[GHC] #9664: Add identity functor to base

[GHC]

#9664: Add identity functor to base
-------------------------------------+-------------------------------------
       Reporter:  hvr                |                   Owner:  ekmett
           Type:  task               |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Core Libraries     |                 Version:
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:
  hour)                              |  None/Unknown
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
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 `base` already provides a `Const`ant functor, but the even more useful
 `Id`entity (or `Identity`) functor does not exist in `base` yet.

 This was already [https://groups.google.com/forum/#!msg/haskell-core-
 libraries/_nReDCbROew/Mdmf0zIloysJ proposed to the CLC back in early
 2013], and already missed inclusion in GHC 7.8

 However, in order to implement this, coordination with `transformers` is
 desirable, as `transformers` currently provides a non-transforming
 `Data.Functor.Identity`, and we'd like to avoid name clashes.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9664>
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