666wman:

>    a slight modification to compile it :
>
>    change:
>    where sumFile = sum . map read . L.words
>    to :
>    where sumFile = sum . map (read . L.unpack) . L.words
>
>    but it's actually _slower_ than the non-bytestring version.

Never unpack a bytestring.

   import qualified Data.ByteString.Lazy.Char8 as S

   main = print . go 0 =<< S.getContents
     where
       go n s = case S.readInt s of
                       Nothing     -> n
                       Just (k,t)  -> go (n+k) (S.tail t)

Assuming you're reading int, integers or doubles.