[Haskell-cafe] Can we come out of a monad?

[Stefan Holdermans]

Jason,

> 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.

How is this cheating?  Or better, how is this breaking out of a monad "without knowing which monad it is"?  It isn't. You know exactly which monad you're breaking out: it's the identity monad.  That's what happens if you put quantifiers in negative positions: here, you are not escaping out of an arbitrary monad (which you can't), but escaping out of a very specific monad.

> The specific function is:
>      > purify :: (forall m. Monad m => ((a -> m b) -> m b)) -> ((a->b)->b)
>      > purify f = \k -> runIdentity (f (return . k))

Cheers,

  Stefan