The order of arguments to the function passed to nubBy got swapped somehow
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
Trac metadata
Trac field | Value |
---|---|
Version | 6.10.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | libraries/base |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | cgibbard@gmail.com |
Operating system | |
Architecture |