On Fri, Dec 12, 2008 at 2:48 AM, Martijn van Steenbergen
I've worried about this but I couldn't find a good code example of when this goes wrong. Can you? Without using any of the unsafeXxx functions, of course.
Maybe I should build my monad on top of the ST monad if that makes things safer.
[1] http://hackage.haskell.org/packages/archive/Yogurt/0.2/doc/html/Network-Yogu...
Here's a simple example: runMud :: Mud a -> a runMud = flip evalState emptyMud main = do let v = runMud (mkVar "hello") let crash = runMud $ do v2 <- mkVar True -- v2 :: Var Bool res <- readVar v -- v :: Var String return res print crash -- boom! Both v2 and v are the same variable index (0), but different types. Since there's nothing preventing the variable from escaping from the first "runMud", we can import it into the second one where it fails. The key is that both use the same "initial state", so you have lost the uniqueness of variables. ST is safer, although you can make these systems just as safe with the phantom "state" type like ST does. It's also faster; variables turn directly into pointer references, instead of traversing an IntMap. But it gets kind of annoying writing all the extra "s" on your type signatures. -- ryan