On Thu, Jul 29, 2010 at 11:48 PM, Lyndon Maydwell <maydw...@gmail.com>wrote:
> You cannot break out of a monad if all you have available to use are > the monad typeclass functions, however there is nothing preventing an > instance from being created that allows escape. Many of these escape > methods come in the form of runX functions, but you can use > constructors to break out with pattern matching if they are exposed. > There is one case where you can break out of a monad without knowing which monad it is. Well, kind of. It's cheating in a way because it does force the use of the Identity monad. Even if it's cheating, it's still very clever and interesting. http://okmij.org/ftp/Computation/lem.html <http://okmij.org/ftp/Computation/lem.html>The specific function is: > purify :: (forall m. Monad m => ((a -> m b) -> m b)) -> ((a->b)->b) > purify f = \k -> runIdentity (f (return . k)) We take some arbitrary monad 'm' and escape from it. Actually, the trick is that f must work for ALL monads. So we pick just one that allows escape and apply f to it. Here we picked Identity. You could have picked Maybe, lists, and any of the others that allow escaping. > As far as I can tell, IO is more of an outlier in this regard. > Yes I agree there. And even with IO we have unsafePerformIO that lets you escape. Jason
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