || { whisper.transcribe_asyncs(recv); });
          Ok(WhisperService{jobs: send})
@@ -142,20 +142,20 @@ struct WhisperImpl {
    engine: WhisperEngine,
    stream_head: f32,
    stream_tail: f32,
-   handler_fn: fn(Result<Whispered, String>),
+   handler_fn: Option<Box<dyn FnMut(Result<Whispered, String>) + Send + 'static>>
 }
 
-fn new_whisper_impl(engine: WhisperEngine, stream_head: f32, stream_tail: f32, handler_fn: fn(Result<Whispered, String>)) -> WhisperImpl {
+fn new_whisper_impl<F>(engine: WhisperEngine, stream_head: f32, stream_tail: f32, handler_fn: F) -> WhisperImpl where F: FnMut(Result<Whispered, String>) + Send + 'static {
    WhisperImpl {
       engine: engine,
       stream_head: stream_head,
       stream_tail: stream_tail,
-      handler_fn: handler_fn,
+      handler_fn: Some(Box::new(handler_fn)),
    }
 }
 
 impl WhisperImpl {
-   fn transcribe_asyncs(&self, recv: std::sync::mpsc::Receiver<AWhisper>) {
+   fn transcribe_asyncs(&mut self, recv: std::sync::mpsc::Receiver<AWhisper>) {
       loop {
          match recv.recv() {
             Ok(job) => {
@@ -172,7 +172,7 @@ impl WhisperImpl {
       }
    }
 
-   fn transcribe(&self, a_whisper: &AWhisper) -> Result<(), ()> {
+   fn transcribe(&mut self, a_whisper: &AWhisper) -> Result<(), ()> {
       match self.engine.transcribe(&a_whisper.data) {
          Ok(result) => {
             self.on_success(&result);
@@ -185,15 +185,15 @@ impl WhisperImpl {
       }
    }
 
-   fn on_success(&self, whispered: &Whispered) {
+   fn on_success(&mut self, whispered: &Whispered) {
       let result = whispered
          .after(&(self.stream_head * 100.0))
          .before(&(self.stream_tail * 100.0));
-      (self.handler_fn)(Ok(result));
+      (self.handler_fn.as_mut().unwrap())(Ok(result));
    }
 
-   fn on_error(&self, msg: String) {
-      (self.handler_fn)(Err(format!("failed to transcribe: {}", &msg)));
+   fn on_error(&mut self, msg: String) {
+      (self.handler_fn.as_mut().unwrap())(Err(format!("failed to transcribe: {}", &msg)));
    }
 }
 
@@ -342,7 +342,7 @@ impl Listener {
       let downsample_ratio = cfg.channels() as f32 * (cfg.sample_rate().0 as f32 / 16_000.0);
       let stream = device.build_input_stream(
          &cfg.clone().into(),
-         move |data: &[f32], _: &cpal::InputCallbackInfo| { // TODO why cant i do this...
+         move |data: &[f32], _: &cpal::InputCallbackInfo| {
             let mut downsampled_data = vec![];
             for i in 0..(data.len() as f32 / downsample_ratio) as usize {
                let mut upsampled = i as f32 * downsample_ratio;