Remove MonadFail (ST s) instance

[Dan Burton]

If someone depends on this behavior, we should give them the courtesy of a
proper deprecation cycle to aid them in fixing their code. That's what a
deprecation cycle is for. Don't skip it.

-- Dan Burton


On Sun, Nov 24, 2019 at 7:57 AM Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> I cannot see any justification for that instance
>
>      -- | @since 4.11.0.0
>      instance Fail.MonadFail (ST s) where
>          fail s = errorWithoutStackTrace s
>
> If someone wants to fail pattern matches in `ST s`, please do that
> explicitly.
>
> I suggest and propose the removal without any deprecation period. If
> someone depends on this behavior, they ought to fix they code
> immediately: it's straight forward compiler type-error driven refactoring.
>
> Note: compatibility package
>
> https://hackage.haskell.org/package/fail-4.9.0.0/docs/Control-Monad-Fail.html
> doesn't have that instance, so I suspect not-that many use(d) that
> instance.
>
> Discussion period: 2 weeks (until Monday 2019-12-09).
>
> - Oleg
>
> P.S. you might run into problems  with
> https://gitlab.haskell.org/ghc/ghc/issues/15681 bug/miss-feature, but
> there's always a workaround to write code using explicit matching
> combinator, so it's not a show stopper.
>
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