Remove MonadFail (ST s) instance

Oleg Grenrus oleg.grenrus at
Sun Nov 24 15:57:27 UTC 2019

I cannot see any justification for that instance

     -- | @since
     instance Fail.MonadFail (ST s) where
         fail s = errorWithoutStackTrace s

If someone wants to fail pattern matches in `ST s`, please do that 

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 
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 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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