[Haskell-cafe] A curios monad
Martijn van Steenbergen
martijn at van.steenbergen.nl
Fri Dec 12 16:57:33 EST 2008
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
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