The implementation of nubBy in Data.List is as follows, where USE_REPORT_PRELUDE refers to [1]: #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 #endif That comment is actually not correct [2], and the report version and the base version don't have the same semantics when used on asymmetric relations: MyReportPrelude> nubBy (<) [1] [1] Data.List> nubBy (<) [1,2] [1,2] ## Proposal Make nubBy and nub obey the report semantics by swapping the arguments to `eq` in elem_by, and defining nub as nubBy (==). This is the 'still easy' variant from [3]. ## Motivation The Report's order is more sensible, since the parameters to the relation stay in the left-to-right order in which they occurred in the list. See [4,5] for user bug reports. Discussion period: 2 weeks Code review: https://phabricator.haskell.org/D238 [1] https://www.haskell.org/onlinereport/list.html#sect17.6 [2] https://ghc.haskell.org/trac/ghc/ticket/2528 [3] https://ghc.haskell.org/trac/ghc/ticket/7913#comment:3 [4] https://ghc.haskell.org/trac/ghc/ticket/3280 [5] https://ghc.haskell.org/trac/ghc/ticket/7913