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

[Ryan Trinkle]

Having a deprecation path for instances would be hugely useful.  It'd also
be extremely useful to be able to do other refactors with typeclasses, such
as splitting or merging them, perhaps using constraint synonyms.

On Mon, Nov 25, 2019 at 11:28 AM 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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