Interleave effects better in chunksOf

Generalize the type of `chunksOf` to allow the inner and outer
streams of the result to use different underlying monads. This
forces effects to be handled within the inner monad, leading
to an arguably better sort of chunking.

Fixes #45

Author: Torgeir Strand Henriksen

src/Streaming/Internal.hs

@@ -722,12 +722,14 @@ takes n = void . splitsAt n
 2
 1
 -}
-chunksOf :: (Monad m, Functor f) => Int -> Stream f m r -> Stream (Stream f m) m r
-chunksOf n0 = loop where
-  loop stream = case stream of
-    Return r  -> Return r
-    Effect m  -> Effect (fmap loop m)
-    Step fs   -> Step (Step (fmap (fmap loop . splitsAt (n0-1)) fs))
+chunksOf :: (Functor f, Monad m, Monad n) => Int -> Stream f m r -> Stream (Stream f m) n r
+chunksOf = go
+  where
+    go !n s = do
+      stage <- yields $ lift . inspect =<< splitsAt n s
+      case stage of
+        Left r -> pure r
+        Right s' -> go n $ wrap s'
 {-# INLINABLE chunksOf #-}        
 
 {- | Make it possible to \'run\' the underlying transformed monad.