Never Mutate What Others Can See: Persistent and Immutable Data Structures

Deep End Era. Engineering with Sankalp · Oct 28, 2026 · 50m

In Edinburgh, a floor-plan editor's undo button summons a staircase nobody drew. In part fifteen of The Deep End's Data Structures series, team lead Priya Raman shows new engineer Oskar Lindqvist how to stop writing undo at all: keep every version, and let each new version share almost everything with the last. From a 1989 paper to Phil Bagwell's hash tries, Rich Hickey's Clojure and the inside of git, this is how immutable data became fast. Priya and Oskar are fictional; the structures, papers and tools are real.

Chapters

00:00:00 Cold open: the undo button that ate the afternoon

00:02:38…