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

13 Sep 2016

      #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 hesiod):

 I tried implementing this, however, simply adding the instance in
 Data.Either leads to an import cycle because
 {{{Data.String(IsString(..))}}} has to be imported:
 {{{
 Module imports form a cycle:
          module ‘Data.Either’ (libraries/base/Data/Either.hs)
         imports ‘Data.String’ (libraries/base/Data/String.hs)
   which imports ‘Data.List’ (libraries/base/Data/List.hs)
   which imports ‘Data.Traversable’ (libraries/base/Data/Traversable.hs)
   which imports ‘Data.Either’ (libraries/base/Data/Either.hs)
 }}}
 However, I can't investigate this further atm, so if anyone wants to pick
 this up, please go ahead. I guess some shuffling around (maybe creating a
 hidden module for IsString) should solve the issue.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12160#comment:12
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