On Mon, Mar 4, 2013 at 12:07 AM, Ertugrul Söylemez <es@ertes.de> wrote:

Arie Peterson <ariep@xs4all.nl> wrote:

> Would anyone have a problem with a deprecation of
> MonadCatchIO-transformers, and a failure to update it to work with a
> base without 'block' and 'unblock'?

Yes. This is a simplified variant of a monad I use:

newtype Continue f m a = Continue (m (Maybe a, f (Continue f a)))

It's related to Cofree and has a valid and very straightforward
MonadCatchIO instance. However, it's probably impossible to write a
valid MonadTransControl/MonadBaseControl instance for it.

So I kindly ask you not to deprecate MonadCatchIO. The reason I'm
hesitant about moving to monad-control is that it's hard to understand
and also very difficult to define for CPS monads. It is commonly
believed to be impossible.

You can always cast the continuation to a dynamic type and cast it back later.

Doing so would typically require additional constraints, however if you're trying to make an instance for MonadTransControl that's unfortunately not possible (you'd need a Typeable constraint on the monad parameter, but it's not in scope). Lacking an appropriate MonadTransControlWithTypeable class, it's certainly possible to fall back to various low-level, highly-dubious constructs. Which I of course implemented without hesitation :grin

Also I've seen at least one article about the incorrectness of
monad-control. That's one further reason I like to avoid it.

I'd appreciate a link if anyone could manage to find it. I haven't seen any criticisms of monad-control.

John L.