There's more to a function than just types. It's not sufficient to know that the function outputs a baz 42.0. You have to understand which one. The oldest? The latest? The one that matches the foo and bar input parameters?

I think that's the part where it remains difficult. Someone has to convey clearly what the semantics and side effects of the function are. Consumers have to read and understand it. Failing that, you get breakage.