Platformer — live coded

Inspired by Bret Victor's Inventing on Principle (2012) · see also worrydream.com.

A 2D platformer whose entire logic lives in the editor below. Edit JUMP-V, GRAVITY, platforms, or step itself — it takes effect next frame without resetting the player. Drag the timeline to rewind; while rewound, a faint yellow arc shows the predicted replay from that point using your recorded keypresses. Now edit a constant — the arc redraws to show what would have happened with the new value. Hold Alt (or Ctrl) and drag any number in the source for a live scrubber.

←/A + →/D move · ↑/W/Space jump · R respawn

What's happening

• (defonce state ...) creates the world state atom exactly once. Re-evaluating the source doesn't touch it.
• (defonce history ...) is a bounded vector of past states — one per frame, last 300. The scrubber reads this to rewind.
• (defn predict ...) runs step forward 45 times from whatever the current display state is, returning [x1 y1 x2 y2 ...]. The JS draws those as dots. Because step is pure and reads live JUMP-V/GRAVITY, editing a constant redraws the dots instantly.
• Every user-facing behavior — physics, gameplay, history, prediction — is Clojure data flowing through a pure function. No hidden state, no hot-reload magic.