appease our forefathers

Author: freckletonj

dunai/src/Data/MonadicStreamFunction/Instances/Profunctor.hs

@@ -11,7 +11,6 @@ import Control.Arrow
 
 #if !MIN_VERSION_base(4,8,0)
 import Data.Traversable (traverse)
-import Control.Applicative (pure)
 #endif
 
 -- profunctors
@@ -35,11 +34,11 @@ instance Monad m => Strong (MSF m) where
 instance Monad m => Profunctor (MSF m) where
   dimap l r p = arr l >>> p >>> arr r
 
-instance (Monad m) => Traversing (MSF m) where
+instance (Functor m, Applicative m, Monad m) => Traversing (MSF m) where
   traverse' (MSF msf) =
     let unMSF' x = StateT (\(MSF m) -> m x)
     in MSF $ \xs -> do
       -- `StateT (MSF m a b) m a` is isomorphic to `a -> m (b, MSF m a b)`, and
       -- `StateT`'s Applicative instance is needed for writing this traversal
       (fb, msf') <- runStateT (traverse unMSF' xs) (MSF msf)
-      pure (fb, traverse' msf')
+      return (fb, traverse' msf')