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

1. If the proposed amendment is rejected, we /still/ have to change template-haskell to implement 425 in its current form. The specification allows invisible wildcards `@_`, which can't be represented in template-haskell at the moment. So I'd like to ask voting members to take that into consideration: this is not an "unforced change" because there is a change coming either way.

Are you sure? We could, if we chose, just (continue to) not support "_" in TH. People generating TH code can always use a fresh variable instead. I'm still leaning towards "do nothing"; and if we don't want that, then "do the minimum" (ie the non-recursive form). We have so much complexity already, I don't want to add more. Simon PS sorry to be slow on this On Mon, 22 Apr 2024 at 21:26, Adam Gundry wrote:

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 mailto:malte.ott@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?

-- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/

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