Hello fellow haskellers, I am wondering if anyone has an idea whether I'd run into trouble if I rebound >>= in order to provide a more restricted monad. My idea is to define a class: class WitnessMonad wm where (>>=) :: wm w w' a -> (a -> wm w' w'' b) -> wm w w'' b (>>) :: wm w w' a -> wm w' w'' b -> wm w w'' b return :: a -> wm w w' a fail :: String -> wm w w' a which obviously just adds a couple of phantom types indicating the monad state prior to and after each operation. I actually rather like this redefined monad, as it seems like it'd be useful in lots of places (e.g. a restricted IO monad that disallows use of a file handle after it's closed). It seems to me like this would be a pretty cool kind of monad. It'd allow us to encode typechecked "properties" in a monad, and there'd be very little restriction on how tricky those properties could be. And we'd get this all using the same do syntax we all love (and the new properties are by definition sequential, so a monad is precisely what we want, only at the type level). One trouble I see is how to move an ordinary monad into the WitnessMonad class. Of course, they differ in kind, so we need to do something, but I'm not sure what, and suspect that folks with experience using monad transformers will immediately see the answer. Of course, I want to be able to use both my fancy witnessed monad and IO in the same module, and I want to be able to use do notation for both of them. I suppose we could easily define data M m w w' c = M (m c) lift = M lower (M m) = m instance WitnessMonad (M m) where f >>= g = lift $ Prelude.(>>=) (lower f) (lower . g) ... but then we'd have to lift all our operations, so an ordinary IO function in a module that used my fancy rebound do notation would look like f :: IO Char f = lower $ do lift $ putStr "Hello world" lift $ getChar which seems rather heavy. Can anyone think of syntax-light way to allow my hypothesized rebound do-notation to also work with ordinary monads? -- David Roundy Department of Physics Oregon State University