Remove MonadFail (ST s) instance

[Andrew Martin]

I'm also in favor of removing this instance. Since there is currently no
deprecation mechanism for instances, I think it should simply be removed in
GHC 8.12.

On Mon, Nov 25, 2019 at 7:14 AM Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> As replied by Henning, fail @IO = throwIO . userError,
> which isn't throwing a pure exception. The IO instance is in the spirit
> of MonadFail proposal, it behaves well.
>
> If you write GHC-7.10..GHC-8.x code, you **should** depend on `fail`
> package, to get MonadFail class for GHC-7.x.
> See
>
> https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Adaptingoldcode
>
> On 25.11.2019 12.28, Bertram Felgenhauer via Libraries wrote:
> > Oleg Grenrus wrote:
> >> I cannot see any justification for that instance
> >>
> >>      -- | @since 4.11.0.0
> >>      instance Fail.MonadFail (ST s) where
> >>          fail s = errorWithoutStackTrace s
> > It's certainly not in the spirit of the MonadFail proposal. Arguably,
> > *all* implementations of `fail` in terms of `error*` are wrong, even
> > the one for IO. (I suspect dropping the instance for IO would cause a
> > lot of pain though...)
> >
> > I'm neutral on the proposal itself.
> >
> >> If someone wants to fail pattern matches in `ST s`, please do that
> >> explicitly.
> > However, I don't like this framing. In practice, there's at least one
> > other use of MonadFail for historical reasons: generic code that may
> > fail. Before MFP, one could write (and people have written) code like
> >
> >      fooM :: Monad m => a -> m b
> >      fooM ... = fail "unsupported case"
> >
> > It is easy enough to change the signature of such a `fooM` to require
> > MonadFail, but then the function becomes impossible to use for monads
> > that don't have a MonadFail instance. So after the MFP, one should
> > provide two functions instead, one using `error` and another one
> > using `fail`...
> >
> >> 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.
> > Or perhaps nobody cares about ghc-7.10 anymore.
> >
> > Cheers,
> >
> > Bertram
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-- 
-Andrew Thaddeus Martin
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