February 2024 - ghc-steering-committee

Proposal #631: Set program exit code by main return type, recommendation: accept something
by Arnaud Spiwack 03 May '24

03 May '24

Dear all, Shea Levy proposes to do something with the values returned by `main` https://github.com/ghc-proposals/ghc-proposals/pull/631 . The problem is that `main` is allowed to be of type `IO A` for any `A`. And GHC will simply drop the value returned by `main`. Shea contends that it's surprising. I agree that dropping a value without the compiler being explicitly instructed to is surprising. But Shea says that when `A` is `ExitCode` this is even more surprising. Namely `main :: IO ExitCode; main = return $ Failure 1` actually terminates with exit code 0. And I doubt that it's what anybody expects when reading the code. The proposal is simple, but I have a lot of comments on it. Sorry about that… Now, this sort of proposal is tricky. When the current behaviour is confusing, we want to change the default. But putting the new default behind an extension doesn't really solve the fact that there's a trap. The extension is, therefore, unlikely to be well tested before it becomes part of the next language edition. Shea's main proposition doesn't actually use an extension though. He adds a type class `ExitStatus`, and if `ExistStatus A`, then `main :: IO A` uses the instance to determine the exit code based on the return value. The only change to the current behaviour is that `main :: IO ExitCode` instead of always terminating with exit code 0 when returning now terminates with the expected error code. The argument for not putting this behind an extension is that virtually anybody affected by the change will actually have the behaviour they were expecting. But maybe the argument isn't strong enough (the changes may be more “interesting” if some library exports some `ExistStatus` instance). This design of this proposal is inspired by Rust's design. I've asked our Rust team, and they certainly seem to have internalised the idea of returning an exit code. It really seems a pretty natural feature to have. So I'm rather in favour of some flavour of the type class implementation. Though have a look at the alternatives, where you'll find other approaches such as restricting the type of `main` to unsurprising types. One caveat with respect to the main proposal: it is proposed that when no `ExistStatus A` is found, then we drop the returned value like today. I don't know that it's quite easy to implement this behaviour. But it can be recovered by a catch-all overlapping instance, so maybe it's a better way to specify the desired behaviour. -- Arnaud Spiwack Director, Research at https://moduscreate.com and https://tweag.io.

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#632: Phased introduction of GHC2024
by Adam Gundry 22 Apr '24

22 Apr '24

Dear committee, In #632, I propose amending the GHC2024 proposal to specify that the default language used by ghc/ghci when run directly will remain GHC2021 for now, since changing to GHC2024 is not backwards compatible. (This does not affect Cabal packages either way, since Cabal specifies its own default.) https://github.com/ghc-proposals/ghc-proposals/pull/632 https://github.com/adamgundry/ghc-proposals/blob/ghc2024-amendment/proposal… On the discussion thread, some people expressed a preference that GHC should default to the latest language edition anyway. There is also Richard's suggestion of wider changes of approach in #636. However, given that the GHC 9.10 fork date is fast approaching, introducing GHC2024 but not making it the default seems like the best short-term solution to me. We can always reassess our approach to this for future releases as part of the wider discussion. If you object to the proposed approach, please speak up ASAP. Otherwise I plan to merge in a week or so. Cheers, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England

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Fine-Grained Unused Warnings (#42)
by Chris Dornan 19 Apr '24

19 Apr '24

Proposal: Fine-Grained Unused Warnings (#42) Author: Jakob Brünker Rendered proposal: https://github.com/JakobBruenker/ghc-proposals/blob/fine-grained-unused/pro… Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/434 Recommendation: Acceptance ## Summary The proposal partitions warning about unused identifiers into a) bindings that are truly unused (not mentioned anywhere) and b) bindings that are mentioned exclusively in code that is itself (transitively) unused, and suppresses the latter unless the -freport-indirectly-unused-bindings flag is specified. The proposal is concise with some well-chosen examples and I recommend everyone at least skim it. I propose that we accept this proposal if nobody objects by the start of next week (Monday, 2024-02-19). Chris

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Please review #617: Add -experimental flag, Shepherd: Simon M
by Adam Gundry 22 Mar '24

22 Mar '24

Dear Committee, Moritz proposes to add a new flag, -experimental, to guard experimental features: https://github.com/ghc-proposals/ghc-proposals/pull/617 https://github.com/angerman/ghc-proposals/blob/std-experimental/proposals/0… I’d like to nominate Simon Marlow as the shepherd. Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process Cheers, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England

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Proposal #606: small syntax change around types in terms; recommendation: vote
by Richard Eisenberg 26 Feb '24

26 Feb '24

Proposal #606 <https://github.com/ghc-proposals/ghc-proposals/pull/606> has been submitted to the committee, by Vlad. This proposal makes a tiny change. Suppose we have `f :: forall (x :: Type) -> x -> ...` (this is part of -XRequiredTypeArguments). Then (as proposed) we can write `y = f (type Int -> Int) abs ...`. The `type` keyword here is to signal that what comes is a type: it should be parsed as a type and name-resolved as a type. This might matter if there is, say, a constructor Int in scope. The proposal at hand is whether to accept the above program or to require extra parentheses, thusly: `y = f (type (Int -> Int)) abs ...`. That's it -- that's the entire proposal. (It says we should _not_ accept the former, without parentheses.) The motivation is to avoid surprising users. Normally when we have `word1 word2 -> word3`, word1 and word2 will associate more closely than the arrow. Yet the first example has `type Int -> Int` where the arrow binds more tightly. Because `type` is a keyword, this is no challenge to parse and is not ambiguous -- it's just perhaps confusing to users. There was some debate in the proposal, but in my perusal, not a clear indication toward any particular direction. The most rigorous statement I could find is that the new syntax is a subset of the original, and so we can easily reverse this decision later. I propose we vote on the matter. We have two choices: 1. Original syntax: allow `(type Int -> Int)` as an argument. 2. Amended syntax: require `(type (Int -> Int))` as an argument. -------------------------------- My vote: 1. There are many keywords in Haskell, and we are used to parsing these differently. For example, if we have `f (do x <- blah ...)`, we know quite well that the <- is within the `do`, not the other way around. Ditto `case`: we don't require scrutinees to be parenthesized. I posit that the strangeness some have felt around `(type Int -> Int)` is (understandable) confusion in the face of novelty. But the `type` herald will not be novel forever, and I think we'll enjoy having fewer parentheses in our code in the long run. (I might be arguing for "2 today, then 1 tomorrow". But let's just skip the intermediate step and do 1 now.) I welcome your opinions and votes. It would be great to conclude this in the next 2 weeks, by Feb 7. Thanks! Richard

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Proposal #608, -XPatternSignatureBinds. Rec: accept
by Richard Eisenberg 26 Feb '24

26 Feb '24

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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Please review #592: Scoped kind variables in standalone kind signatures
by Adam Gundry 25 Feb '24

25 Feb '24

Dear Committee, Vlad proposes to amend StandaloneKindSignatures to support scoped kind variables: https://github.com/ghc-proposals/ghc-proposals/pull/592 https://github.com/int-index/ghc-proposals/blob/scoped-saks/proposals/0054-… I'd like to nominate Eric as the shepherd. Please guide us to a conclusion as outlined in https://github.com/ghc-proposals/ghc-proposals#committee-process Cheers, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England

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Proposal #612: Change semantics of -fhpc to not accumulate data over multiple runs, rec: accept
by Moritz Angermann 25 Feb '24

25 Feb '24

Dear Committee members, In Proposal #612 <https://github.com/ghc-proposals/ghc-proposals/pull/612>, David Binder suggests changing the semantics from `-fhpc` to auto-ingest existing .tix files for accumulation, towards _not_ auto-ingesting them and instead overwriting the .tix file with the coverage of the latest run. I agree with his assessment that the current behaviour easily leads to unexpected surprises. He suggests adding a flag --read-tix-file= to control this behaviour and defaulting that to _no_, with a grace and deprecation period prior informing the user that the currently accumulating feature will change in a future GHC release. Adding the flag to control this behaviour is fairly uncontroversial I hope, however I'd like you to. Weight in on the default. Should we change the default behaviour, or only add the flag? I'd recommend changing the default to --read-tix-file=no after a deprecation period. Best, Moritz

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Shepherding
by Simon Peyton Jones 22 Feb '24

22 Feb '24

Dear shepherds We still have eight GHC proposals in our inbox, some dating back quite a long way. I know that progress is being made on some (thank you), but others seem to have stalled. Please could you make time to push our debate forward? It need not take long. But if the shepherd does not push onward, no one else does, and we really owe our authors a timely response. I would much prefer not to have to nag you. I feel bad doing so; but I also feel bad not doing so. Thanks! Spreadsheet here <https://docs.google.com/spreadsheets/d/1e6GdwHmAjeDEUhTvP-b18MDkpTfH3SMHhFu…>. Do keep it up to date. Simon #434 Unused warnings Chris 2 Jul 2023 #608 -XPatternSignatures without implicit binds Richard 17 Aug 2023 #612 -fhpc tix file changes Moritz 27 Sep 2023 #606 Amendment to #281: atype not ktype Richard 29 Nov 2023 #617 Add -experimental flag Simon M 25 Jan 2024 #631 Type of main fixes return code Arnaud 27 Jan 2024 #592 Scoped kind variables in SAKS Eric 8 Feb 2024 #632 Phased introduction of GHC2024 Adam 15 Feb 2024

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Proposal #624 on linear let-bindings; recommendation: accept
by Richard Eisenberg 13 Feb '24

13 Feb '24

Hi all, I've reviewed Arnaud's https://github.com/ghc-proposals/ghc-proposals/pull/624 <https://github.com/ghc-proposals/ghc-proposals/pull/624> and wish to recommend acceptance. The proposal is an amendment to the proposal for linear types, adding support for linear let bindings. Today, if you have f :: T %1-> T f t = let t2 = t in t2 you'll get an error because t2 is not linear. The only way to bind a linear variable is via a `case`, never a `let` or `where`. This is annoying. With this proposal, the little program above is accepted, with an inferred linearity restriction on t2. Users can also annotated their lets like `let %1 x = ... in ...`. Bindings in `where` clauses can also be inferred or annotated as linear. There is a downside, of course: linear bindings have various restrictions, chiefly that they must be strict bindings (because projections are hard with linear types) and that bindings cannot be generalized. I'm a little unsure that the choices in the proposal (particularly around generalization) are the best for users, but I think the best way to learn is to experiment. In my understanding, the community knows that -XLinearTypes is subject to revision, and so I think we should just blast ahead, revising if and when necessary. Please share your thoughts! Thanks, Richard

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