Hi,
The FFI doesn't allow using type signatures in exported functions.
Imagine this simplified example
class Foo a where
action1 :: a -> IO()
action2 :: a -> IO()
exportable :: Foo a => StablePtr a -> IO()
exportable ptr = do deref <- deRefStablePtr ptr
action1 deref
Currently the FFI doesn't allow to export exportable due to the type
signature. But I don't really understand why isn't it feisable.
In my code I solved the problem by quantifying existentially
data CFoo = forall a.Foo a => CFoo a
instance Foo CFoo where
action1 (CFoo a) = action1 a
action2 (CFoo a) = action2 a
exportable' :: StablePtr CFoo -> IO()
exportable' ptr = do deref <- deRefStablePtr ptr
action1 a
Another way to fix it would be, unlike standard Haskell98, allowing
dictionaries to be packaged in data constructors
data CFoo' a = Foo a => CFoo' a
instance Foo (CFoo' a) where
action1 (CFoo' x) = action1 x
action2 (CFoo' x) = action2 x
exportable'' :: StablePtr (CFoo' a) -> IO()
exportable'' ptr = do deref <- deRefStablePtr ptr
action1 deref
As far as I know, the later option has only been recently made
available for GADTs in GHC, does anyone know away of doing it with
normal ADT?
Does anyone know a better workaround?
Thanks in advance,
Alfonso Acosta
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