Hi.
So the natural question here is if we can employ the type system to enforce this correspondence. Phantom types immediately come to mind, as this problem is almost the same as ensuring that STRefs are only ever used in a single ST computation. The twist is that the inotify interface has nothing analogous to runST, which does the "heavy lifting" of the type magic behind the ST monad.
This twist is very simple to deal with if you have real existential types, with the relevant part of the interface looking approximately like
init :: exists a. IO (Inotify a) addWatch :: Inotify a -> FilePath -> IO (Watch a) rmWatch :: Inotify a -> Watch a -> IO ()
You can still do the ST-like encoding (after all, the ST typing trick is just an encoding of an existential), with init becoming "like runST":
init :: (forall a. Inotify a -> IO b) -> IO b addWatch :: Inotify a -> FilePath -> IO (Watch a) rmWatch :: Inotify a -> Watch a -> IO ()
Looking at your inotify.hs, the code of init becomes:
init :: (forall a. Inotify a -> IO b) -> IO b init k = do nextWatchRef_ <- newIORef 0 currentWatchesRef_ <- newIORef [] k $ Inotify { nextWatchRef = nextWatchRef_ , currentWatchesRef = currentWatchesRef_ }
And the code of main becomes:
main = init $ \ nd0 -> do wd0 <- addWatch nd0 "foo" wd1 <- addWatch nd0 "bar" init $ \ nd1 -> do wd3 <- addWatch nd1 "baz" printInotifyDesc nd0 printInotifyDesc nd1 rmWatch nd0 wd0 rmWatch nd1 wd3 -- These lines cause type errors: -- rmWatch nd1 wd0 -- rmWatch nd0 wd3 printInotifyDesc nd0 printInotifyDesc nd1
Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com