And those who do... do they really use it in a way that would manifest this breakage?

Never mind, this is obviously the case; I just had to refresh my mind on what the difference was between the two implementations. (Continuation gets called with original vs latest state.)

-- Dan Burton

On Tue, Jan 20, 2015 at 10:19 AM, Dan Burton <danburton.email@gmail.com> wrote:
Do people actually *use* the MonadCont class? And those who do... do they really use it in a way that would manifest this breakage?

basically every existing user of the instance is pretty much guaranteed to have breakage.

It seems the answer to these questions is "yes" and "yes." I'm curious to hear more.

-- Dan Burton

On Tue, Jan 20, 2015 at 10:02 AM, Ross Paterson <R.Paterson@city.ac.uk> wrote:
On Wed, Jan 21, 2015 at 12:09:13AM +1100, Ivan Lazar Miljenovic wrote:
> This is what I thought: I just wanted to make sure that I wasn't
> missing something (especially since there's no real documentation that
> I could find as to _how_ liftCallCC' fails to satisfy the laws of a
> monad transformer).

Any lifting of callCC should satisfy

        lift (f k) = f' (lift . k) => lift (callCC f) = liftCallCC callCC f'

and liftCallCC' doesn't.
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