Upgrade MaybeT to AMP, and otherwise clean it up

[David Feuer]

Yes, I see now that that's impossible. Sorry! I guess just drop the
redundant constraint.
On Jan 3, 2015 9:19 PM, "Ross Paterson" <R.Paterson at city.ac.uk> wrote:

> On Sat, Jan 03, 2015 at 03:31:29PM -0500, David Feuer wrote:
> > http://stackoverflow.com/questions/22750315/
> > applicative-instance-for-maybet-m-assumes-monad-m noted that
> >
> > instance (Functor m, Monad m) => Applicative (MaybeT m)
> > instance (Functor m, Monad m) => Alternative (MaybeT m)
> >
> > is not exactly ideal. An answer to the question explained why MaybeT
> isn't
> > really so important for Applicative, but that doesn't seem to be a good
> reason
> > to leave the status quo in place. If I'm not very much mistaken, there
> are two
> > issues:
> >
> > 1. The Functor constraint is never used, and of course every valid monad
> is
> > also a valid functor with fmap=liftM. So for pre-AMP base, the signatures
> > should have had only a Monad constraint, and not a Functor one.
>
> I guess the unused Functor constraint might as well be dropped -- it
> will be redundant in the AMP world anyway.
>
> > 2. For AMP, the constraints should be changed to require Applicative
> > instead of Monad.
>
> This instance will still require Monad even if Applicative is a
> superclass.  We expect that applicatives that are also monads will satisfy
> (<*>) = ap (though sometimes (<*>) will be more efficient, it should be
> semantically equivalent).  Since ap doesn't execute the second action if
> the first one fails, this requires that the inner constructor is a monad.
> (The instance proposed at the above link doesn't meet this expectation;
> it is equivalent to Errors ().)
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