Proposal: add ordNub somewhere in containers

Henning Thielemann lemming at
Tue Oct 17 09:32:27 UTC 2017

On Tue, 17 Oct 2017, Eric Torreborre via Libraries wrote:

> As a tangent I am interested in knowing what would be the typeclass for this sort of operation:
>  - take all the elements out of `f a`
>  - do something with them with a function a -> b
>  - insert the results back to get the same "original shape" `f b`

I guess, this one can only be Functor.

> In this case "do something with them" is "order them"

A function of type (a -> b) would not be able to order elements.

ordNub could be implemented using Traversable:

    catMaybes (traverse ??? xs :: [Maybe a])

You would only need a generalization for catMaybes like mfilter.

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