Sequencer — live coded

In the spirit of TidalCycles and Sonic Pi: rhythms and melodies are vectors of data in Clojure. Edit the source, the grid and the drum machine update. Companion to the subtractive synth demo.

A 16-step, 4-track step sequencer. Three percussion tracks (kick / snare / hat) plus a melody line whose pitches come from the cljrs.music library — try changing the scale from :minor to :dorian or :pentatonic and watch the melody re-harmonize on the next bar. Web Audio handles sample-accurate scheduling; the cljrs runtime owns the score, and the entire grid below is rendered by cljrs.ui from the source.

Hold Alt (or Ctrl) and drag any number — try BPM — to scrub it live. Press Cmd/Ctrl+Enter to apply immediately. Click a cell to toggle; click Randomize melody to regenerate the melody from the current scale.

What's happening

• The Clojure source defines the score (BPM + four 16-element vectors) and the entire control panel as a cljrs.ui hiccup tree. Editing the file re-evals it, then (boot) re-mounts the panel — Preact diffs against the live DOM.
• Click any kick/snare/hat cell to toggle that step; Randomize melody picks fresh notes from the current (cljrs.music/scale ROOT SCALE) using a deterministic LCG (no rand in cljrs yet). The kick/snare/hat randomize buttons each have their own density default.
• Web Audio scheduling: a setInterval-driven lookahead walks the next ~100 ms of steps and queues drum hits / oscillator notes with audioCtx.currentTime, the standard Chris Wilson scheduling pattern. The cljrs interpreter is fast enough for per-frame logic but sample-accurate timing belongs in the audio thread.
• The playhead column is just (reset! STEP n) from JS once per audio step — cljrs.ui re-renders to highlight the current column.