[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        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 There seems to be the low hanging fruit of a MonadFail instance for Either
 String.

 {{{#!hs
 instance MonadFail (Either String) where
     fail = Left
 }}}

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