Does Unison have stack traces? Using the hash of the ast as the only identifier seems like it would lose some useful runtime debugging information.

While hashes are the primary identifier, things also generally have names associated with them. (Unless you deliberately remove the names that is.)

Sounds oddly much like git. Git-commits are identified by hashes.

And git commits "are" (versions of) programs.

So what does Unison have that git does not?

Mostly, it heavily normalizes code, to the point where a name change will not register as a change to the program.

Yes, we have stack traces. You might see some hashes in the middle of the stack traces, but whenever we do have a name, we'll show you the name. So if you are calling into an anonymous clousure, but functions in your call stack will typically have names available