don't force function value, but make accumulation strict. add docs to foldMap

Author: chessai

src/Streaming/Prelude.hs

@@ -1383,23 +1383,31 @@ mappedPost :: (Monad m, Functor g) => (forall x . f x -> m (g x)) -> Stream f m
 mappedPost = mapsMPost
 {-# INLINE mappedPost #-}
 
-{-| Fold streamed items into their monoidal sum
+{-| Map each element of the stream to a monoid, and take the monoidal sum of the results.
 
->>> S.mconcat $ S.take 2 $ S.map (Data.Monoid.Last . Just) (S.stdinLn)
-first<Enter>
-last<Enter>
-Last {getLast = Just "last"} :> ()
+>>> S.foldMap Sum $ S.take 2 (S.stdinLn)
+1<Enter>
+2<Enter>
+3<Enter>
+Sum {getSum = 6} :> ()
 
  -}
-
 foldMap :: (Monad m, Monoid w) => (a -> w) -> Stream (Of a) m r -> m (Of w r)
-foldMap f = fold (\acc a -> let !fa = f $! a in mappend acc fa) mempty id
+foldMap f = fold (\ !acc a -> mappend acc (f a)) mempty id
 {-# INLINE foldMap #-}
 
 foldMap_ :: (Monad m, Monoid w) => (a -> w) -> Stream (Of a) m r -> m w
-foldMap_ f = fold_ (\acc a -> let !fa = f $! a in mappend acc fa) mempty id
+foldMap_ f = fold_ (\ !acc a -> mappend acc (f a)) mempty id
 {-# INLINE foldMap_ #-}
 
+{-| Fold streamed items into their monoidal sum
+
+>>> S.mconcat $ S.take 2 $ S.map (Data.Monoid.Last . Just) (S.stdinLn)
+first<Enter>
+last<Enter>
+Last {getLast = Just "last"} :> ()
+
+ -}
 mconcat :: (Monad m, Monoid w) => Stream (Of w) m r -> m (Of w r)
 mconcat = fold mappend mempty id
 {-# INLINE mconcat #-}