[GHC] #9984: Show, Read, Eq, Ord instances for Control.Applicative.Const

[GHC]

#9984: Show, Read, Eq, Ord instances for Control.Applicative.Const
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              Reporter:  fumieval    |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  low         |           Version:  7.8.4
             Component:              |  Operating System:  Unknown/Multiple
  libraries/base                     |   Type of failure:  None/Unknown
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 As mentioned in
 https://www.haskell.org/pipermail/libraries/2013-October/021531.html, the
 following instances can be derived clearly:

 * `Show a => Show (Const a b)`
 * `Read a => Read (Const a b)`
 * `Eq a => Eq (Const a b)`
 * `Ord a => Ord (Const a b)`

 The absence of these instances makes debugging harder.

 I suggest handcrafted Show/Read instances for neat representation.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9984>
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