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

Andreas Abel andreas.abel at
Mon Nov 25 16:28:13 UTC 2019

Incidently, I recently asked for a "instance warning" feature in GHC:

(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:
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