I am trying to write a union data structure which can store vectors - a simplified definition of data structure is below: --------------------- -- V is Data.Vector.Storable data Elems = I {-# UNPACK #-} !GHC.Int.Int32 | S {-# UNPACK #-} !GHC.Int.Int32 {-# UNPACK #-} !(Ptr CChar) | T {-# UNPACK #-} !(V.Vector Elems) --------------------- What I can't figure out is how to store Vector elements when defining Storable instance for Elems type - the main stumbling block is that my approach requires defining a Storable instance of ForeignPtr. Snippets of code below: --------------------- instance Storable Elems where sizeOf _ = ... alignment _ = ... {-# INLINE poke #-} poke p x = case x of ... T x -> do poke (castPtr p :: Ptr Word8) 3 -- store a tag for data constructor T. We will check it when doing peek let (fp,_,n) = V.unsafeToForeignPtr x poke (castPtr p1) n poke (castPtr (p1 `plusPtr` 8)) fp where p1 = (castPtr p :: Ptr Word8) `plusPtr` 1 -- get pointer to start of the element. First byte is type of element {-# INLINE peek #-} peek p = do let p1 = (castPtr p::Ptr Word8) `plusPtr` 1 -- get pointer to start of the element. First byte is type of element t <- peek (castPtr p::Ptr Word8) case t of .... -- handle all data constructors except T here _ -> do x <- peek (castPtr p1 :: Ptr Int) y <- peek (castPtr (p1 `plusPtr` 8) :: Ptr (ForeignPtr Elems)) return (T (V.unsafeFromForeignPtr y 0 x)) -- return vector elements --------------------- If I use above approach, I will need to define Storable instance of ForeignPtr for Elems data type. Is it possible to define peek/poke operations for storing vectors, using something other than ForeignPtr? I prefer using ForeignPtr because it has finalizer associated with it. I guess it might be possible to store Ptr through withForeignPtr, but then how do you convert back from the Ptr to Vector in peek? If storing the ForeignPtr is a cleaner approach, how would Storable definition for ForeignPtr look like? GHC code for ForeignPtr seems to export only ForeignPtr constructor. I am aware above code is not portable (e.g., assumption that Int is 8 bytes). That is not really an issue because I am writing code for a very specific platform. If there is a library defined somewhere which has already dealt with similar issue, I will very much appreciate pointers.