Adding binary to the Haskell Platform

[Felipe Lessa]

On Wed, Aug 05, 2009 at 06:48:02AM -0600, Denis Bueno wrote:
> [...] This is because of the [] instance.  I, myself, have been
> bitten by this, but I don't think it's a flaw in the library -- there
> are plenty of people using the library who haven't complained about
> the instance.
>
> If the instance isn't right for you, write a new one.  That's what I did.

The problem is that people shouldn't be reinventing the wheel
everytime.  I propose adding something along the lines of this
snippet to binary (untested):


import Control.Applicative
import Control.Monad (replicateM_)

-- | Provides a new instance of 'Binary' to lists that 'put's and
--   'get's using chunks instead of forcing the whole spine of
--   the list before writing the first byte.  This enconding is
--   less space-efficient in the disk, though, having an overhead
--   of @1 + (length xs `div` 255)@ bytes instead of only four
--   bytes (independently of list size), the overhead of the
--   default instance.
newtype Chunked a = Chunked {unChunked :: [a]}

instance Binary (Chunked a) where
    -- This 'put' should be good enough.
    put = mapM_ putChunk . chunks 255 . unChunked
        where chunks 255 [] = [(0,[])] -- not []!
              chunks _   [] = []
              chunks _   xs = let (i,f) = splitAt 255 xs
                                  len   = length i
                              in (len,i) : chunks len f
              putChunk (len,xs) = putWord8 len >> mapM_ put xs

    -- I don't know if this get works nicely, though.
    get = getWord8 >>= go []
        where go acc 0   = return $ concat $ reverse acc
              go acc len = do xs   <- replicateM_ len get
                              len' <- if len == 255 then getWord8 else return 0
                              go (xs:acc) len'

--
Felipe.