#7913: Argument order not preserved by nubBy -------------------------+-------------------------------------------------- Reporter: paullik | Owner: Type: bug | Status: new Priority: normal | Component: Prelude Version: 7.6.3 | Keywords: nubBy Os: Linux | Architecture: Unknown/Multiple Failure: None/Unknown | Blockedby: Blocking: | Related: -------------------------+-------------------------------------------------- Hello. I recently wanted to know how the element 4 in [2,4] is ruled out by: {{{ nubBy (\x y -> x `mod` y == 0) [2,4] }}} and discussing this on #haskell we discovered that the documentation or the code is buggy. The [http://hackage.haskell.org/packages/archive/base/latest/doc/html/src /Data-List.html#nubBy numBy source] states that ''we keep the call to `eq` with arguments in the same order as in the reference implementation'', which, comparing the following two lines, is not true: {{{ nubBy eq (x:xs) = x : nubBy eq (filter (\ y -> not (eq x y)) xs) elem_by eq y (x:xs) = y `eq` x || elem_by eq y xs }}} Also this is easily proved by defining: {{{ nubBy' eq [] = [] nubBy' eq (x:xs) = x : nubBy' eq (filter (\ y -> not (eq x y)) xs) }}} Then running: {{{ nubBy (\x y -> x `mod` y == 0) [2,4] }}} which yields [2] because of '''eq y x''' and {{{ nubBy' (\x y -> x `mod` y == 0) [2,4] }}} which yields [2,4] because of '''eq x y'''. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7913 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler