July 2023 - ghc-devs - Haskell.org

Thread on Discourse - HIE file processing
by David Christiansen 10 Aug '23

10 Aug '23

Dear GHC devs, I think that having automated security advisory warnings from build tools is important for Haskell adoption in certain industries. This can be done based on build plans, but a package is really the wrong granularity - a large, widely-used package might export a little-used definition that is the subject of an advisory, and it would be good to warn only the users of said definition (cf base and readFloat). Tristan is exploring using HIE files to do this check, but I don't know if you read Discourse, where he posted the question: https://discourse.haskell.org/t/rfc-using-hie-files-to-list-external-declar… Thanks! David

3 4

[ANNOUNCE] GHC 9.8.1-alpha1 is now available
by Ben Gamari 28 Jul '23

28 Jul '23

The GHC developers are very pleased to announce the availability of the first alpha prerelease of GHC 9.8.1. Binary distributions, source distributions, and documentation are available at https://downloads.haskell.org/ghc/9.8.1-alpha1 GHC 9.8 will bring a number of new features and improvements, including: * Preliminary support the `TypeApplications` language extension [type-binders], allowing types to be bound in type declarations. * Support for the `ExtendedLiterals` extension, providing syntax for non-word-sized numeric literals in the surface language [extended-literals] * Improved rewrite rule matching behavior, allowing limited matching of higher-order patterns * Better support for user-defined warnings by way of the `WARNING` pragma [warnings] * The introduction of the new `GHC.TypeError.Unsatisfiable` constraint, allowing more predictable user-defined type errors [unsatisfiable] * Implementation of the export deprecation proposal, allowing module exports to be marked with `DEPRECATE` pragmas [deprecated-exports] * The addition of build semaphore support for parallel compilation; with coming support in `cabal-install` this will allow better use of parallelism in multi-package builds [jsem] * More efficient representation of info table provenance information, reducing binary sizes by over 50% in some cases when `-finfo-table-map` is in use A full accounting of changes can be found in the [release notes]. We would like to thank GitHub, IOG, the Zw3rk stake pool, Well-Typed, Tweag I/O, Serokell, Equinix, SimSpace, the Haskell Foundation, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do give this release a try and open a [ticket] if you see anything amiss. Happy compiling, ~ Ben [type-binders]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-d… [extended-literals]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0451-s… [unsatisfiable]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0433-u… [warnings]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0541-w… [deprecated-exports]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0134-d… [jsem]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0540-j… [release notes]: https://downloads.haskell.org/ghc/9.8.1-alpha1/docs/users_guide/9.8.1-notes… [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

1 0

NamedDefaults and relaxed defaults?
by Benjamin Redelings 26 Jul '23

26 Jul '23

Hi, If I understand correctly, the traditional defaulting rules prevent defaulting variables with constraints like (Num a, Convertible a Double), but the NamedDefaults proposal would allow defaulting a ~ Double in this case due to the relaxed defaulting rules in section 2.5 of the proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0409-e… 1. Is there any plan to start implementing NamedDefaults? I saw the comment from Simon P-J that it would not be fun to implement because it might require orphan default declarations... so perhaps there's no plan to implement this? 2. Would it be worth adding a separate LANGUAGE option that just implements the relaxed defaulting rules in section 2.5? Specifically (a) allowing variables with multiparameter constraints and (b) allowing variables with constraints that are not in the Prelude. 3. Am I correct in assuming that the relaxed defaulting rules require NamedDefaults to be enabled in the importing module, and not just in the imported model? -BenRI

2 3

GHC 9.8 release status
by Ben Gamari 25 Jul '23

25 Jul '23

Hi all, As you may have noticed, the GHC 9.8 alpha series is a bit behind the expected schedule due to an unfortunate constellation of CI issues, bugs, and backports. I am currently trying to get a set of releasable artifacts through CI although things are somewhat stalled until we find a way forward in resolving ghc/ci-config#6 [1], caused by a regression in Docker. My hope is that we can have a set of release artifacts by Thursday. When this happens I will make any necessary amendments to the release schedule [2] to account for this delay. Thank you for your patience! Cheers, - Ben [1]: https://gitlab.haskell.org/ghc/ci-config/-/issues/6 [2]: https://gitlab.haskell.org/ghc/ghc/-/milestones/379#tab-issues

1 0

Coercing a `ByteArray#` into a `MutableByteArray#`
by Adithya Kumar 24 Jul '23

24 Jul '23

Hello all, I have a use-case where I want to convert `ByteArray#` to `MutableByteArray#` without the copy overhead. Is the underlying memory structure of `ByteArray#` and `MutableByteArray#` the same? If I can guarantee that I won't mutate my `MutableByteArray#`, can I use `unsafeCoerce#` on `ByteArray#` and treat it as `MutableByteArray#`? Best, Adithya

4 3

RFC Or patterns syntax: (p1 | p2) vs. (p1; p2)
by Sebastian Graf 24 Jul '23

24 Jul '23

Hi devs, I would like to invite you to provide arguments for or against the Or patterns syntax RFC `(p1; p2)` vs. `(p1 | p2)` over at this GH issue <https://github.com/ghc-proposals/ghc-proposals/issues/598>. *In particular, `(p1 | p2)` has a small lead over `(p1; p2)`*, but the latter will steal syntax from a hypothetical guards-in-patterns extension `(p | e)` as described here <https://gitlab.haskell.org/ghc/ghc/-/wikis/view-patterns-alternative>. I dismissed this point until Vlad made me aware <https://github.com/ghc-proposals/ghc-proposals/issues/598#issuecomment-1600…> of the fact that `f (a -> b)` *could* mean "a pattern match on the type constructor `(->)`" in a Dependent Haskell future. Apparently, the existence of the hypothetical guards-in-patterns extension was reason enough <https://github.com/ghc-proposals/ghc-proposals/issues/598#issuecomment-1600…> to exclude view patterns from GHC2021. Of course, `(p1; p2)` has issues of its own. Given how close this vote is, I want to make sure that you, the GHC devs, are aware of this issue and perhaps have a few minutes to formulate a (counter) argument or just leave your vote before we submit the winner in the actual amendment proposal, at which point it will be up to the GHC steering committee to decide. Thanks, Sebastian

1 0

ghc-9.4.6 is being prepared
by Matthew Pickering 24 Jul '23

24 Jul '23

Hi all, Zubin is preparing the 9.4.6 release so we expect to release it within the next two weeks. The focus of the release ot back-port the quite large number of fixes which are marked for backport to 9.4. https://gitlab.haskell.org/ghc/ghc/-/merge_requests?scope=all&state=opened&… Cheers, Matt

1 0

Request for approval of account
by Mark Seemann 18 Jul '23

18 Jul '23

Hello I’d like to report a bug in GHC because an error message tells me to do so. In order to do that, I’m trying to follow the guidelines in https://gitlab.haskell.org/ghc/ghc/-/wikis/report-a-bug, so I’ve created an account. The next step, as I understand it, is to use this mailing list to request approval for he account. I’m hereby doing that. TIA Mark Seemann

1 0

Why do the reverse binder swap transformation?
by Rodrigo Mesquita 14 Jul '23

14 Jul '23

Dear GHC devs, I’m wondering about the reverse binder swap transformation, the one in which we substitute occurrences of the case binder by occurrences of the scrutinee (when the scrut. is a variable): case x of z { r -> e } ===> case x of z { r -> e[x/z] } My question is: why do we do this transformation? An example in which this transformation is beneficial would be great too. The Note I’ve found about it, Note [Binder-swap during float-out], wasn’t entirely clear to me: 4. Note [Binder-swap during float-out] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the expression case x of wild { p -> ...wild... } we substitute x for wild in the RHS of the case alternatives: case x of wild { p -> ...x... } This means that a sub-expression involving x is not "trapped" inside the RHS. And it's not inconvenient because we already have a substitution. Note that this is EXACTLY BACKWARDS from the what the simplifier does. The simplifier tries to get rid of occurrences of x, in favour of wild, in the hope that there will only be one remaining occurrence of x, namely the scrutinee of the case, and we can inline it. Many thanks, Rodrigo

3 3

Exact-print info in the the HsSyn syntax tree
by Simon Peyton Jones 13 Jul '23

13 Jul '23

Dear GHC developers Could you please look at #23447 Where should "tokens" live <https://gitlab.haskell.org/ghc/ghc/-/issues/23447>? In brief, the question is whether we want to have: data HsExpr p = .... | HsLet (XLet p) (HsLocalBinds p) (LHsExpr p) or data HsExpr p = .... | HsLet (XLet p) (HsToken "let" p) (HsLocalBinds p) (HsToken "in" p) (LHsExpr p) In the former, if a client wants HsTokes to track the precise source locations of the "let" and "in" keywords, they'd have to put it in the TTG extension field; in the latter, this information is in *every* syntax tree. At the moment we have some of each, which is not satisfactory. We need to decide a policy and stick to it. If you use HsSyn, HsExpr, HsPat etc, in any way, you should have an opinion. Please do express it. At the moment we have only a few voices so we risk deciding without enough evidence and use-cases. Comments with specific use-cases and examples would be particularly helpful. Thanks! Simon

2 3