[GHC] #9588: Add `MonadPlus {IO, Either e}` and `Alternative (Either e)` instances

[GHC]

#9588: Add `MonadPlus {IO,Either e}` and `Alternative (Either e)` instances
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              Reporter:  hvr         |            Owner:  hvr
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by hvr):

 I'm hitting a problem while trying to implement this.

 While the `IO` instances are rather straightforward
 (and btw, there's also an `Alternative IO` instance defined in
 `transformers`):

 {{{#!hs
 instance MonadPlus IO where
     mzero       = ioError (userError "mzero")
     m `mplus` n = m `catchIOError` \ _ -> n

 instance Alternative IO where
     empty = mzero
     (<|>) = mplus
 }}}

 The instances for `Either`, however, depend on an `Error` class in
 `transformers` to produce an `empty`/`mzero` value:

 {{{#!hs
 instance (Error e) => Alternative (Either e) where
     empty        = Left noMsg
     Left _ <|> n = n
     m      <|> _ = m

 instance (Error e) => MonadPlus (Either e) where
     mzero            = Left noMsg
     Left _ `mplus` n = n
     m      `mplus` _ = m
 }}}

 how shall this issue be resolved?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:1>
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