[jfengel]

You should avoid "any" like the plague. I strongly recommend putting that into your linter.

Unknowns are also a code smell. I'm not sure they're ever necessary, but they can save a lot of grief when fighting with the type system isn't the best use of your time.

As for exceptions... I agree, but a lot of programmers hate checked exceptions.

I will add one beef: fetch should return unknown rather than any.