I would like to create a data structure that uses an unboxed array as one of its components. I would like the data structure to be parameterized over the type of the elements of the array. Further, I'd like to build the array using runSTUArray. I can't make the code work though. My naive approach: data Ring a = Ring (UArray Int a) and the code to make the array: makeArray :: [a] -> Ring a makeArray ls = Ring (ST.runSTUArray (ST.newListArray (0, length ls - 1) ls)) But that doesn't work. I get from GHC (6.8.1): Could not deduce (MArray (STUArray s) a (ST s)) from the context () arising from a use of `newListArray' I am pretty sure I need to constrain 'a' to primitive types only, but how? Can I do it in the data definition? Thanks in advance for any help! Justin