June 2021 - ghc-steering-committee

Please review #392: Clarify modifiers design principle, Shepherd: Alejandro
by Joachim Breitner 28 Jan '22

28 Jan '22

Dear Committe, Clarify modifiers design principle has been proposed by Richard https://github.com/ghc-proposals/ghc-proposals/pull/392 This is an amendmend to #370, see the PR description for links to diffs etc. I propose Alejandro as the shepherd, as he shepherded #370 before. 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/

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Please review #319: NoFallibleDo proposal, Shepherd: Eric Seidel
by Joachim Breitner 08 Jan '22

08 Jan '22

Dear Committee, this is your secretary speaking: NoFallibleDo proposal has been proposed by Cale https://github.com/ghc-proposals/ghc-proposals/pull/319 https://github.com/obsidiansystems/ghc-proposals/blob/no-fallible-do/propos… I propose Eric Seidel 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/

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Proposal #281: Visible "forall" in terms; rec: accept
by Richard Eisenberg 01 Nov '21

01 Nov '21

Hi committee, Proposal #281 has been submitted for our consideration. Proposal PR: https://github.com/ghc-proposals/ghc-proposals/pull/281 <https://github.com/ghc-proposals/ghc-proposals/pull/281> Proposal text: https://github.com/int-index/ghc-proposals/blob/visible-forall/proposals/00… <https://github.com/int-index/ghc-proposals/blob/visible-forall/proposals/00…> The text of the proposal is long and detailed, but do not be daunted: it is simpler in practice than it appears. The main payload of the proposal is: Introduce a new extension -XRequiredTypeArguments. With this extension enabled, the `forall ... ->` syntax, currently in existence in kinds, is now available in types (of ordinary functions). This means that function definitions and call sites are sometimes required to write a type argument. The type argument is *not* preceded by @ or any other syntactic marker. This is useful for defining what would otherwise be ambiguous types. Example: > sizeof :: forall a -> Sizeable a => Int > sizeof = ... > > intWidth = sizeof Int There are further examples and motivation in the proposal. The rest of the proposal is simply about dealing with odd corner cases that come up with the main payload. In particular, mixing types in with terms with no syntactic signifier means that we must be careful about parsing and namespaces. If a type argument is written in the syntax that is shared between types and terms (including function application!) and uses identifiers in scope in only one of the two namespaces, nothing unusual can be observed. But, of course, there are corner cases. Here are some of the salient details: - Define type-syntax and term-syntax, where the choice of syntax is always driven by syntactic markers, such as :: or @. See the dependent types proposal <https://github.com/goldfirere/ghc-proposals/blob/dependent-types/proposals/…> for more details. Parsing and name-resolution are controlled by whether a phrase is in type-syntax or term-syntax. For name resolution, if a lookup fails in the first namespace (the term-level namespace in term-syntax or the type-level namespace in type-syntax), we try the other namespace before failing. - Because term- vs type-syntax is controlled by syntax, a required type argument is in *term*-syntax and gets name-resolved *as a term*. In the absence of punning, this works out fine, but it is possible that a punned identifier will cause confusion. The proposal includes section 4.3 allowing users to write `type` to signify a switch to type-syntax. - The proposal also includes a way to avoid punning for the built-in types with privileged syntax: lists and tuples. This method allows users to specify -XNoListTupleTypeSyntax to disable the list and tuple syntax in types (but still allows it for terms). The proposal also suggests exporting type List = [] from Data.List and other synonyms for tuples from Data.Tuple. --------------- I recommend acceptance. When doing type-level programming, the lack of this feature is strange, leading to ambiguous types and easy-to-forget arguments and easy-to-make type errors. The design space here is difficult, but this proposal is very much in keeping with the design sketch of our recently-accepted #378, in particular its section on this point <https://github.com/goldfirere/ghc-proposals/blob/dependent-types/proposals/…>. I believe the design described here is both backward compatible with what we have today (users who do not use this feature will not notice a difference) and forward compatible with a cohesive design for dependent types. There are several optional pieces: - The `type` herald <https://github.com/int-index/ghc-proposals/blob/visible-forall/proposals/00…>. I am unsure about this one, but others have felt strongly in favor, and I have no reason to object. - Types-in-terms <https://github.com/int-index/ghc-proposals/blob/visible-forall/proposals/00…>. I think this is necessary in order to avoid annoying definitions of type synonyms for one-off usage sites. It is a straightforward extension of the term-level parser to allow previously type-level-only constructs. It is necessary in order for us to achieve the vision of dependent types in #378. The only challenge here is that this requires us to make `forall` an unconditional keyword in terms. This does pose a backward-compatibility problem. I see, for example, that the sbv package exports a function named `forall`, so we may need to think more carefully about how to proceed here -- possibly by guarding the keyword-ness of `forall` behind the extension for some number of transitionary releases. - Lists and Tuples <https://github.com/int-index/ghc-proposals/blob/visible-forall/proposals/00…>. This section describes the -XNoListTupleTypeSyntax extension. I am not convinced that this change needs to be part of this proposal (thinking it belongs more in #270), but I do think we'll need it in the end. Is it OK to export new type synonyms from Data.List and Data.Tuple? Not sure, though I'd like these exported from some central place. What do others think? Thanks, Richard

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#283: Local modules (again), recommendation: accept
by Spiwack, Arnaud 18 Oct '21

18 Oct '21

Dear all, It is time to review the local modules proposal, from our own Richard Eisenberg, again. In fact it’s been time for a while, and I must apologise to Richard for not having understood that. Sorry, Richard. Link: https://github.com/ghc-proposals/ghc-proposals/pull/283 Since the last time, the proposal has been both simplified and made more precise. The main issues solved by this proposal are - The ability to export qualified names from modules - The ability to define functions with qualified names within a module ( *e.g.* you could have both T.f and U.f). - The ability to locally refer to qualified data as unqualified (suppose Set.map is in scope, then the proposal lets you write let import module Set in map f (map g some_set). I think these are three important issues, and the proposal makes a good case that they ought to be solved together. I also think that the specification is clear and doesn’t leave any dark corner. Caveat: I have contributed to writing the current specification, so I may be missing some flaws. The proposal does introduce a somewhat byzantine semantic difference between module X and module qualified X in export lists. This is to keep the current meaning of module X which is already not very intuitive (cf https://ro-che.info/articles/2012-12-25-haskell-module-system-p1 ). I think this is unavoidable. There are also 3 optional pieces of specification that we are asked to rule on (§4). I’m personally in favour of option 1. I’m weakly in favour of option 3 (with a preference for the variant described in Alternative 7). On the other hand option 2 seems to add a bit too much syntactic subtlety to be worth it (though if we choose option 2, I see no reason not to accept Alternative 3 as well). /Arnaud

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Please review #409: Exportable named defaults, Shepherd: Eric
by Joachim Breitner 30 Sep '21

30 Sep '21

Dear Committe, Exportable named defaults has been proposed by Mario https://github.com/ghc-proposals/ghc-proposals/pull/409 https://github.com/blamario/ghc-proposals/blob/exportable-named-default/pro… I propose Eric as the Shepherd. This did not gather a lot of attention on Github, or rather none, so Eric, maybe also consider whether this needs to be advertised more, or maybe who should be pointed to it. 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/

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#390: Fine-grained pragmas, recommendation: accept
by Vitaly Bragilevsky 18 Sep '21

18 Sep '21

Dear Committee, Our own Alejandro has been proposed Fine-grained pragmas for classes, families, and instances https://github.com/ghc-proposals/ghc-proposals/pull/390 https://github.com/serras/ghc-proposals/blob/instance-pragmas/proposals/000… His idea is to bring the flexibility of overlaps/overlapping/overlappable pragmas to termination checking, type inference, and constraint solving. Alejandro proposes to introduce the following %-modifiers (as in already accepted #370, https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0370-m… ): %NoTerminationCheck %LiberalCoverage %LiberalInjectivity %Overlapping %Overlappable %Overlaps to liberate conditions for classes and instances, type families, forall-types, etc. The first three modifiers can be used instead of the scary-sounding UndecidableInstances extension. The last three modifiers are supposed to be used instead of the overlap*-pragmas for instances we already have. Note that this proposal doesn't suggest deprecating those extensions and pragmas. I think that this proposal goes in the right direction and recommend accepting it. As far as I understand, all the committee members, including those with terms about to expire, have the right to either support this proposal or to raise a voice against it either here or at https://github.com/ghc-proposals/ghc-proposals/pull/390. Regards, Vitaly

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Proposal #412: Explicit Splice Imports; rec: accept
by Vladislav Zavialov (int-index) 14 Sep '21

14 Sep '21

Dear Committee, Proposal #412 "Explicit Splice Imports” by Matthew Pickering has been submitted for our consideration. Read it here: https://github.com/mpickering/ghc-proposals/blob/splice-imports/proposals/0… Discussion here: https://github.com/ghc-proposals/ghc-proposals/pull/412 The goals of this proposal are to (1) improve parallel compilation, (2) speed up -fno-code and IDEs that rely on it, (3) reduce binary sizes or at least reduce linking overhead, (4) create phase separation useful for cross-compilation. All of the above are achieved by introducing a new form of import called a splice-import. Compare: import M import splice H Under this proposal, names imported from M can *not* be used in top-level Template Haskell splices, whereas names imported from H can be used *only* in Template Haskell splices. At this point I will note that I do not particularly like the proposed syntax, which looks as if we’re importing a splice rather than importing names for use in splices. In the “Alternatives” section, you will find several other options such as “import {-# SPLICE #-} M”, “import for splice H”, “$(import H)”, and so on. That said, let us not get bogged down in the discussion of syntax before we agree on the semantics. If we choose to accept the proposal, I will organize a vote on the syntax. Regarding the semantics, I believe the general idea behind the proposal is good, and I recommend acceptance. It provides a way for the programmer to specify the intended usage of an import (whether it is for normal programming or metaprogramming), and GHC can use this information to build the program more effectively (more parallelism, less linking). In ways that I do not entirely understand, it is allegedly good for future work on cross-compilation. Let me know what you think. - Vlad

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Please review #400: Constrained COMPLETE sets, Shepherd: Cale
by Joachim Breitner 14 Sep '21

14 Sep '21

Dear Committee, this is your secretary speaking: Constrained COMPLETE sets has been proposed by Sebastian Graph https://github.com/ghc-proposals/ghc-proposals/pull/400 https://github.com/sgraf812/ghc-proposals/blob/constrained-complete-sigs/pr… This proposal tries to solve the same issue as Cale’s #399, and essentially has slightly different syntax. I therefore suggest that Cale is the shepherd, and hashes out with Sebastian the details of syntax so that they can both get behind it, and then makes a recommendation. 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/

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GHC proposals
by Simon Peyton Jones 12 Jul '21

12 Jul '21

Friends I'm a bit concerned that we are falling down on our commitment to decide about GHC proposals in a timely manner. Part of the problem is that at any moment I don't have a clear snapshot in my head of what decisions are pending, and who is driving them. I know that Joachim hates manual solutions, but I have spent a few minutes digging through my email to build * this spreadsheet giving the current status<https://docs.google.com/spreadsheets/d/1tt2BUtr6ws4YtdkM7f986pb8rid26-TyKxs…> You all have edit permissions. It covers only the handful of proposals that are in our court. Can I suggest that we all use it to keep ourselves on the ball? E.g. as a shepherd you can use it to record who you are waiting for, as I have done for #302. You'll notice that we are behind on every one of them. Remember, if there edits we want the author to make, we push it back, out of our court. It can re-enter when the author re-submits. If our commitments are over-ambitious, let's review them. Tom: you are our official nudger. Would you like to make you weekly nudge into an email to the full committee, with a pointer to the spreadsheet and your current understanding of who is responsible for driving? I hope this is helpful. If not, let's think of something else! Simon

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Proposal #302: `\of`
by Richard Eisenberg 01 Jul '21

01 Jul '21

Hi all, Proposal #302 was submitted to the committee and assigned to Cale. He has made a recommendation on the GitHub trail, but I don't believe the committee has discussed this among ourselves. PR: https://github.com/ghc-proposals/ghc-proposals/pull/302 Proposal: https://github.com/JakobBruenker/ghc-proposals/blob/patch-1/proposals/0000-… Cale's recommendation: https://github.com/ghc-proposals/ghc-proposals/pull/302#issuecomment-666075… The idea, in brief, is to introduce a new syntax (guarded behind -XMultiWayLambda) \of. Here is an example, which gives you the idea: mplus :: Maybe Int -> Maybe Int -> Maybe Int mplus = \of Nothing _ -> Nothing _ Nothing -> Nothing (Just x) (Just y) -> Just (x + y) The new keyword allows us to use a syntax similar to function definitions, but without repeating the name of the function. It is also like \case, but it allows multiple arguments. Guards are allowed, as usual. I really like this new syntax -- mostly because I find it very strange that we have to repeat the function name on every line. And then change the spacing when we change the function name. And I like the mnemonic "lambda of". And it allows me to write a where clause that is accessible in multiple different patterns, or an indented where clause that is usable in just one. If it didn't confuse readers, I would use this syntax all the time. Even so, I agree with Cale's recommendation to reject. We just have too much syntax! If someone were to come along and draft a concrete proposal of how we could, for example, use this syntax to replace both \case and if|, with a migration strategy, etc., then I might be in favor. Until then, I think we've spent our budget for cute, obscure bits of syntax. Richard

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