[GHC] #9664: Add identity functor to base

[GHC]

#9664: Add identity functor to base
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              Reporter:  hvr         |            Owner:  ekmett
                  Type:  task        |           Status:  patch
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:  Phab:D313   |
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Comment (by hvr):

 @ross,

 I noticed you already reversed the Identity<->Class import-dependency in
 recent `transformers` commmits

 I've adapted

 http://git.haskell.org/packages/transformers.git/commitdiff/refs/heads/wip/T9664

 which is now a rather minimal change to `transformers` (i.e. moving the
 `Identity.hs` file into a different source folder, and adapting `.cabal`)

 You write you "like to avoid a release with conditional exports". What
 problems do you anticipate that could occur by making use of such a
 conditional (non)export of `Data.Functor.Identity`?

 Specifically, I used `impl(ghc>=7.9)` instead of Cabal flags in order to
 avoid any Cabal-related problems I could think of.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9664#comment:8>
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