IO inside Get Monad

Question

So my problem is this. I am trying to implement a streaming parser for RDB files (dump files that Redis produces). I want to implement a function similar to mapM_, in which I can, say, print each object represented in the dump file as it is parsed. However, I cannot get it to work in a constant space. I find that what happens is that I create a large IO () thunk inside the Get monad, returning from the Get monad, and then do the IO. Is there anyway to pass my objects when they are parsed for printing and then discarded? I tried counters and channels, but I did not see any real benefits. Here is what I still have:

loadObjs_ :: (Monad m) => (Maybe Integer -> BL8.ByteString -> RDBObj -> Get (m a)) -> Get (m a)
loadObjs_ f = do
             code <- lookAhead getWord8
             case code of
                0xfd -> do
                 skip 1
                 expire <- loadTime
                 getPairs_ f (Just expire)
               0xfc -> do
                 skip 1
                 expire <- loadTimeMs
                 getPairs_ f (Just expire)
               0xfe -> f Nothing "Switching Database" RDBNull
               0xff -> f Nothing "" RDBNull
               _ -> getPairs_ f Nothing

getPairs_ :: (Monad m) => (Maybe Integer -> BL8.ByteString -> RDBObj -> Get (m a)) -> Maybe Integer -> Get (m a)
getPairs_ f ex = do
                !t <- getWord8
                !key <- loadStringObj False
                !obj <- loadObj t
                !rest <- loadObjs_ f
                !out <- f ex key obj
                return (out >> rest)


(loadObj does the actual parsing of a single object but I believe that whatever I need to fix the streaming to operate in constant or near-constant memory is at a higher level in the iteration than loadObj)

getDBs_ :: (Monad m) => (Maybe Integer -> BL8.ByteString -> RDBObj -> Get (m a)) -> Get (m a)
getDBs_ f = do
           opc <- lookAhead getWord8
           if opc == opcodeSelectdb
              then do
                  skip 1
                  (isEncType,dbnum) <- loadLen
                  objs <- loadObjs_ f
                  rest <- getDBs_ f
                  return (objs >> rest)
              else f Nothing "EOF" RDBNull

processRDB_ :: (Monad m) => (Maybe Integer -> BL8.ByteString -> RDBObj -> Get (m a)) -> Get (m a)
processRDB_ f = do
                header <- getBytes 9
                dbs <- getDBs_ f
                eof <- getWord8
                return (dbs)

printRDBObj :: Maybe Integer -> BL8.ByteString -> RDBObj -> Get (IO ())
printRDBObj (Just exp) key obj = return $ (print ("Expires: " ++ show exp) >>
                                           print ("Key: " ++ (BL8.unpack key)) >> 
                                           print ("Obj: " ++ show obj))
printRDBObj Nothing key RDBNull = return $ (print $ BL8.unpack key)
printRDBObj Nothing key obj = return $ (print ("Key: " ++ (BL8.unpack key)) >> 
                                        print ("Obj: " ++ show obj))


main = do
       testf <- BL8.readFile "./dump.rdb"
       runGet (processRDB_ printRDBObj)  testf

Thanks to everyone in advance.

Best, Erik

EDIT: , IO .

processRDB :: Get [RDBObj]

processRDB = do
                header <- getBytes 9
                dbs <- getDBs
                eof <- getWord8
                return (dbs)

main = do
       testf <- BL8.readFile "./dump.rdb"
       mapM_ (print . show) $ runGet processRDB testf

Answer 1

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