June 2022 - ghc-steering-committee

#511 Deep Subsumption, recommendation: accept
by Spiwack, Arnaud 02 Jan '23

02 Jan '23

Dear all, The Deep Subsumption proposal [ https://github.com/ghc-proposals/ghc-proposals/pull/511 ] proposes a new extension, -XDeepSubsumption, which, when activated, partially reverts the changes from the Simplified Subsumption proposal. The Simplified Subsumption breaks more programs than anticipated. Many don't see any benefit from Simplified Subsumption, just the breakage, and don't like the eta-expansion that it forces. -XDeepSubsumption, when activated, restores deep skolemisation and co/contra-variance of the function arrow (but not deep instantiation, which doesn't affect the observed breakage). The patch already exists for it, and is about 400loc. There are two interesting highlights for me. - It is proposed that -XDeepSubsumption is activated by default in Haskell98 and Haskell2010, but not GHC2021. -XDeepSubsumption is orthogonal to Haskell2010, as far as I can tell, but it gives a cut-off point from which the recommended behaviour (-XNoDeepSubsumption) is the default. - Even with -XDeepSubsumption, the Quick Look algorithm assumes that the function arrow is invariant. The consequences of that are difficult to anticipate, but there is no known example of a bad behaviour due to that interaction yet. The authors also have one unresolved question that I'm bringing to the committee's attention: should `-XDeepSubsumption` be backported to GHC 9.2? --- Despite the fact that this extension is decidedly fork-like, and that it's a real possibility to see the community split around this (after all, the motivation for -XDeepSubsumption is a few libraries which were designed to leverage GHC's deep subsumption, and may very well stay that way in the foreseeable future). I recommend acceptance. Providing a path to backward compatibility seems to me like the right thing to do. I also recommend backporting to GHC 9.2. It should essentially be backward compatible, and providing an update path that doesn't go directly from 9.0 to 9.4 feels better to me.

5 8

Please review #500: Add implicit import proposal, Shepherd: Baldur
by Joachim Breitner 05 Sep '22

05 Sep '22

Dear Committee, Add implicit import proposal has been submitted by Tristan de Cacqueray This is an interesting one: At first glance it looks quite un- haskellish, but it’s hard to poing out what’s, if anything, is wrong with it. I expect that the discussion will revolve not so much about technical issues, but more about best practices and our vision for Haskell’s look and feel. https://github.com/ghc-proposals/ghc-proposals/pull/500 https://github.com/TristanCacqueray/ghc-proposals/blob/implicit-import/prop… I suggest that Baldur, as our newest committee member, gets to shepherd this one. Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process Thanks, Joachim -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

6 9

Modern Scoped Type Variables #448: recommendation (mostly) accept
by Spiwack, Arnaud 11 Jul '22

11 Jul '22

Dear all, Let me bring to your attention the Modern Scoped Type Variables proposal, by our own Richard https://github.com/ghc-proposals/ghc-proposals/pull/448 . The proposal is a touch intimidating, because the text is large. But most of it comes from other, already accepted, proposals that this proposal is tidying up and tying together. What this proposal tries to achieve is to make a consistent text about all the recent changes to binding type variables. The proposal adds new principles to the `principles.rst` files which inform the changes proposed, of these, I have the following comments: - The Visibility Orthogonality Principle doesn't seem to have a very clear definition. It may be a sign that it's not something that is so important - The Explicit Binding Principle says that we need to be able to enforce that all type variables have a binding site. I think it's a bit strong: I would like Haskell to let me write a binding site for every type variable, but not necessarily to error out when that doesn't happen. That being said, I'm happy with warnings to help me along the way, but I don't think that this Explicit Binding Principle should be phrased in a way that requires adding extensions to control this behaviour. - The Contiguous Scoping Principle, which states that a binder binds in one contiguous region sounds dubious to me. I don't see a particular reason for this to be. --- In the proposed changes themselves: up to Section 5, the proposed changes are mostly existing accepted proposals, cleaned up with what was learnt since they were written, as well as to adhere to the new principles. These are mostly uncontroversial and the specification makes sense to me. I'm in favour of accepting all that. With the caveat that this proposal introduces quite a few extensions. And at this point, I'm still not quite sure what Richard recommends is the set of extensions that I should use (and I'm slightly dismayed that I believe that it will be a set of cardinal more than 1). I think this reflects a vision of extensions as switches to customise the behaviour of GHC. This vision, as I've stated before, is very alien to me: I see extensions as staging areas for features to become an integral part of Haskell. So I don't know what to think of all these extensions. I'm definitely not against splitting -XScopedTypeVariables into smaller components, if it is done so that they are reassembled in a different way in an alternative extension that would now be the recommended default (or at least is to become the next recommended default). Finally, there are Sections 6 to 8. These are entirely new. Though they are working towards the new principles (well, as far as I can tell, Section 6 doesn't contribute to the principles, but it is a stepping stone for both Sections 7 and 8). These sections are concerned with adding local let-bindings of type variables, in particular inside types and patterns. By the way, Section 7 proposes two syntaxes for let binders in patterns, and I *strongly* prefer the second syntax, which reads something like `f (let b = Bool) (True :: Bool) = …`. Anyway, these are new, I feel that they are a bit out of place in a proposal that is about tidying up the existing designs. That being said, they are here, and they seem like fairly uncontroversial to me, (except, probably the syntax `(let b = _)` to bind a variable to a type to be filled by the compiler). I'm fine with accepting these, though they may require a bit more scrutiny than the rest. Best, Arnaud

6 20

Please review #496: Nothing {}, Shepherd: Eric
by Joachim Breitner 29 Jun '22

29 Jun '22

Dear Committee, Empty records with {} have been proposed by John Ericsson https://github.com/ghc-proposals/ghc-proposals/pull/496 https://github.com/Ericson2314/ghc-proposals/blob/empty-record-wildcards/pr… This seems to be about straightening a corner case, so to say. I suggest Eric as the shepherd. Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process Thanks, Joachim -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

6 6

Please review: #454 Custom type warnings
by Tom Harding 25 Jun '22

25 Jun '22

Hi all, As Joachim noted, #454 <https://github.com/ghc-proposals/ghc-proposals/pull/454> is extracted from Adam’s previous efforts with the Unsatisfiable constraint <https://github.com/ghc-proposals/ghc-proposals/pull/433> proposal (which has since been accepted). In short, it covers an interface for custom warnings, and I can already think of plenty of ways I’d use it. I’d therefore like to recommend acceptance, and discuss any issues or changes. Thanks, Tom

6 9

GHC Steering Committee Status
by Joachim Breitner 13 Jun '22

13 Jun '22

Dear Committee, it’s about time for my ~monthly~ ~quarterly~ bimonthly status update So what has happened since the last one? * I proposed that we proposals upon submission ought to have some indication that there is someone willing and able to implement it, please join the discussion on https://github.com/ghc-proposals/ghc-proposals/pull/517 * we were asked to review these proposals: #500: Add implicit import proposal, Shepherd: Baldur #473: First-class existential types, Shepherd: Simon PJ #496: Nothing {}, Shepherd: Eric * we have a recommendation from the shepherd about: #496: Nothing {}, rec: accept * we have sent the following proposals back to revision none, it seems * we decided about the following proposals #451: Sized literals proposal (accept) We currently have to act on the following 6 proposals, up 2 since last update: ## Waiting for committee decision #454: Custom type warnings, Shepherd: Tom 2022-01-13: Recommendation to accept 2022-06-01: Arnaud says objections are minor. Tom, do you think we have consensus here? #448: Modern Scoped Type Variables, Shepherd: Arnaud 2022-03-29: Arnaud makes a non-comprehensive recommendation 2022-06-07: Ongoing discussion #496: Nothing {}, Shepherd: Eric 2022-06-12: Eric suggests acceptance ## Waiting for Shepherd action #270: Support pun-free code, Shepherd: Chris 2022-01-01: Assigned to Chris Chris, kindly please leap into action! #500: Add implicit import proposal, Shepherd: Baldur 2022-05-05: Assignd to Baldur #473: First-class existential types, Shepherd: Simon PJ 2022-05-16: Assigned to Simon Cheers, Joachim -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

2 2

Please review #473: First-class existential types, Shepherd: Simon PJ
by Joachim Breitner 07 Jun '22

07 Jun '22

Dear Committee, First-class existential types has been submitted by Richard https://github.com/ghc-proposals/ghc-proposals/pull/374 https://github.com/goldfirere/ghc-proposals/blob/existentials/proposals/047… Exciting stuff! This feels, at least to me, like a proposal with major implications – not least because I believe it means that type equality will now involve some form of term equivalence, and may thus affect what kind of Core transformations are still type-preserving. Therefore I would suggest Simon PJ to shepherd this proposal. Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process Thanks, Joachim -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

2 1