[GHC] #9004: Add sortOn function to Data.List

[GHC]

#9004: Add sortOn function to Data.List
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        Reporter:              |            Owner:
  JohnWiegley                  |           Status:  patch
            Type:  feature     |        Milestone:  7.10.1
  request                      |          Version:  7.8.2
        Priority:  normal      |         Keywords:
       Component:              |     Architecture:  Unknown/Multiple
  libraries/base               |       Difficulty:  Easy (less than 1 hour)
      Resolution:              |       Blocked By:
Operating System:              |  Related Tickets:
  Unknown/Multiple             |
 Type of failure:              |
  None/Unknown                 |
       Test Case:              |
        Blocking:              |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"44512e3c855d8fb36ab6580f4f97f842ebcf4c6c/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="44512e3c855d8fb36ab6580f4f97f842ebcf4c6c"
 Add Data.List.sortOn function (re #9004 and #2659)

 `sortOn` sorts a list by comparing the results of a key function applied
 to each
 element.  `sortOn f` is equivalent to `sortBy . comparing f`, but has the
 performance advantage of only evaluating `f` once for each element in the
 input list.

 Historical note: This was already proposed in 2008 as part of

   http://www.haskell.org/pipermail/libraries/2008-October/010797.html

 It was, however, the recent re-attempt

   http://www.haskell.org/pipermail/libraries/2014-April/022489.html

 that let `sortOn` make it into base at last. Maybe the other functions
 mentioned in #2659 might be worth reconsidering as well.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9004#comment:3>
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