[Haskell-cafe] Future of MonadCatchIO

[John Lato]

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

> Arie Peterson <ariep at 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.
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