[Haskell-cafe] Writing a Storable instance for a C union

[Simon Marechal]

On 12/14/2013 05:46 PM, Nikita Karetnikov wrote:
> Yes, but I also want to poke, so SomeUnion must be an instance of 
> Storable.  Could you show how to define it properly or provide a 
> workaround?

You do not need to store the "key", or have an additional C'MyStruct
data type, as you already have this information in Haskell :

data SomeUnion = UnionFoo C'Foo
               | UnionWord64 Word64
               | UnionVoid (Ptr ())
               deriving (Show, Eq)

instance Storable SomeUnion where
  sizeOf _ = 12
  alignment _ = 4
  peek p = do
    v0 <- peekByteOff p 0
    case v0 of
        1  -> UnionFoo `fmap` peekByteOff p 4
        4  -> UnionWord64 `fmap` peekByteOff p 4
        20 -> UnionVoid `fmap` peekByteOff p 4
        _  -> fail "you should probably handle this case"
  poke p (UnionFoo    v1) = pokeByteOff p 0  1 >> pokeByteOff p 4 v1
  poke p (UnionWord64 v1) = pokeByteOff p 4  1 >> pokeByteOff p 4 v1
  poke p (UnionVoid   v1) = pokeByteOff p 20 1 >> pokeByteOff p 4 v1