[Haskell-cafe] Lazy object deserialization

[Jeff Shaw]

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)