An issue with many languages is the presence of mutually dependant parts of code, because how do you generate a content-hash for something whose constituent part's content-hashes depend on it? Merkle trees can be generalized to DAGs, but not directed graphs with cycles.

As far as I understand, Unison's syntax has been designed specifically to avoid such cycles, although it supports recursive functions and inductive data types which refer to themselves such as lists. A self-cycle can be handled, but I'm not sure if, and if so how, it would support mutually recursive functions or data types. I suspect they're not supported for the reason that content-hashing becomes non-trivial.

If you wanted to generalize the solution to arbitrary languages, you would need to somehow sever the cycles, which is likely to require a specific solution for each language, or come up with another solution to content-hashing which does not have the problem of Merkle-DAGs. Either way, you are looking at something non-trivial compared to having a language which simply forbids the cycles.

kind of. For example, in Java, build tools will normally cache the hash of the source file that generated a class file. That class file is a cached version of the source, basically. IF you could keep both the class file and the hashes of the source around, you could avoid ever re-compiling the source unless you've changed it. This is just incremental compilation, in essence.

However, you'll need quite a lot of tooling around this simple idea: where to store the class files and the hashes, should you commit it into source control, how to distribute the cache, can this be made multi-platform so it works on all developers' machines?

All solvable, but Unison already does all that, and more, with its single tool, the `ucm`.

This is one of things Bazel tries to do.