[GHC] #9330: Introduce shortcut for "on (==)" much like there's a shortcut for "on compare".

[GHC]

#9330: Introduce shortcut for "on (==)" much like there's a shortcut for "on
compare".
-------------------------------------+-------------------------------------
              Reporter:  frerich     |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.8.2
             Component:              |          Keywords:
  libraries/base                     |  Operating System:  Unknown/Multiple
            Resolution:              |   Type of failure:  None/Unknown
Differential Revisions:              |         Test Case:
          Architecture:              |          Blocking:
  Unknown/Multiple                   |
            Difficulty:  Unknown     |
            Blocked By:              |
       Related Tickets:              |
-------------------------------------+-------------------------------------

Comment (by frerich):

 A lot of `*By` functions taking an `a -> a -> Bool` are in `Data.List`,
 e.g. `groupBy`, `nubBy`, `deleteBy`, `intersectBy`, `unionBy`. Hence, it
 seems plausible to define `equating` in `Data.List`.

 This is the same reasoning as why `comparing` is in `Data.Ord`: because
 the module exposes a lot of `*By` functions taking an `a -> a ->
 Ordering`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9330#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler