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

[Richard Eisenberg]

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 <mailto: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 <mailto: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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