I am trying to write a union data structure which can store vectors - a simplified definition of data structure is below:<div>---------------------</div><div>-- V is Data.Vector.Storable</div>
<div><div><div>data Elems = I {-# UNPACK #-} !GHC.Int.Int32</div><div>             | S {-# UNPACK #-} !GHC.Int.Int32 {-# UNPACK #-} !(Ptr CChar)</div><div>             | T {-# UNPACK #-} !(V.Vector Elems)</div></div></div>

<div>---------------------</div><div><br></div><div>What I can&#39;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:</div>

<div><br></div><div>---------------------</div><div><div><div>instance Storable Elems where</div><div>  sizeOf _ = ...</div><div>  alignment _ = ...</div><div><br></div><div><div> {-# INLINE poke #-}</div><div>  poke p x = case x of</div>

<div>      ...</div><div>      T x -&gt; do</div><div>              poke (castPtr p :: Ptr Word8) 3 -- store a tag for data constructor T. We will check it when doing peek</div><div>              let (fp,_,n) = V.unsafeToForeignPtr x</div>

<div>              poke (castPtr p1) n</div><div>              poke (castPtr (p1 `plusPtr` 8)) fp</div><div><br></div><div>      where  p1 = (castPtr p :: Ptr Word8) `plusPtr` 1 -- get pointer to start of the element. First byte is type of element</div>

</div><div><br></div><div><br></div><div>  {-# INLINE peek #-}</div><div>  peek p = do</div><div>            let p1 = (castPtr p::Ptr Word8) `plusPtr` 1 -- get pointer to start of the element. First byte is type of element</div>

<div>            t &lt;- peek (castPtr p::Ptr Word8)</div><div>            case t of</div><div>             .... -- handle all data constructors except T here</div><div>              _ -&gt; do</div><div>                    x &lt;- peek (castPtr p1 :: Ptr Int)</div>

<div>                    y &lt;- peek (castPtr (p1 `plusPtr` 8) :: Ptr (ForeignPtr Elems))</div><div>                    return (T (V.unsafeFromForeignPtr y 0 x)) -- return vector elements</div><div><br></div><div> </div>

</div></div><div>---------------------</div><div><br></div><div>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?</div>

<div><br></div><div>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. </div><div><br></div><div>
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.</div>