light refactor

Author: freckletonj

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

@@ -1,5 +1,6 @@
 {-# LANGUAGE CPP                 #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# OPTIONS_GHC -ddump-simpl #-}
 
 -- | Instances of 'Profunctor' and 'Traversing' for Monadic Stream Functions ('MSF').
 --
@@ -35,11 +36,12 @@ instance Monad m => Strong (MSF m) where
 instance Monad m => Profunctor (MSF m) where
   dimap l r p = arr l >>> p >>> arr r
 
+-- | This 'Traversing' instance will step the internal state of the 'MSF' once
+-- for every element of the input 'Traversable'.
 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
+  traverse' msf =
+    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)
+      (fb, msf') <- runStateT (traverse (StateT . flip unMSF) xs) msf
       return (fb, traverse' msf')