On Sat, Nov 10, 2007 at 11:09:54AM -0800, Justin Bailey wrote:

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!

What you're trying to do deliberately can't be done. Polymorphism has a runtime cost in GHC, and UArrays must specialize. That said, have you heard of Data.Array.IArray.listArray? Stefan