[GHC] #12160: MonadFail instance for (Either String)?

[GHC]

#12160: MonadFail instance for (Either String)?
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        Reporter:  lexi.lambda       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries         |              Version:  8.0.1
  (other)                            |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by lexi.lambda):

 Your point about just using `IsString` makes sense to me, so maybe that
 would be better, instead. One of the reasons `MonadFail` might sometimes
 be desirable over `MonadError` is that `MonadError` does not make it
 possible to make a single expression generic over both `Either` ''and''
 `IO`, unless the `Either` type is `Either IOException a`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12160#comment:6>
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