#7913: Argument order not preserved by nubBy ---------------------------------+------------------------------------------ Reporter: paullik | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Prelude | Version: 7.6.3 Keywords: nubBy | Os: Linux Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Changes (by simonpj): * difficulty: => Unknown Comment: OK, here is the implementation of `nubBy`, both reference model and actual: {{{ nubBy :: (a -> a -> Bool) -> [a] -> [a] #ifdef USE_REPORT_PRELUDE nubBy eq [] = [] nubBy eq (x:xs) = x : nubBy eq (filter (\ y -> not (eq x y)) xs) #else nubBy eq l = nubBy' l [] where nubBy' [] _ = [] nubBy' (y:ys) xs | elem_by eq y xs = nubBy' ys xs | otherwise = y : nubBy' ys (y:xs) -- Not exported: -- Note that we keep the call to `eq` with arguments in the -- same order as in the reference implementation -- 'xs' is the list of things we've seen so far, -- 'y' is the potential new element elem_by :: (a -> a -> Bool) -> a -> [a] -> Bool elem_by _ _ [] = False elem_by eq y (x:xs) = y `eq` x || elem_by eq y xs }}} In `nubBy'` the `xs` is the list of all earlier elements that have been accepted into the result list. Indeed when we compare a later item `y` with these, the `y` parameter should be second, as it is in the reference implementation. So if I'm right the fix should be to swap the order of arguments to `eq` in the definition of `elem_by`. And document all this more clearly in the defn of `nubBy`. And if we were paranoid we might do the same thing for `nub`, really, just in case `(==)` isn't symmetrical. Does everyone agree? Easy to do. Would someone like to make a patch? Simon -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/7913#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler