id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3280	The order of arguments to the function passed to nubBy got swapped somehow	guest		"According to the Report:
{{{
  nubBy            :: (a -> a -> Bool) -> [a] -> [a]
  nubBy eq []      =  []
  nubBy eq (x:xs)  =  x : nubBy eq (filter (\y -> not (eq x y)) xs)
}}}
Hence, we should have that
{{{
nubBy (<) (1:2:[])
= 1 : nubBy (<) (filter (\y -> not (1 < y)) (2:[]))
= 1 : nubBy (<) []
= 1 : []
}}}
However in ghc-6.10.3:
{{{
Prelude Data.List> nubBy (<) [1,2]
[1,2]
}}}
The order of the parameters to the function which is passed to nubBy is *important* since the function might not be an equivalence relation. nubBy is more generally useful for sieving even when the relation is not symmetric. groupBy, for a similar reason, has applications for grouping beyond those provided by equivalence relations, and I think we should be able to rely on its behaviour.

Moreover, I contend that 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. 

 - Cale"	bug	closed	normal	6.12.1	libraries/base	6.10.3	wontfix		cgibbard@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown