play statements. The interpreter evaluates the cell and renders each play to audio, with a grid of every hit and note, or the same as sheet music.
This is What Happens Now? by Porcupine Tree. The play holds two blocks in brackets, so they play in order: an accented rim as a pickup, and after it the groove with a crash in its first bar. Each line of groove repeats at its own length: 7 steps for the kick and rim, 3 for the ride bell, 10 for the guitar. They only meet again after 15 bars, and play for 3 bars stops it after three.
Why instructions
- You state the rules, the interpreter writes the bars. Two lines of different lengths make a polyrhythm that would take pages to write out, and you hear where they meet without working it out.
- Every edit is a new take. Change a step, a length or the time signature, evaluate, and listen.
- Parts are values. A groove is named once and reused under every section, a song is its sections in brackets, in order, and a cell is source you can diff, version and generate from Python.
Run it
- Playground. Open composer-nb.vercel.app, select a cell and press Shift+Enter. Nothing to install.
- Jupyter.
pip install composer-nb. See Jupyter. - An assistant. Connect Claude or Cursor to the MCP server and ask for a groove in words. See Assistants.
The reference
Language covers what every cell has: settings, names and types, blocks, patterns andplay. The drums and the pitched instruments are two separate parts, and a cell can use either or both. Drums covers the kit pieces and their steps type. Pitched instruments covers the instruments, the key, and their notes and chords types.