[GHC] #13593: Unexpected behavior from Data.List.groupBy

[GHC]

#13593: Unexpected behavior from Data.List.groupBy
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        Reporter:  dsf               |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by svenpanne):

 The Haskell report is very explicit about these functions:

 > When the "By" function replaces an Eq context by a binary predicate, the
 predicate is assumed to define an equivalence; when the "By" function
 replaces an Ord context by abinary predicate, the predicate is assumed to
 define a total ordering.

 And the `Data.List` documentation repeats these requirements, so I propose
 to close this issue as "invalid".

 As a side note, I wouldn't call this an "infelicity": If your `Eq`/`Ord`
 instances don't obey the required laws, you would have similar fun with
 plain `group`. The `By` functions just make an implicit argument explicit,
 and I would be very surprised if they behaved differently.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13593#comment:9>
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