2 files changed, 4 insertions, 1 deletions

diff --git a/lib/Data/Packed/Internal/Vector.hs b/lib/Data/Packed/Internal/Vector.hs
index 7360f93..ba68909 100644
--- a/lib/Data/Packed/Internal/Vector.hs
+++ b/lib/Data/Packed/Internal/Vector.hs
@@ -387,6 +387,7 @@ foldVectorG f s0 v = foldLoop g s0 (dim v)
 -------------------------------------------------------------------
 
 -- | monadic map over Vectors
+--    the monad @m@ must be strict
 mapVectorM :: (Storable a, Storable b, Monad m) => (a -> m b) -> Vector a -> m (Vector b)
 mapVectorM f v = do
     w <- return $! unsafePerformIO $! createVector (dim v)
@@ -419,6 +420,7 @@ mapVectorM_ f v = do
 {-# INLINE mapVectorM_ #-}
 
 -- | monadic map over Vectors with the zero-indexed index passed to the mapping function
+--    the monad @m@ must be strict
 mapVectorWithIndexM :: (Storable a, Storable b, Monad m) => (Int -> a -> m b) -> Vector a -> m (Vector b)
 mapVectorWithIndexM f v = do
     w <- return $! unsafePerformIO $! createVector (dim v)
diff --git a/lib/Data/Packed/Vector.hs b/lib/Data/Packed/Vector.hs
index 23fe37f..e79e237 100644
--- a/lib/Data/Packed/Vector.hs
+++ b/lib/Data/Packed/Vector.hs
@@ -99,7 +99,8 @@ state_put :: s -> State s ()
 state_put s = State $ \_ -> ((),s)
 
 evalState :: State s a -> s -> a
-evalState m s = fst $ runState m s
+evalState m s = let (a,s') = runState m s
+                in seq s' a
 
 newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }