[GHC] #9664: Add identity functor to base

[GHC]

#9664: Add identity functor to base
-------------------------------------+-------------------------------------
              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   |
-------------------------------------+-------------------------------------
Changes (by hvr):

 * status:  new => patch
 * differential:   => Phab:D313


Comment:

 @ross: please take a look at
 http://git.haskell.org/packages/transformers.git/commitdiff/refs/heads/wip/T9664
 ... does that look ok to you?

 I had to conditionally expose the `Data.Functor.Identity` module, as
 otherwise it gets more complicated to tweak GHC's build-system to cope
 with a `transformers` that would only work with GHC>=7.9 & base>=4.8, as
 the in-tree `transformers` packages needs to be compiled multiple times,
 once with the bootstrapping compiler (which is either GHC 7.6 or GHC 7.8
 for GHC HEAD), and later-on on with GHC HEAD.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9664#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler