Hi everybody, I know this sounds like an issue many have faced before me, but good old google didn't come up with any helpful results. The basic problem is that I need to marshal a union so I can pass it to a C function via FFI. So to make sure I have my head wrapped around this correctly, say I have a union like so: typedef union { int i; char c; } my_union; This would typically be represented in Haskell as: data myUnion = I CInt | C CChar My question is how would you marshall (define an Storable instance for) myUnion into my_union? It's my understanding that an instance my_union would take up sizeof(int) bytes in memory, i.e. the size of it's largest member. So to store this we would write something along the lines of: instance Storable myUnion where size _ = #{size my_union} -- <-- hsc2hs shortcut alignment _ = alignment undefined::CInt -- <-- What should this really be? peek ptr = do -- <-- How are you supposed to know which element to extract? poke ptr (I i) = poke ptr i -- <-- Or should this be #{poke my_union, i} ptr i ? poke ptr (C c) = poke ptr c I hope this clear. I appreciate all the help. If this question has already been answered, which I'm sure it has, please point me to right spot. Thanks again for everyone's time. Nabil Alsharif.