We had a very similar discussion this time last year (https://mail.haskell.org/pipermail/libraries/2016-February/026678.html), with many people on both sides of the issue. There wasn't enough agreement reached to change the status quo.

I'll just repeat two things from that discussion:
- This is *not* just an issue of "beginners may be confused": Andrew Farmer describes encountering what I think is the most disconcerting part of having these instances: you can no longer have confidence in the process of change-the-type-then-just-fix-the-type-errors: https://mail.haskell.org/pipermail/libraries/2016-February/026782.html

- The tl;dr of my argument last time: "Part of the power of a type system is which program it rejects, and I'm arguing we're cluttering the space of valid programs."

The last time this was discussed, we planned to implement a flag to forbid certain instances. Has anyone taken that up?

Tom

On Thu, Mar 2, 2017 at 5:35 PM, Oleg Grenrus <oleg.grenrus@iki.fi> wrote:

Anecdotally, today I wrote `foldMap toList`, to use as `Maybe (NonEmpty
a) -> [a]`.
Also I rely heavily in lens-based code on the `folded :: Fold (Maybe a) a`.

So :-1: for me.

OTOH :+1: for using non-default preludes for teaching.

- Oleg

On 02.03.2017 19:35, Francesco Ariis wrote:
> On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:
>> Personally, I think it would be a shame to lose foldMap on EIther. I
>> frequently foldMap over Maybe values (where mempty is suitable in case of
>> "failure"), and I can certainly see myself doing the same thing with Either.
> I am not trying to be polemic, just to see where the community
> stands: regarding Either/Maybe: do you have use cases for length
> (sum) too?
>
> _______________________________________________
> Libraries mailing list
> Libraries@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________
Libraries mailing list
Libraries@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries