Remove MonadFail (ST s) instance

Carter Schonwald carter.schonwald at
Wed Feb 5 18:21:49 UTC 2020

This does seem like a reasonable request,
Though of course it’d be really cool to evaluate what code may be
incidentally using the monad fail instance by accident atm!
(is there any way we could do that?)

On Sun, Nov 24, 2019 at 12:35 PM Henning Thielemann <
lemming at> 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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