[GHC] #9623: Use Data.List.dropWhileEnd

[GHC]

#9623: Use Data.List.dropWhileEnd
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              Reporter:  dfeuer      |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  libraries   |          Version:  7.8.3
  (other)                            |         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:              |
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Comment (by dfeuer):

 Replying to [comment:9 refold]:
 > Replying to [comment:8 dfeuer]:
 > > However, the above `dropWhileEndLE` appears to be faster than the
 double reverse in all cases. So I'll add it to the utilities module and
 use that instead.
 >
 > Since `dropWhileEndLE` is both faster and has less surprising semantics
 (spine-strict instead of element-strict), I think that the standard
 library should be changed to use this version. And we should also add
 `takeWhileEnd` while we're at it.

 It seems to be faster in the common case of trimming spaces from fairly
 short strings. It would be pretty bad for cutting a little bit off the end
 of a lazily-read file. I would be in favor of adding `dropWhileEndLE` (by
 some name) to `Data.List`, and perhaps deprecating the name `dropWhileEnd`
 in favor of something less confusing like `takeUntilLast = dropWhileEnd .
 not`, but just changing the semantics of an existing function seems
 generally frowned upon. That said, the semantics of
 `Data.Text.Lazy.dropWhileEnd`, that someone suggested I check into, are
 much more confusing to me so far, so there may be a bit of room to wiggle
 things around.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9623#comment:10>
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