It's currently hard to describe that a type should be usable with UArray or IOUArray, and almost impossible to describe without QualifiedConstraints that it should be usable with STUArray. And it's actually impossible to use GeneralizedNewtypeDeriving to derive new instances of IArray or MArray. It also means that it's difficult to keep the three sets of instances in sync. Therefore, there should be (taking inspiration from primitive) a class that describes types which can be used in unboxed arrays. It'd be something along the lines of: class Unboxed e where unsafeNewUArray :: Int# -> e -> State# s -> (# State# s, MutableByteArray# s #) unsafeNewUArray_ :: Const (Int# -> State# s -> (# State# s, MutableByteArray# s #)) e unsafeReadUArray :: MutableByteArray# s -> Int# -> State# s -> (# State# s, e #) unsafeWriteUArray :: MutableByteArray# s -> Int# -> e -> State# s -> State# s unsafeIndexUArray :: ByteArray# -> Int# -> e And then the instances for UArray, STUArray, and IOUArray would be: instance Unboxed e => IArray UArray e where ... instance Unboxed e => MArray IOUArray e IO where ... instance Unboxed e => MArray (STUArray s) e (ST s) where ... Incidentally, this also means it'd be less effort to add instances like Unboxed CInt, Unboxed a => Unboxed (Complex a), and Unboxed Ordering.