[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:  #9588             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 The danger here is that the `(Either String)` specific instance proposed
 here rules out reasoning uniformly about any other instance someone might
 want to put on `Either` that works for more types, which could get to be
 be common in things like test suites.

 This lets one 'pointwise' instance override anybody's ability to grab a
 more general one. e.g. the old `Error` class from `MonadError` approach.
 I'm not advocating for that class per se, as there is no need for `noMsg`
 for `MonadFail`, making that class "too big".

 `IsString` on the other hand, does have the remarkable property that it
 has precisely the right size.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12160#comment:8>
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