Haskell Foldable Wats

[Mark Roberts]

>
> these warning are supposed to be opt-in at the use sites,
> aren't they?


As I read it, Phil's suggestion of a FORBIDINSTANCE language extension
would be opt-in per module. Lennart's suggestion of attaching warnings to
methods in the instance does not seem opt-in. Lennart, have I
misinterpreted?

People have circulated somewhat convincing examples that show how these
> Functor instances lead to real bugs.


And there are those who are fine with these instances, too. You could
imagine many more disagreements which are a consequence of polymorphism. So
this feels more like a code-style/linting issue to me, and I think that
using a tool like HLint would be more appropriate than introducing a
language extension or other change to GHC.



On Thu, Feb 25, 2016 at 1:35 PM, Kosyrev Serge <_deepfire at feelingofgreen.ru>
wrote:

> Mark Roberts <markandrusroberts at gmail.com> writes:
> > Perhaps such warnings could be added to a tool like HLint? I do not
> > think they belong in GHC.
>
> People have circulated somewhat convincing examples that show how these
> Functor instances lead to real bugs.
>
> I doubt they would agree.
>
> Besides -- these warning are supposed to be opt-in at the use sites,
> aren't they?
>
> --
> с уважениeм / respectfully,
> Косырев Сергей
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20160225/acba8071/attachment.html>