Add ability to listen (tap) the input

By doing something like
```
fn input_output_loop(input_output: Subject(List(String))) -> Nil {
  let output = process.receive_forever(input_output)

  echo output

  input_output_loop(input_output)
}
```

src/input/key.gleam

@@ -1,5 +1,7 @@
 import gleam/erlang/atom
+import gleam/erlang/process.{type Subject}
 import gleam/list
+import gleam/string
 
 import input/internal as internal_input
 
@@ -22,7 +24,7 @@ pub const esc = "\u{001B}"
 pub const csi = "["
 
 // input introducer
-const input_introducer = "$"
+pub const input_introducer = "::"
 
 pub fn from_list(l: List(String)) -> Key {
   case l {
@@ -35,9 +37,10 @@ pub fn from_list(l: List(String)) -> Key {
 
     [ci] | [ci, _] if ci == input_introducer -> Continue(l)
     [ii, cmd, tail] if ii == input_introducer -> {
-      case tail == "\r" {
+      case tail {
-        True -> Input(cmd)
+        "\r" -> Input(cmd)
-        False -> Continue([ii, cmd <> tail])
+        "\u{007F}" -> Continue([ii, string.drop_end(cmd, 1)])
+        _ -> Continue([ii, cmd <> tail])
       }
     }
 
@@ -55,14 +58,20 @@ pub fn start_raw_shell() {
   internal_input.shell_start_interactive(#(no_shell, raw))
 }
 
-pub fn read_input_until_key(l: List(String)) -> Key {
+pub fn read_input_until_key(
+  l: List(String),
+  tap_input: Subject(List(String)),
+) -> Key {
   case
     internal_input.read_input()
     |> list.wrap
     |> list.append(l, _)
     |> from_list
   {
-    Continue(l) -> read_input_until_key(l)
+    Continue(l) -> {
+      process.send(tap_input, l)
+      read_input_until_key(l, tap_input)
+    }
     k -> k
   }
 }

src/mpv/control.gleam

@@ -10,6 +10,8 @@ import tcp/tcp.{type Socket}
 pub type Control {
   TogglePlayPause
 
+  Search
+
   Exit
 }
 
@@ -17,7 +19,12 @@ pub type ControlError {
   ControlError(details: String)
 }
 
+// TODO this should also have a context:
+// `/` in "artist list" "context will should be`control.Search`
+// `<some char>` in "create new playlist" context should be `control.Input`
+// `q` in most contexts should be `Exit`, but in a popup it should be `Close`
 pub fn from_key(key: Key) -> Result(Control, Nil) {
+  echo key
   case key {
     key.Char(char) -> char_control(char)
     _ -> Error(Nil)
@@ -28,6 +35,7 @@ fn char_control(char: String) -> Result(Control, Nil) {
   case char {
     " " -> Ok(TogglePlayPause)
     "q" -> Ok(Exit)
+    "/" -> Ok(Search)
     _ -> Error(Nil)
   }
 }

src/mpv/mpv.gleam

@@ -4,13 +4,18 @@ import gleam/otp/actor
 import gleam/result
 import gleam/string
 
-import input/key
+import input/key.{type Key}
 import mpv/control.{type Control}
 import tcp/reason
 import tcp/tcp.{type Socket}
 
-type State(socket, exit) {
+type State(socket, inject_input, tap_input, exit) {
-  State(socket: Socket, exit: Subject(Nil))
+  State(
+    socket: Socket,
+    inject_input: Subject(Key),
+    tap_input: Subject(List(String)),
+    exit: Subject(Nil),
+  )
 }
 
 pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
@@ -20,8 +25,17 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
   case tcp.connect(socket_path) {
     Error(r) -> Error("Could not connect to mpv: " <> reason.to_string(r))
     Ok(socket) -> {
+      // `inject_input` is created by name to allow the process that
+      // owns `read_input` to be able to register it, while the agent
+      // also have a reference to it to be able to inject input
+      let inject_input_name = process.new_name("inject_input")
+      let inject_input = process.named_subject(inject_input_name)
+
+      let tap_input_name = process.new_name("tap_input")
+      let tap_input = process.named_subject(tap_input_name)
+
       case
-        actor.new(State(socket, exit))
+        actor.new(State(socket, inject_input, tap_input, exit))
         |> actor.on_message(handle_message)
         |> actor.start
       {
@@ -30,7 +44,20 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
         Ok(actor.Started(data:, ..)) -> {
           echo "waiting for input"
           key.start_raw_shell()
-          process.spawn(fn() { read_input(data) })
+
+          process.spawn(fn() {
+            let assert Ok(_) =
+              process.register(process.self(), inject_input_name)
+
+            read_input(data, inject_input, tap_input)
+          })
+
+          process.spawn(fn() {
+            let assert Ok(_) = process.register(process.self(), tap_input_name)
+
+            input_output_loop(tap_input)
+          })
+
           Ok(Nil)
         }
       }
@@ -39,10 +66,14 @@ pub fn new(exit: Subject(Nil)) -> Result(Nil, String) {
 }
 
 fn handle_message(
-  state: State(socket, exit),
+  state: State(socket, inject, input_output, exit),
   control: Control,
-) -> actor.Next(State(socket, exit), Control) {
+) -> actor.Next(State(socket, inject, input_output, exit), Control) {
   case control {
+    control.Search -> {
+      process.send(state.inject_input, key.Continue([key.input_introducer]))
+      actor.continue(state)
+    }
     control.TogglePlayPause -> {
       echo "toggling play/pause"
 
@@ -65,14 +96,28 @@ fn handle_message(
   }
 }
 
-fn read_input(subject: Subject(Control)) -> Nil {
+/// `read_input` operates by reading from input until a `Key` can be created.
+/// It is possible to create a `Key` without the users input by sending
+/// messages to `inject_input` which will initialize the "input to key" sequence.
+/// This is useful to ultimately create a `Control` without the user having to
+/// input all of the character(s) needed.
+fn read_input(
+  agent: Subject(Control),
+  inject_input: Subject(Key),
+  tap_input: Subject(List(String)),
+) -> Nil {
+  let buffer = case process.receive(inject_input, 1) {
+    Ok(key.Continue(buffer)) -> buffer
+    Ok(_) | Error(_) -> []
+  }
+
   case
-    key.read_input_until_key([])
+    key.read_input_until_key(buffer, tap_input)
     |> control.from_key
   {
     Error(_) -> Nil
-    Ok(control) -> process.send(subject, control)
+    Ok(control) -> process.send(agent, control)
   }
 
-  read_input(subject)
+  read_input(agent, inject_input, tap_input)
 }

test/input/input_test.gleam

@@ -1,7 +1,7 @@
 import gleam/list
 import gleeunit
 
-import input/key.{type Key, Char, csi, esc}
+import input/key.{type Key, Char, csi, esc, input_introducer as ii}
 
 pub fn main() -> Nil {
   gleeunit.main()
@@ -27,10 +27,10 @@ pub fn key_from_list_test() {
   ]
 
   let input_tests = [
-    TestCase(["$"], key.Continue(["$"])),
+    TestCase([ii], key.Continue([ii])),
-    TestCase(["$", "a"], key.Continue(["$", "a"])),
+    TestCase([ii, "a"], key.Continue([ii, "a"])),
-    TestCase(["$", "a", "b"], key.Continue(["$", "ab"])),
+    TestCase([ii, "a", "b"], key.Continue([ii, "ab"])),
-    TestCase(["$", "ab", "\r"], key.Input("ab")),
+    TestCase([ii, "ab", "\r"], key.Input("ab")),
   ]
 
   let test_cases = [base_tests, char_tests, escape_tests, input_tests]