fix type issue

Author: Rewbert

lib/Primitives.hs

@@ -389,10 +389,10 @@ primTryPutMVar = _primitive "IO.tryputmvar"
 primTryReadMVar :: MVar a -> IO b {-(Maybe a)-}
 primTryReadMVar = _primitive "IO.tryreadmvar"
 
-primWaitWriteFD :: Int -> IO ()
+primWaitWriteFD :: Int -> IO Int
 primWaitWriteFD = _primitive "IO.waitwrfd"
 
-primWaitReadFD :: Int -> IO ()
+primWaitReadFD :: Int -> IO Int
 primWaitReadFD = _primitive "IO.waitrdfd"
 
 primThreadDelay :: Int -> IO ()