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.ht…
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.
