[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:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:  8.0.1
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was hoping that
 {{{#!hs
 let notBoth1 a b = not (a == 1 && b == 1) in
 groupBy notBoth1  [1,1,2,3,1,1,4,5,6,1]
 }}}
 would give me
 {{{
 [[1],[1,2,3,1],[1,4,5,6,1]]
 }}}
 but instead I get
 {{{
 [[1],[1,2,3],[1],[1,4,5,6],[1]]
 }}}
 It seems that groupBy assumes transitivity in the argument function. I
 have a new implementation that does not make this assumption. Of course,
 the implications of changing this function's behavior are troubling.
 {{{#!hs
 groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
 groupBy' p (x : xs) = go [x] xs
     where
       go (x : xs) (y : zs) | p x y = go (y : x : xs) zs
       go g (y : zs) = reverse g : go [y] zs
       go g [] = [reverse g]
 }}}

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