Consider adding `classify`.

[Roel van Dijk]

I think your function is a possible candidate for the
decorate-sort-undecorate paradigm, like sortOn in base [1].

classifyByProjection ∷ Ord π ⇒ (a → π) → [a] → [[a]]
classifyByProjection f = (map . map) snd . List.groupBy ((==) `on` fst) .
List.sortBy (comparing fst) . map (\x -> let y = f x in y `seq` (y, x))

That way the function f is only evaluated once for each element in the
input list.
I have not benchmarked this.

1 -
https://hackage.haskell.org/package/base-4.14.0.0/docs/Data-List.html#v:sortOn

Op ma 24 aug. 2020 om 14:49 schreef Ignat Insarov <kindaro at gmail.com>:

> Andreas, then how do you feel about adding that one?
>
>     classifyByProjection ∷ Ord π ⇒ (a → π) → [a] → [[a]]
>     classifyByProjection f = List.groupBy ((==) `on` f) . List.sortBy
> (compare `on` f)
>
> Having thought about it, I have come to agree that it is usually not
> hard to define a suitable projection function.
>
> On Mon, 24 Aug 2020 at 12:39, Andreas Abel <andreas.abel at ifi.lmu.de>
> wrote:
> >
> > This cannot be done efficiently.  I'd extend the equivalence relation to
> > a total order and then use sort and group.
> >
> > -1 to adding to the standard libraries.  How about publishing this as a
> > (well discoverable) package on hackage and see how popular it gets?
> >
> > On 2020-08-20 19:59, Ignat Insarov wrote:
> > > Hello.
> > >
> > > There has been [a question on Stack Overflow][1] asking for a way to
> group a
> > > list by an equivalence relation. Several answers were proposed over
> time, and I
> > > too [have offered a variant][2]. [I also wrote a benchmark.][3]
> > >
> > > I propose that the function be added to the standard libraries.
> > >
> > > [1]: https://stackoverflow.com/q/8262179
> > > [2]: https://stackoverflow.com/a/57761458
> > > [3]: https://github.com/kindaro/classify-benchmark.git
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