On Thursday 08 September 2011, 02:07:39, Cale Gibbard wrote: > > If we reimplement it in the obvious way: > ghci> let nubBy f [] = []; nubBy f (x:xs) = x : filter (not . f x) > (nubBy f xs) > ghci> nubBy (>=) [1,2,3,4] > [1,2,3,4] > > I'm aware that the Report (strangely!) explicitly leaves the behaviour > of nubBy unspecified for functions which are not equivalence > relations,
I find that not so strange, another obvious reimplementation is the one you get if you compile Data.List with -DUSE_REPORT_PRELUDE, nubBy eq [] = [] nubBy eq (x:xs) = x : nubBy eq (filter (\ y -> not (eq x y)) xs) If the relation defined by the predicate argument is not transitive, you get a different result than with your definition above. Of course, the report could make a choice and say that behaviour is authoritative, but while the expected behaviour is clear for an equivalence relation (if you don't distinguish different bottoms), it's not so obvious otherwise. > but the behaviour given by the Report implementation (the > opposite of the current behaviour in GHC) is useful and desirable > nonetheless. Yes, although I don't think nubBy is a perfect name for that function - and, it's also only useful for transitive relations, as far as I can see. > > I'm sure I've written about this before. I'm not entirely sure what > happened to the previous thread of discussion about this, but it just Nobody made a formal proposal, so it withered and died. > came up again for me, and I decided that I was sufficiently irritated > by it to post again. Sufficiently irritated to make a formal proposal? If nobody does that, it probably won't change (and I'm not interested enough in the matter to make one, but would support). > > Another thing perhaps worth pointing out is that the parameters to > mapAccumR have always been backwards (compare it with foldr). Few > enough people use this function that I'm fairly sure we could just > change it without harm. Supposing that the last assessment is not wildly incorrect, +1. _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
