[GHC] #12044: Remove sortWith in favor of sortOn

[GHC]

#12044: Remove sortWith in favor of sortOn
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        Reporter:  cblp              |                Owner:  cblp
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  7.10.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #2659             |  Differential Rev(s):  Phab:D2587
       Wiki Page:                    |
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Changes (by bgamari):

 * owner:  vkonton => cblp


Comment:

 It's really not clear to me that this is actually a refactoring that we
 would want to undertake. While `sortOn` and `sortWith` are semantically
 equivalent, they are operationally much different. Namely, for "cheap"
 mapping functions (e.g. projections like `fst`) `sortOn` will allocate
 significantly more than `sortWith`. Consider the case of,
 {{{#!hs
 x :: [(Int, String)]
 x = ...

 by = sortBy (comparing fst) x
 on = sortOn fst
 }}}

 The evaluation of `on` will allocate a list of type `[(Int, (Int,
 String))]`, `sortBy (comparing fst)` that intermediate list, and then
 project back to a `[(Int, String)]`. It seems to me like this would be
 quite undesirable.

 While we may not want to deprecate `GHC.Exts.sortWith` in favor of
 `sortOn` in particular, perhaps we want to deprecate it nevertheless. The
 more standard form `sortBy (comparing f)` is only a bit longer and
 arguably you should be forced to consider whether or not you want to use
 `sortOn` instead.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12044#comment:5>
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