Proposal: add ordNub somewhere in containers

[Henning Thielemann]

On Tue, 17 Oct 2017, Henning Thielemann wrote:

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

ordNub :: (Traversable f, MonadPlus f, Ord a) => f a -> f a
ordNub =
    join . snd .
    Trav.mapAccumL
       (\s x ->
          if Set.member x s
            then (s,mzero)
            else (Set.insert x s, return x))
       Set.empty


We don't need mplus, only Monad and mzero.