This is a little weird:

Prelude> :m + Data.Array.Unboxed Prelude Data.Array.Unboxed> let f () = array (1, 5) [] :: UArray Int Int Prelude Data.Array.Unboxed> f () == f () False

Are we giving away purity for efficiency? ;) If this behaviour is intended, maybe it should be documented?

Maybe it would be a good idea to define default values for types which can be unboxed?

That's a very good point :) Fixed now, thanks. Cheers, Simon