writeChunk and writeChunkNonBlock are not finished?

I ran into a runtime error today because the definition of writeChunk and writeChunkNonBlocking look unfinished. However, I don't know from the TODO what they are supposed to do.

 867 writeChunk :: Handle__ -> Ptr Word8 -> Int -> IO ()                        
 868 writeChunk h_@Handle__{..} ptr bytes                                       
 869   | Just fd <- cast haDevice  =  RawIO.write (fd::FD) ptr bytes            
 870   | otherwise = error "Todo: hPutBuf"                                      
 871                                                                            
 872 writeChunkNonBlocking :: Handle__ -> Ptr Word8 -> Int -> IO Int            
 873 writeChunkNonBlocking h_@Handle__{..} ptr bytes                            
 874   | Just fd <- cast haDevice  =  RawIO.writeNonBlocking (fd::FD) ptr bytes 
 875   | otherwise = error "Todo: hPutBuf"