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

[Oliver Charles]

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
>> > > _______________________________________________
>> > > Libraries mailing list
>> > > Libraries at haskell.org
>> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>> > _______________________________________________
>> > Libraries mailing list
>> > Libraries at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries