You can peek the "key", and then, depending on its value, return a different constructor :
data MyStruct = MyStructOne Foo | MyStructFour Integer | MyStructTwenty [Blah]
OK, but how can I pick one of the union fields? Also, I’d rather change MyStruct to Union in the above since I already defined MyStruct, which has a Storable instance that looks like so (omitting sizeOf, alignment, and poke): instance Storable MyStruct where peek p = do v0 <- peekByteOff p 0 return $ MyStruct v0 This allows to get the value of key, and I could change it to get one of the union fields: instance Storable MyStruct where peek p = do v0 <- peekByteOff p 0 v1 <- peekByteOff p 4 return $ MyStruct v0 v1 But that would require to define a Storable instance for the Union type, which is exactly the point of the thread. How can I do so? (Note that Foo, Word64, and (Ptr a) are instances of Storable themselves.) The other question that remains unanswered is how to make such code portable. Should I rely on hsc2hs to determine things like sizeOf and alignment?