[GHC] #12481: Add MonadFail instance for Either String

[GHC]

#12481: Add MonadFail instance for Either String
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        Reporter:  hesiod            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hesiod):

 I just realized that this instance has already been mentioned in
 https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail, not in
 `base` however, but rather in `transformers`. I'm not quite sure why
 having that instance in `transformers` would be better, since it would
 make it an orphan.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12481#comment:1>
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