I once made a prototype lisp interpreter where you could call previous versions of definitions. A fun experiment, but I didn't carry it further.

Unison is a language/platform that incorporates some ideas from version control, but I don't know how far along they are.

Yeah, I'm doing that. :)

I've got this pure functional language and rather than deal with impure IO I just assume a git repo as a data oracle and then file and string operations are all nice and pure!

Instead of regular IO you operate on 3-tuples of (hash, offset, length) where the hash is the git hash of a data blob, and offset and length specify a subset of the bytes in it.

Conceptually these 3-tuples represent lists of ints but the runtime is free to do whatever is most efficient in terms of representing and processing them "under the hood".

slightly relevant: https://thedailywtf.com/articles/the-inner-json-effect