[ghc-steering-committee] #371: Stop treating ~ magically. Rec: weak accept

[Spiwack, Arnaud]

I'm still not sure what problem this proposal solves. The proposal alludes
to some code simplification but doesn't give me a good idea of how much
simpler this would make the code.

Also note that -XGADT becomes mostly vacuous with this proposal:
-XGADTSyntax + -XExistentialTypes imply GADTs (both are in -XGHC2021). Not
that I mind, really, I actually probably prefer it this way. But it's
worthy of notice.

On Thu, Apr 22, 2021 at 4:10 AM Richard Eisenberg <rae at richarde.dev> wrote:

> Hi committee,
>
> This proposal has been updated to include exporting (~) from the Prelude.
> This means that migration is much easier than it was previously, likely not
> requiring any import changes (or just in a project's custom Prelude). Users
> will have to enable -XTypeOperators (and could choose to disable -XGADTs or
> -XTypeFamilies). With this new export, I'm more firmly in favor of cleaning
> up this one little corner: let's accept.
>
> I will wait one week for any comments, and then will accept this proposal.
>
> Thanks,
> Richard
>
> On Apr 1, 2021, at 2:37 PM, Richard Eisenberg <rae at richarde.dev> wrote:
>
>
>
> On Apr 1, 2021, at 3:59 AM, Vladislav Zavialov (int-index) <
> vlad.z.4096 at gmail.com> wrote:
>
> Exporting from Prelude is not a panacea: without a compatibility fallback,
> users of custom preludes will be affected. The proposed migration story
> means that for 8 releases, no existing code will break at all.
>
>
> Right. I wasn't suggesting a change to the migration plan -- just that (~)
> would additionally be exported from the Prelude. The warning would trigger
> only when (~) isn't in scope, meaning that many users won't need to react
> to this change (given that -XTypeOperators is now on by default).
>
> As for the specialness of ~: yes, it's special, but we export other
> special things from the Prelude, like $ (which automatically enables
> -XImpredicativeTypes).
>
> On Apr 1, 2021, at 9:59 AM, Alejandro Serrano Mena <trupill at gmail.com>
> wrote:
>
> I meant the usual convention that type constructors must start with a
> capital letter or the : symbol. But in this case I had forgotten that with
> TypeOperators on, those conventions behave differently for types than for
> terms.
>
>
> This used to be true, but the requirement that the first letter be a
> capital letter or : was dropped some time ago -- before I started using
> Haskell, in any case. It is independent of -XTypeOperators, which is
> necessary for any infix type level syntax.
>
>
>
> But those custom preludes would have to be updated anyway, right?
>
>
> Yes, they would.
>
> Richard
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