Improving efficiency & compatibility of traverse implementation for b…

…oxed vectors. (gh pr #221, fixes bug #220)

previously traverse used unknown size fromList rather fromListN for constructing Boxed vectors.
In the presence of compact regions the implementation strategy for fromList results in program crashes. Now traverse on Boxed vectors uses the input vector size for constructing the result vector.

Data/Vector.hs

@@ -425,7 +425,12 @@ instance Foldable.Foldable Vector where
 
 instance Traversable.Traversable Vector where
   {-# INLINE traverse #-}
-  traverse f xs = Data.Vector.fromList Applicative.<$> Traversable.traverse f (toList xs)
+  traverse f xs =
+      -- Get the length of the vector in /O(1)/ time
+      let !n = G.length xs
+      -- Use fromListN to be more efficient in construction of resulting vector
+      -- Also behaves better with compact regions, preventing runtime exceptions
+      in  Data.Vector.fromListN n Applicative.<$> Traversable.traverse f (toList xs)
 
   {-# INLINE mapM #-}
   mapM = mapM