fix correctness issue with swap2 that was making QuickSort fail on prim-mutable-arrays

Author: michaelborkowski

src/ArrayOperations.hs

@@ -66,10 +66,10 @@ swap xs i j = let !xi   = get xs i
                               left xs == left ys && right xs == right ys &&
                               ys == swap xs i j } @-}
 swap2 :: HasPrim a => Int -> Int -> (Array a -. Array a)
-swap2 i j xs =
+swap2 i j xs = 
   let
-    !(Ur xi, xs1) = get2 i xs
-    !(Ur xj, xs2) = get2 j xs1
+    !((Ur !xi), xs1) = get2 i xs
+    !((Ur !xj), xs2) = get2 j xs1
     xs3 = setLin i xj xs2
     xs4 = setLin j xi xs3
   in {- xi `pseq` xj `pseq` -} xs4