But Haskell (and GHC) have existential types, and your prototype code works with GHC after a couple of trivial changes:
> main = do > W nd0 <- init > wd0 <- addWatch nd0 "foo" > wd1 <- addWatch nd0 "bar" > W nd1 <- init > 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 The only change is that you have to write W nd <- init and that's all. The type-checker won't let the user forget about the W. The commented out lines do lead to type errors as desired. Here is what W is > data W where > W :: Inotify a -> W > init :: IO W [trivial modification to init's code] I must say though that I'd rather prefer Adres solution because his init > init :: (forall a. Inotify a -> IO b) -> IO b ensures that Inotify does not leak, and so can be disposed of at the end. So his init enforces the region discipline and could, after a trivial modification to the code, automatically do a clean-up of notify descriptors -- something you'd probably want to do. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
