Is there any way to implement a JE instance for [a]?

Question

I'm writing a network processing program, and having some troubles. I decided to implement my directed graph as a Judy Array of linked lists of arcs. That is

JudyL [Arc]

where,

type Id  = Int
type Day = Int

data Arc = Arc { aid  :: Id,
                 time :: Day} deriving (Eq, Show)

However, Judy arrays require that their inputs can be converted to a Word value, or a word-sized pointer. In particular I must implement,

instance JE [a] where
     toWord :: [a] -> IO Word
     fromWord :: Word -> IO [a]

I however, have no idea whatsoever how to deal with pointers in Haskell. Any wisdom would be greatly appreciated. Perhaps I should just be using an array instead, but I fear that my dataset is very large (a terabyte uncompressed, though a lot of that data is extraneous) and have convinced myself that additional reads/writes on the (admittedly solid state) drive could seriously slow this down. IDK though.

Answer 1

You can just copy and paste the ByteString instance, which essentially allocates a new StablePtr and then casts it to Word. Keep in mind that you will need to do your own deallocation when (if) the time comes for that.