[Haskell-cafe] StateT and ContT

Dan Burton danburton.email at gmail.com
Tue Jan 20 18:19:47 UTC 2015

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 at city.ac.uk>

> 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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