Lazy encoding / decoding of [a] using Data.Binary in Haskell

Lazy.hs

{-# LANGUAGE FlexibleInstances #-}

--
-- head $ decode $ encode [1..]                      -- : infinite loop
-- head $ runLazily $ decode $ encode $ Lazily [1..] -- : 1
--

module Data.Binary.Lazy (
  Lazily(..),
) where

import Data.Binary
import Data.Binary.Get
import Data.Functor ((<$>))
import Data.List    (unfoldr)
import qualified Data.ByteString.Lazy as L

newtype Lazily a = Lazily { runLazily :: a }

chunkSize = 65535

instance Binary a => Binary (Lazily [a]) where
    get = Lazily . concat . unfoldr get1 <$> getRemainingLazyByteString
        where get1 s | L.null s  = Nothing
              get1 s | otherwise = Just $ drop3rd $ runGetState get s 0
    put = mapM_ put . group chunkSize . runLazily

group :: Int -> [a] -> [[a]]
group n = unfoldr group1
  where group1 [] = Nothing
        group1 xs = Just $ splitAt n xs

drop3rd :: (a, b, c) -> (a, b)
drop3rd    (x, y, _) =  (x, y)