[ghc-steering-committee] Please review #641: Wildcard binders in type declarations

[Adam Gundry]

I find Vlad's argument convincing: if we are already adding support for 
@_ then at the very least it's worth adding _ at the same time, and it 
seems to involve no more breakage or implementation cost than #425 
unamended. So I vote to accept.

I'm on the fence as to whether to prefer the recursive version (more 
general and consistent with term syntax) or the non-recursive version 
(since it is simpler, and in practice the more general forms seem 
unlikely to be useful).

Adam


On 19/04/2024 17:17, Vladislav Zavialov wrote:
> That's exactly right. We are not choosing between change / no change, we 
> are choosing between three possible changes:
> 
> 1. Current proposal: only add support for @_
> 2. Amendment sans recursion (if revised): add support for @_, @(_ :: k), 
> _, and (_ :: k)
> 3. Amendment with recursion: add support for arbitrary combinations 
> of @, _, ::, and ( ... )
> 
> It's going to be breaking in all three scenarios, unless we come up with 
> a compatibility layer using pattern synonyms as Adam suggests (I have 
> not investigated the feasibility of that).
> 
> Vlad
> 
> On Fri, Apr 19, 2024 at 5:59 PM Malte Ott <malte.ott at maralorn.de 
> <mailto:malte.ott at maralorn.de>> wrote:
> 
>     Thanks for the input Vlad. Regarding the breaking change to TH:
>     Do I understand you correctly that the required changes from 425
>     have not landed
>     in 9.10 and therefor accepting this proposal will not create anymore
>     breakage,
>     even between 9.10 and 9.12?


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