[monoideism]

He was working in OCaml, which even now isn't fortunate to have a persistent O(1) hashmap like Clojure (and Scala, I think) does in the standard library.

It's true that there are a few 3rd party implementations that I've noticed before, but not sure how robust and well-tested these are. I'd hesitate to use them or something I wrote in production without careful review.

In any case, persistent HAMTs are a relatively new phenomenon that are just starting to catch on, and this story is described as being "back in the day". It's unlikely this option was available to him even if he were willing to write an implementation himself or use an untested 3rd-party implementation.

Maybe one will be added eventually to one of the OCaml standard libraries, which would be great.