STUArray with polymorphic type

Question

I want to implement an algorithm using the ST monad and STUArrays, and I want it to be able to work with both Float and Double

Answer 1

So here's the workaround I'm going with for now - creating a new typeclass for types for which (forall s. MArray (STUArray s) a (ST s)):

class IArray UArray a => Unboxed a where
  newSTUArray :: Ix i => (i, i) -> a -> ST s (STUArray s i a)
  readSTUArray :: Ix i => STUArray s i a -> i -> ST s a
  writeSTUArray :: Ix i => STUArray s i a -> i -> a -> ST s ()

instance Unboxed Float where
  newSTUArray = newArray
  readSTUArray = readArray
  writeSTUArray = writeArray

instance Unboxed Double where
  newSTUArray = newArray
  readSTUArray = readArray
  writeSTUArray = writeArray

While I'm not perfectly satisfied with this, I prefer it on rules because:

• rules depend on optimizations
• rules are not really supposed to change the algorithm (?). where in this case they would as boxed arrays have very different behaviour regarding lazyness etc.