nubBy seems broken in recent GHCs

Cale Gibbard cgibbard at
Sat Jun 6 01:21:45 EDT 2009

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]

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. It would be good if this could get
fixed for 6.10.4.

 - Cale

PS: There is actually another implementation of groupBy which would
*also* be useful to have, but which is exactly the same for
equivalence relations as groupBy, and it compares adjacent elements
rather than the first element of a group with successive ones.
(groupByAdjacent or something would be a reasonable name.)

An implementation of it can be found here:

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