Yes, we have a way of hashing literally all values in the language, including arbitrary data types, functions, continuations, etc. For instance, here, I'm hashing a lambda function:[1]

    > crypto.hash Sha3_256 (x -> x + 1)
    ⧩
    0xs704e9cc41e9aa0beb70432cff0038753d07ebb7f5b4de236a7a0a53eec3fdbb5

We only do this caching for pure tests (which are deterministic and don't need to be re-run over and over), enforced by the type system. You can have regular I/O tests as well, and these are run every time. Projects typically have a mix of both kinds of tests.

It is true that you can only hash things which are "closed" / have no free variables. You might instead hash a function which takes its free variables as parameters.

Overall I think Unison would be a nice implementation language for really anything that needs to make interesting use of hashing, since it's just there and always available.

[1]: https://share.unison-lang.org/@unison/base/code/releases/7.4... [2]: https://share.unison-lang.org/@unison/base/code/releases/7.4...