echo example: make actors less assuming what the next message type is

I.e before this change, Input, Damper actors assumed they are fed into Mixer,
but Delay assumed it is fed into something that expects naked `Chunk`.
This made no sense, and made actors dependent on how they are wired in the
system, which is an antipattern.

This is an artificial change that demonstrates problems outlined in
#11 (comment)

examples/echo.rs

@@ -43,19 +43,39 @@ type Chunk = Arc<[Sample; CHUNK_SAMPLES]>;
 /// Number of chunks the "Delay" actor (effect) has.
 const DELAY_CHUNKS: usize = 60;
 
+/// A chunk of samples that represents the "dry" (original, authentic) signal.
+struct DryChunk(Chunk);
+
+/// A chunk of sample that represents the "wet" (processed, edited) signal.
+struct WetChunk(Chunk);
+
+/// DryChunk converts to (unspecified) Chunk (but not the other way around).
+impl From<DryChunk> for Chunk {
+    fn from(orig: DryChunk) -> Self {
+        orig.0
+    }
+}
+
+/// WetChunk converts to (unspecified) Chunk (but not the other way around).
+impl From<WetChunk> for Chunk {
+    fn from(orig: WetChunk) -> Self {
+        orig.0
+    }
+}
+
 /// Dummy trigger for [`Input`] to read next chunk.
 struct ReadNext;
 
 fn silence_chunk() -> Chunk {
     Arc::new([[0i16; 2]; CHUNK_SAMPLES])
 }
 
-/// Actor to read and decode input stream (stdin) and produce sound chunks.
-struct Input {
-    next: Recipient<MixerInput>,
+/// Actor to read and decode input stream (stdin) and produce sound [`DryChunk`]s.
+struct Input<M> {
+    next: Recipient<M>,
 }
 
-impl Actor for Input {
+impl<M: From<DryChunk>> Actor for Input<M> {
     type Error = Error;
     type Message = ReadNext;
 
@@ -76,14 +96,14 @@ impl Actor for Input {
         trace!("[Input] decoded chunk: {:?}...", &chunk[..5]);
 
         // Send the parsed chunk to the next actor.
-        self.next.try_send(MixerInput::Dry(chunk))?;
+        self.next.try_send(DryChunk(chunk))?;
 
         // Trigger a loop to read the next chunk.
         context.myself.try_send(ReadNext).map_err(Error::from)
     }
 }
 
-/// Actor to encode and write sound chunks to output stream (stdout).
+/// Actor to encode and write sound chunks to output stream (stdout). Consumes [`Chunk`]s,
 struct Output;
 
 impl Actor for Output {
@@ -108,27 +128,45 @@ impl Actor for Output {
 /// A chunk that knows whether it is dry or wet.
 enum MixerInput {
     /// The original signal.
-    Dry(Chunk),
+    Dry(DryChunk),
     /// Signal from an effect.
-    Wet(Chunk),
+    Wet(WetChunk),
+}
+
+impl From<DryChunk> for MixerInput {
+    fn from(orig: DryChunk) -> Self {
+        Self::Dry(orig)
+    }
+}
+
+impl From<WetChunk> for MixerInput {
+    fn from(orig: WetChunk) -> Self {
+        Self::Wet(orig)
+    }
 }
 
 /// Audio mixer actor. Mixes 2 inputs (dry, wet) together, provides 2 equal outputs.
-struct Mixer {
-    out_1: Recipient<Chunk>,
-    out_2: Recipient<Chunk>,
-    dry_buffer: Option<Chunk>,
-    wet_buffer: Option<Chunk>,
+/// Consumer either [`DryChunk`]s or [`WetChunk`]s and produces [`Chunk`]s.
+struct Mixer<M1, M2> {
+    out_1: Recipient<M1>,
+    out_2: Recipient<M2>,
+    dry_buffer: Option<DryChunk>,
+    wet_buffer: Option<WetChunk>,
 }
 
-impl Mixer {
-    fn new(out_1: Recipient<Chunk>, out_2: Recipient<Chunk>) -> Self {
+impl<M1, M2> Mixer<M1, M2> {
+    fn new(out_1: Recipient<M1>, out_2: Recipient<M2>) -> Self {
         // Start with buffers filled, so that output is produced right for the first message.
-        Self { out_1, out_2, dry_buffer: Some(silence_chunk()), wet_buffer: Some(silence_chunk()) }
+        Self {
+            out_1,
+            out_2,
+            dry_buffer: Some(DryChunk(silence_chunk())),
+            wet_buffer: Some(WetChunk(silence_chunk())),
+        }
     }
 }
 
-impl Actor for Mixer {
+impl<M1: From<Chunk>, M2: From<Chunk>> Actor for Mixer<M1, M2> {
     type Error = Error;
     type Message = MixerInput;
 
@@ -146,8 +184,9 @@ impl Actor for Mixer {
         // if both buffers are full, mix them and send out.
         if let (Some(dry), Some(wet)) = (&self.dry_buffer, &self.wet_buffer) {
             let mixed_slice: Arc<[Sample]> = dry
+                .0
                 .iter()
-                .zip(wet.iter())
+                .zip(wet.0.iter())
                 .map(|(a, b)| [a[0].saturating_add(b[0]), a[1].saturating_add(b[1])])
                 .collect();
             let mixed: Chunk = mixed_slice.try_into().expect("sample count is correct");
@@ -163,20 +202,21 @@ impl Actor for Mixer {
 }
 
 /// Delay audio effect actor. Technically just a fixed circular buffer.
-struct Delay {
-    next: Recipient<Chunk>,
+/// Consumes [`Chunk`]s and produces [`WetChunk`]s.
+struct Delay<M> {
+    next: Recipient<M>,
     buffer: Vec<Chunk>,
     index: usize,
 }
 
-impl Delay {
-    fn new(next: Recipient<Chunk>) -> Self {
+impl<M> Delay<M> {
+    fn new(next: Recipient<M>) -> Self {
         let buffer: Vec<Chunk> = repeat(silence_chunk()).take(DELAY_CHUNKS).collect();
         Self { next, buffer, index: 0 }
     }
 }
 
-impl Actor for Delay {
+impl<M: From<WetChunk>> Actor for Delay<M> {
     type Error = Error;
     type Message = Chunk;
 
@@ -191,16 +231,16 @@ impl Actor for Delay {
         self.index = (self.index + 1) % self.buffer.len();
 
         // Send out the least recent chunk.
-        self.next.try_send(Arc::clone(&self.buffer[self.index])).map_err(Error::from)
+        self.next.try_send(WetChunk(Arc::clone(&self.buffer[self.index]))).map_err(Error::from)
     }
 }
 
-/// Audio damper actor. Attenuates audio level a bit.
-struct Damper {
-    next: Recipient<MixerInput>,
+/// Audio damper actor. Attenuates audio level a bit. Consumes [`Chunk`]s and produces [`WetChunk`]s.
+struct Damper<M> {
+    next: Recipient<M>,
 }
 
-impl Actor for Damper {
+impl<M: From<WetChunk>> Actor for Damper<M> {
     type Error = Error;
     type Message = Chunk;
 
@@ -214,7 +254,7 @@ impl Actor for Damper {
         let chunk: Chunk = chunk_slice.try_into().expect("sample count is correct");
 
         // Pass it right on.
-        self.next.try_send(MixerInput::Wet(chunk)).map_err(Error::from)
+        self.next.try_send(WetChunk(chunk)).map_err(Error::from)
     }
 }