7.10.15. STRUDEL-15 — Live-Reloading Patterns

This is the killer feature. daslang-live.exe compiles your script, runs it, then watches the file on disk — every save triggers a recompile and a hot-reload of the new code without stopping the audio stream. Edit a pattern, save, hear the change. No process restart, no decoder cost, no audio glitch.

Run this tutorial under the live host:

bin/Release/daslang-live.exe tutorials/daStrudel/daStrudel_15_live_reloading.das

It also runs under plain daslang.exe — the file works in both hosts. In standalone mode there is no hot-reload and the lifecycle collapses to main().

7.10.15.1. Part A: Lifecycle Functions

Scripts that export init / update / shutdown run in lifecycle mode under daslang-live:

[export] def init() { ... }      // called on start and after every reload
[export] def update() { ... }    // called every frame
[export] def shutdown() { ... }  // called on exit and before every reload

The audio thread and any OS resources (windows, sample banks) live outside the reload scope — only the daScript side recompiles.

7.10.15.2. Part B: Preserving Player State

require strudel/strudel_live is all it takes to keep the strudel player state alive across reloads. The module registers [before_reload] and [after_reload] hooks that serialise the player (tracks, SID, BPM, timing) and the loaded SF2 data into the persistent byte store:

require strudel/strudel public
require strudel/strudel_player
require strudel/strudel_live   // auto-persists player state
require live_host

def strudel_main() {
    strudel_add_track(s("bd [hh hh] sd [hh cp]"))
    strudel_set_bpm(120.0lf)
    strudel_play()
}

[export] def init() {
    load_samples()
    strudel_init(@@strudel_main)
}

Edit the pattern inside strudel_main and save — the audio system keeps rolling, strudel_init runs again, and the new pattern takes over on the next cycle boundary. The SID (sound identifier) stays the same, so external audio routing is unaffected.

7.10.15.3. Part C: Live Commands (REST API)

[live_command] annotations expose daScript functions over HTTP. daslang-live starts a REST server on port 9090; any external tool can drive the running script via POST /command:

require live/live_commands
require daslib/json_boost

struct CmdBpmArgs {
    bpm : double = 120.0lf
}

[live_command(description="Set tempo in BPM")]
def cmd_set_bpm(input : JsonValue?) : JsonValue? {
    let args = from_JV(input, type<CmdBpmArgs>)
    strudel_set_bpm(args.bpm)
    return JV("bpm set to {args.bpm}")
}

Call it from the shell:

curl -X POST http://localhost:9090/command \
  -d '{"name":"cmd_set_bpm","args":{"bpm":140}}'

The MCP server exposes these commands through the live_command tool so AI assistants can steer a running strudel patch directly. See Annotations and Options for annotation syntax.

7.10.15.4. Part D: Custom Persistent State

strudel_live handles the player. Your own state needs its own hooks — use [before_reload] / [after_reload] and the persistent byte store to keep globals alive:

var g_reload_count : int = 0

[before_reload]
def save_my_state() {
    var data : array<uint8>
    data |> resize(4)
    unsafe { *reinterpret<int?>(addr(data[0])) = g_reload_count; }
    live_store_bytes("tutorial15_reload_count", data)
}

[after_reload]
def restore_my_state() {
    var data : array<uint8>
    if (live_load_bytes("tutorial15_reload_count", data) && length(data) >= 4) {
        unsafe { g_reload_count = *reinterpret<int?>(addr(data[0])); }
        g_reload_count ++
    }
}

The store takes arbitrary byte arrays keyed by string. For structured data, serialise with daslib/archive (which is how strudel_live preserves the player itself).

7.10.15.5. Workflow Tips

Save the file to reload — the file-watcher does the rest.
Hit POST /reload for a manual reload.
POST /reload/full does a deeper recompilation if module-level structure changed.
is_live_mode() returns true under daslang-live, false under daslang.exe — branch on it for dual-mode scripts.
A compile error freezes the running version — fix, save again, and the new code takes over. The audio never stops.