[ghc-steering-committee] Proposal #608, -XPatternSignatureBinds. Rec: accept

[Eric Seidel]

The motivation for this change does not make sense to me. ScopedTypeVariables
is used everywhere and is even part of GHC2021. The goal of this change appears
to be to extract out a controversial piece of PatternSignatures such that it
could be later removed entirely, but without addressing the issue of
ScopedTypeVariables.

That's going about it the wrong way IMO. If we want to remove the behavior
this proposal calls PatternSignatureBinds, the Committee should first adopt
a resolution that we will remove (or alter the meaning of) ScopedTypeVariables
in the next GHC20xx. If we get agreement on that, then this proposal could make
sense as a step towards the broader goal.

Outside of that broader agreement though, this feels more like a fork-inducing
proposal and I would vote against.

On Wed, Jan 24, 2024, at 11:32, Richard Eisenberg wrote:
> John Ericson has submitted proposal #608 
> <https://github.com/ghc-proposals/ghc-proposals/pull/608> about adding 
> -XPatternSignatureBinds. The proposal is an amendment and is thus a 
> little harder to consume. I recommend reading the summary here.
>
> Proposed change:
>
> The proposed -XPatternSignatureBinds takes a piece out from 
> -XPatternSignatures: with only -XPatternSignatures and not 
> -XPatternSignatureBinds, a pattern signature  can mention in-scope type 
> variables but can never bind fresh ones. With -XPatternSignatureBinds, 
> an appearance of an out-of-scope type variable in a pattern signature 
> will cause the variable to be bound to the type it unifies with.
>
> -XScopedTypeVariables will imply -XPatternSignatures 
> -XPatternSignatureBinds (and a few more), so the behavior of 
> -XScopedTypeVariables is completely unchanged. Note that 
> -XPatternSignatures is currently deprecated (and has been so for a long 
> time), so a change in behavior there is not so bad.
>
> Motivation for the change:
>
> - It is awkward to have a rule in a language where scoping behavior 
> depends on what else is in scope. This can confound e.g. tools meant to 
> find the bindings sites of a variable occurrence.
> - The implicit binding we have today plays poorly with the plan to 
> allow users to pretend GHC has a unified namespace. That is, suppose x 
> is in scope as a term variable, and then we have f (... :: ... x ...) = 
> ... . Should that x be bound implicitly there or not? It's pretty 
> unclear. By separating out -XPatternSignatureBinds from 
> -XPatternSignatures, users of the latter are insulated from worrying 
> about this potential future change.
>
> My opinion:
>
> - John has argued that this will make future namespace changes easier. 
> I disagree with this assessment, because -XScopedTypeVariables is 
> utterly pervasive. So the problem that this change is supposed to solve 
> becomes only a tiny bit smaller, benefiting only those users who 
> carefully enable -XPatternSignatures but not, say, 
> -XScopedTypeVariables.
> - But in the end, I don't care all that much. I've come to the opinion 
> that we shouldn't worry about the distinction between warnings and 
> extensions. The net effect of this proposal is to promote a warning 
> into an extension (because we previously had 
> -Wpattern-signature-binds). So if we shouldn't worry about this 
> distinction, then this proposal is a no-op on the aspects of the 
> language we should care about, and thus accepting and rejecting have 
> the same effect. (This little analysis only makes sense because the 
> features are so new -- there's no broad re-education to do or 
> back-compat issues.)
> - Accepting will make at least one person (John) happy. And I don't 
> note anyone who would become unhappy. More happy people is more better. 
> So I recommend acceptance. :)
>
> Please let us know your thoughts!
> Richard
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