On Sun, Jul 14, 2013 at 7:31 AM, Clark Gaebel <cgaebel@uwaterloo.ca> wrote:

Similarly, I've always used:

import qualified Data.HashSet as S

nub :: Hashable a => [a] -> [a]
nub = S.toList . S.fromList

And i can't think of any type which i can't write a Hashable instance, so this is extremely practical.

This won't yield results lazily (e.g. nub (repeat 'x') = _|_ instead of 'x' : _|_), but Niklas' ordNub will. His ordNub can be translated directly to HashSet and still have the stability and laziness properties.

A difficulty with putting ordNub in Data.List is that it depends on containers, which is outside of the base package. Some options:

* Move the implementation of Set to base.

* Implement a lean version of Set in base that only provides 'insert' and 'member'.

* Define ordNub in Data.Set instead.

Adding a Hashable-based nub to base would be even more problematic, since you'd need Hashable in base.