Instance warnings [Re: Remove MonadFail (ST s) instance]

[David Feuer]

That definitely sounds like a good approach to me!

On Mon, Nov 25, 2019, 5:04 PM Oliver Charles <ollie at ocharles.org.uk> wrote:

> I wonder if like TypeError we could have TypeWarning? Then you could
> have something like
>
> instance TypeWarning "Please use Fooble instead" => Show Foo where
>
> which would compile, but produce a warning whenever the `Show Foo`
> constraint was solved?
>
> On Mon, Nov 25, 2019 at 4:28 PM Andreas Abel <andreas.abel at ifi.lmu.de>
> wrote:
> >
> > Derailing...
> > Incidently, I recently asked for a "instance warning" feature in GHC:
> >
> >    https://gitlab.haskell.org/ghc/ghc/issues/17485
> >
> > (In my case I could go with a TypeError constraint instead.)
> >
> > Maybe instance warnings could be a useful feature.  It could be
> > implemented along the TypeError mechanism.
> >
> > On 2019-11-24 18:35, Henning Thielemann wrote:
> > >
> > > On Sun, 24 Nov 2019, Dan Burton wrote:
> > >
> > >> We're talking about an instance in `base`, so if there's no other way,
> > >> it could at least be a flag akin to
> > >> -Wmissing-monadfail-instances that gets included in -Wcompat.
> > >> But relatedly: should there be a way to deprecate instances? I would
> > >> say yes.
> > >
> > > Sounds like another application of instance warnings that we already
> > > talked about:
> > >     https://gitlab.haskell.org/ghc/ghc/issues/11796
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