#13593: Unexpected behavior from Data.List.groupBy -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- 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] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13593 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler