Please apply the comparison function given to nubBy to elements of the list in the order in which they occur in the list.

[Cale Gibbard]

I just tried this in ghci-7.0.3:

ghci> nubBy (>=) [1,2,3,4]
[1]

Think about what this is doing: it is excluding 2 from the list
because 2 >= 1, rather than including it because 1 >= 2 fails.

I think an important convention when it comes to higher order
functions on lists is that to the extent which is possible, the
function parameters take elements from the list (or things computed
from those) in the order in which they occur in the original list.

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, but the behaviour given by the Report implementation (the
opposite of the current behaviour in GHC) is useful and desirable
nonetheless.

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
came up again for me, and I decided that I was sufficiently irritated
by it to post again.

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.

 - Cale