So you can fix it for example by using a specialized version of freezeArr inside accumArray, of type (Ix i, IArray a e) => STArray s i e -> ST s (a i e) This will give quite general type of accumArray: arbitrary immutable array from the IArray class.
freezeArr2:: (Ix i, IArray.IArray a e) => STArray s i e -> ST s (a i e) freezeArr2 = MArray.freeze
If the immutable array type used was particularly UArray, it would be more efficient to use the corresponding STUArray instead of STArray, so freezing could just copy a memory block (there are magic specializations in ghc's libraries for such case). But if the element type was to remain generic, the type would have to be constrained over STUArray
freezeArr3:: (Ix i, MArray.MArray (MArray.STUArray s) e (ST s), IArray.IArray a e) => (MArray.STUArray s i e) -> ST s (a i e) freezeArr3 = MArray.freeze Is this the way to do it? I wonder a bit if this triggers the optimization if 'a' is of type STUArray. (By the way I still have problems to see where unboxed values are a good thing and were you should better avoid them.) In addition I wonder why there are so many array types. Are both STArray and IOArray really necessary and what's the difference between them? What about DiffArray? I'm still surprised by operations like 'unsafeFreezeArray' and 'freezeArray'. Shouldn't the garbage collector juge if there are still other references to a distinct object? Regards, Thomas