[Haskell-cafe] Lazy object deserialization

[Jeff Shaw]

On 3/13/2013 6:14 PM, Ben wrote:
> that's too bad, i used lazy deserialization for an external sort thing i did aeons ago.
>
> http://www.haskell.org/pipermail/haskell-cafe/2007-July/029156.html
>
> that was an amusing exercise in lazy IO.  these days it's probably better off doing something with pipes et al instead of unsafeInterleaveIO.
>
> b
>
> On Mar 13, 2013, at 2:54 PM, Scott Lawrence wrote:
>
>> I tried it, but it still goes and reads the whole list. Looking at the `binary` package source code it seems that strict evaluation is hard-coded in a few places, presumably for performance reasons. It also seems to necessarily read the bytestring sequentially, so complex tree-like data structures would presumably encounter problems even if it worked for a list.
>>
>> Ah well. As long as I'm not duplicating someone else's work, I'm more than happy to go at this from scratch.
>>
>> On Wed, 13 Mar 2013, Jeff Shaw wrote:
>>
>>> On 3/13/2013 12:15 AM, Scott Lawrence wrote:
>>>> Hey all,
>>>> All the object serialization/deserialization libraries I could find (pretty much just binary and cereal) seem to be strict with respect to the actual data being serialized. In particular, if I've serialized a large [Int] to a file, and I want to get the first element, it seems I have no choice but to deserialize the entire data structure. This is obviously an issue for large data sets.
>>>> There are obvious workarounds (explicitly fetch elements from the "database" instead of relying on unsafeInterleaveIO to deal with it all magically), but it seems like it should be possible to build a cereal-like library that allows proper lazy deserialization. Does it exist, and I've just missed it?
>>>> Thanks,
>>> I haven't tested this, but I suspect something like this could give you lazy binary serialization and deserialization. It's not tail recursive, though.
>>>
>>> newtype LazyBinaryList a = LazyBinaryList [a]
>>>
>>> instance Binary a => LazyBinaryList a where
>>>    put (LazyBinaryList []) = putWord8 0
>>>    put (LazyBinaryList (x:xs)) = putWord8 1 >> put x >> put (LazyBinaryList xs)
>>>    get = do
>>>        t <- getWord8
>>>        case t of
>>>            0 -> return (LazyBinaryList [])
>>>            1 -> do
>>>                x <- get
>>>                (LazyBinaryList xs) <- get
>>>                return $ LazyBinaryList (x:xs)
>>>
>> -- 
>> Scott Lawrence
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> Haskell-Cafe at haskell.org
>> http://www.haskell.org/mailman/listinfo/haskell-cafe
I tried really hard to make Binary read a list in lazily, but I failed. 
Here's a full program that demonstrates the idea I had earlier. I used 
binary-0.7.0.1*.*

import Data.Binary
import Data.Binary.Get
import System.Environment
import qualified Data.ByteString.Lazy as L

newtype LazyBinaryList a = LazyBinaryList [a]
	deriving (Show)

instance Binary a => Binary (LazyBinaryList a) where
	put = putLazy
	get = getLazy

putLazy :: Binary a => LazyBinaryList a -> Put
putLazy (LazyBinaryList []) = putWord8 0
putLazy (LazyBinaryList (x:xs)) = putWord8 1 >> put x >> putLazy (LazyBinaryList xs)

getLazy :: Binary a => Get (LazyBinaryList a)
getLazy = aux []
	where aux accum = do
		t <- getWord8
		case t of
			0 -> return (LazyBinaryList (reverse accum))
			1 -> do
				x <- get
				aux (x:accum)

list :: LazyBinaryList Int
list = LazyBinaryList [1..10000]

main = do
	args <- getArgs
	case args !! 0 of
		"encode" -> encodeFile (args !! 1) list
		"decode" -> do
			bs <- L.readFile $ args !! 1
			let lOrR = decodeOrFail bs :: Either (L.ByteString, ByteOffset, String) (L.ByteString, ByteOffset, LazyBinaryList Int)
			case lOrR of
				Left l -> print l
				Right (_,offset,LazyBinaryList l) -> putStrLn $ "at offset " ++ show offset ++ " read value " ++ show (head l)


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20130313/3b05f7ef/attachment.htm>