The vector package offers createT :: Traversable t => (forall s. ST s (t (MVector s a))) -> t (Vector a) This is a generalization of create, which is very similar to runSTArray in the array package. I suggest we add functions runSTArrays :: Traversable t => (forall s. ST s (t (STArray s i e))) -> t (Array i e) runSTUArrays :: Traversable t => (forall s. ST s (t (STUArray s i e))) -> t (UArray i e) Why do I think it's worth the trouble? While it's occasionally useful to create multiple arrays of the same type in one go, I think the Maybe and (a,) Traversable instances, and their compositions, are likely more important. I can use (a,) to record some extra information while building an array. I can use Maybe to give up and not produce an array. And the compositions let me do both in different ways.