It's hard to write something that's generic over a uniqueness type and a normal type, which is indeed a bit of a pain. It's not completely impossible: we have a kind 'Type*' which covers both unique and normal types, which can be used to make some polymorphic functions which preserve uniqueness, but that's all at the minute.

On the other hand, I see them as something you use when you want more precise control over memory management, control of in-place update and so on.

As ever, there's a trade off between efficiency and abstraction, which is where the partial evaluator comes in. This is still experimental and we're working on it, but I'm wondering if we can use partial evaluation to turn nicely abstract high level code into something efficient based on uniqueness types at run time.