Proposal: add ordNub somewhere in containers

[Michael Walker]

> ordNub could be implemented using Traversable:
>
>    catMaybes (traverse ??? xs :: [Maybe a])
>
> You would only need a generalization for catMaybes like mfilter.

The witherable package
(https://hackage.haskell.org/package/witherable) has a typeclass
generalising Traversable to containers which can remove elements.  So
I suspect Witherable is exactly the abstraction needed to write a
polymorphic function:

ordNub :: (Witherable f, Ord a) => f a -> f a
ordNub = catMaybes . ordNub'

ordNub' :: (Traversable f, Ord a) => f a -> f (Maybe a)
ordNub; = ...

-- 
Michael Walker (http://www.barrucadu.co.uk)