barline

A song in barline is not a project file — it is a TypeScript program that imports @barline/core. for, if, classes, and your own helpers all work. The library gives you pattern primitives and a runtime; you bring the composition.

import { r, at, backbeat, notes, beats, bars, fx } from "@barline/core";
import type { Song } from "@barline/runtime";

export default function (song: Song) {
  const kick = song.track({
    name: "kick",
    output: { kind: "synth", synth: { id: "kick" } },
    effects: [fx.distortion({ drive: 0.35, wet: 0.6 }), fx.limiter({ threshold: -2 })],
  });
  const hats = song.track({
    name: "hats",
    output: { kind: "synth", synth: { id: "hat", params: { volume: -12 } } },
  });
  const rumble = song.track({
    name: "rumble",
    output: { kind: "synth", synth: { id: "bass", params: { volume: -6 } } },
    effects: [fx.sidechain({ source: "kick", amount: 0.8 })], // the pump
  });

  // INTRO — a filtered heartbeat. Edit while it plays; changes land next bar.
  song.section("intro", bars(16), () => {
    kick.play(r`x . . . x . . . x . . . x . . .`).gain(0.9);
    hats.play(at([2, 6, 10, 14])).gain(0.7);
    rumble.play(notes(["F1", null, "F1", "Ab1"], beats(0.5))).gain(0.5);
  });

  song.arrange([{ section: "intro" }]);
}

Type that, press play, and a 148 BPM techno loop comes out of the browser. Drag a note in the piano roll and the code updates. Ask Claude to make the drop harder and it edits the same document you are editing. Nothing is config.

It's real TypeScript

Imports, classes, loops, third-party npm packages. Pattern is a three-member interface — your own class is as first-class as r or euclidean.

Code is the source of truth

Timeline, beat grid, and piano roll are projections of the AST, never independent state. Edit either side; both update.

Performance is flags, not fast typing

MIDI pads and knobs bind to typed flags. Section code branches on them. The controller is the performance surface; the code is the composition.

The AI hears the output

An MCP-connected model edits the song structurally and reads a live audio-feature stream: change → render → listen → iterate.

The Studio

Every canvas is a different projection of the same executable project:

Compose

Code-forward composition, Arrangement and Clip Detail, one canonical musical selection, bounded visual write-back, and Source Lens context.

Sound

Instruments, effects, mixer, managed audio, recording, vocals, references, stems, and audio I/O.

Perform

Session scenes, typed flags, transport, live mixing, MIDI input/output/clock, and external-instrument returns.

Publish

Production Proof and Musical Change Requests, snapshots, branches, registry packages, publication, and remix lineage.

The AI collaborator

The external MCP collaborator can read and edit the complete project, work from the current Source Lens selection, discover devices/presets/assets, control transport and mixer state, inspect Audio Session health, render and measure, manage history and branches, and compose the same Production Proof used by the Studio. Start with MCP setup and the complete tool reference.