Fix some warnings.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

Database/MongoDB/Query.hs

@@ -561,7 +561,7 @@ newCursor :: (MonadIO m, MonadBaseControl IO m) => Database -> Collection -> Bat
 newCursor db col batchSize dBatch = do
 	var <- newMVar dBatch
 	let cursor = Cursor (db <.> col) batchSize var
-	mkWeakMVar var (closeCursor cursor)
+	_ <- mkWeakMVar var (closeCursor cursor)
 	return cursor
 #if !MIN_VERSION_base(4,6,0)
   where mkWeakMVar = addMVarFinalizer

System/IO/Pipeline.hs

@@ -2,7 +2,7 @@
 
 A pipeline closes itself when a read or write causes an error, so you can detect a broken pipeline by checking isClosed.  It also closes itself when garbage collected, or you can close it explicitly. -}
 
-{-# LANGUAGE DoRec, RecordWildCards, NamedFieldPuns, ScopedTypeVariables #-}
+{-# LANGUAGE RecursiveDo, RecordWildCards, NamedFieldPuns, ScopedTypeVariables #-}
 {-# LANGUAGE CPP #-}
 
 module System.IO.Pipeline (
@@ -69,7 +69,7 @@ newPipeline stream = do
 	rec
 		let pipe = Pipeline{..}
 		listenThread <- forkIO (listen pipe)
-	mkWeakMVar vStream $ do
+	_ <- mkWeakMVar vStream $ do
 		killThread listenThread
 		closeStream stream
 	return pipe