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

[George Wilson]

Some kind of instance warning like this would enable library authors
to migrate type class instances. The Core Libraries Committee would
likely be able to use this for good. I would definitely like to see
that happen.
Type class instances are "stuck" in Haskell today.

On Tue, 26 Nov 2019 at 08:10, Oliver Charles <ollie at ocharles.org.uk> wrote:
>
> Perhaps I have found motivation for my first proposal!
>
> On Mon, Nov 25, 2019 at 10:06 PM David Feuer <david.feuer at gmail.com> wrote:
> >
> > 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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