<div dir="ltr"><div><div><div><span style="font-family:arial,helvetica,sans-serif">We had a very similar discussion this time last year (<a href="https://mail.haskell.org/pipermail/libraries/2016-February/026678.html">https://mail.haskell.org/pipermail/libraries/2016-February/026678.html</a>), with many people on both sides of the issue. There wasn't enough agreement reached to change the status quo.<br><br></span></div><span style="font-family:arial,helvetica,sans-serif">I'll just repeat two things from that discussion:<br>  - 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: <a href="https://mail.haskell.org/pipermail/libraries/2016-February/026782.html">https://mail.haskell.org/pipermail/libraries/2016-February/026782.html</a><br><br></span></div><div><span style="font-family:arial,helvetica,sans-serif">  - The tl;dr of my argument last time: "</span><span style="font-family:arial,helvetica,sans-serif">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."</span><span style="font-family:arial,helvetica,sans-serif"></span></div><div><span style="font-family:arial,helvetica,sans-serif"><br></span></div><span style="font-family:arial,helvetica,sans-serif">The last time this was discussed, we planned to implement a flag to forbid certain instances. Has anyone taken that up?<br><br></span></div><span style="font-family:arial,helvetica,sans-serif">Tom<br></span><div><div><span style="font-family:arial,helvetica,sans-serif"><br></span></div></div><div class="gmail_extra"><span style="font-family:arial,helvetica,sans-serif"><br></span><div class="gmail_quote"><span style="font-family:arial,helvetica,sans-serif">On Thu, Mar 2, 2017 at 5:35 PM, Oleg Grenrus <span dir="ltr"><<a target="_blank" href="mailto:oleg.grenrus@iki.fi">oleg.grenrus@iki.fi</a>></span> wrote:<br></span><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote"><span style="font-family:arial,helvetica,sans-serif">Anecdotally, today I wrote `foldMap toList`, to use as `Maybe (NonEmpty<br>
a) -> [a]`.<br>
Also I rely heavily in lens-based code on the `folded :: Fold (Maybe a) a`.<br></span>
<span style="font-family:arial,helvetica,sans-serif"><br>
So :-1: for me.<br></span>
<span style="font-family:arial,helvetica,sans-serif"><br>
OTOH :+1: for using non-default preludes for teaching.<br></span>
<span style="font-family:arial,helvetica,sans-serif"><span class="gmail-HOEnZb"><font color="#888888"><br>
- Oleg<br>
</font></span></span><div class="gmail-HOEnZb"><div class="gmail-h5"><span style="font-family:arial,helvetica,sans-serif"><br>
On 02.03.2017 19:35, Francesco Ariis wrote:<br>
> On Thu, Mar 02, 2017 at 05:19:26PM +0000, Oliver Charles wrote:<br>
>> Personally, I think it would be a shame to lose foldMap on EIther. I<br>
>> frequently foldMap over Maybe values (where mempty is suitable in case of<br>
>> "failure"), and I can certainly see myself doing the same thing with Either.<br>
> I am not trying to be polemic, just to see where the community<br>
> stands: regarding Either/Maybe: do you have use cases for length<br>
> (sum) too?<br>
><br>
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