Re: [Haskell-cafe] A curios monad

Ryan Ingram wrote:

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.

Ah, yes, of course, thank you. :-) I was trying to break it from within the monad. Martijn.