A git-like datastore for your AST+callgraph.
Let's say you have definitions like that:
f: Nat -> Nat
g: Nat -> Nat
h: Nat -> Nat
h x = g (x * 2)
g x = f (x * 3)
f x = x < 0 ? 1 : h (x / 4)
f x = x < 1 ? 1 : h (x / 4)
EDIT: nevermind, recursion is a special case handled differently.
when you change a function implementation the system has to walk the callers graph backwards starting from all the places where the function was called updating all the implementations with the new hash, then callers of these with the new implementation and so on up to main (or whatever it's called).
I had a chance to implement something like this in a system that used jbpm 3 graph language (basically process X version 1 called process Y version 1 and I updated process Y to version 2). It's nontrivial especially with recursion, I'm wondering how they are dealing with that.