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

[GHC]

#9588: Add `MonadPlus (Either e)` and `Alternative (Either e)` instances
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        Reporter:  hvr               |                Owner:  hvr
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Core Libraries    |              Version:
      Resolution:                    |             Keywords:  report-impact
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10755 #12160     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 intended?

 {{{#!hs
 $ ghci-8.0.1
 GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /apps/strats/.ghci
 > import Data.Foldable
 > import Control.Applicative
 > import Control.Monad.Trans.Error

 <interactive>:1:1: warning: [-Wdeprecations]
     Module ‘Control.Monad.Trans.Error’ is deprecated:
       Use Control.Monad.Trans.Except instead
 > Left "A" <|> Left "B"
 Left "B"
 > asum [Left "A", Left "B"]
 Left ""
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:11>
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