- ghc-devs - Haskell.org

NamedDefaults and relaxed defaults?
by Benjamin Redelings 27 Jul '23

27 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

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GHC 9.8 release status
by Ben Gamari 26 Jul '23

26 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

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Coercing a `ByteArray#` into a `MutableByteArray#`
by Adithya Kumar 25 Jul '23

25 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

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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

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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

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Why do the reverse binder swap transformation?
by Rodrigo Mesquita 15 Jul '23

15 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

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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

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New spurious failure bot is running on GitLab
by Bryan Richter 13 Jul '23

13 Jul '23

Today I've deployed a new version of my retry service. I think I've worked out most of the kinks, so I'm going to leave it running overnight and see how it goes. In the worst case, it might crash and die, in which case spurious failures will not get retried until I come back and fix it tomorrow morning, Europe time. In the best case, the service will just keep doing what the old one did, with two improvements: 1. If the same job keeps failing, it will give up retries. 2. If sending a retry command to GitLab itself fails, those will also be retried a few times. -Bryan "Who retries the retryer?" - Juvenal, probably

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Usage of Template Haskell quotes in GHC source tree vs. usage of GHC as a library
by Gergő Érdi 13 Jul '23

13 Jul '23

Hi, A recent commit 983ce55815f2dd57f84ee86eee97febf7d80b470 starts using TemplateHaskellQuotes in the GHC codebase. It seems this is at odds with using GHC as a library, a la ghc-lib. The `ghc-lib` approach is to basically take the module hierarchy from the `compiler/` subtree, and compile it as a completely vanilla Haskell library, with no direct attachment to the host GHC version. This enables using e.g. GHC 9.4 to compile a program using the GHC 9.6 API, and so on. In particular, it also makes it very easy to apply patches to the version of GHC used as a library, since in this setup it doesn’t need to be able to bootstrap. So what is the problem with using TemplateHaskellQuotes? The problem is the dependency on the template-haskell package. When a module inside GHC-as-a-library containing TH quotes is compiled, the quotes are translated into applications of the constructors defined by the *host* GHC’s TH package. But because GHC is tightly coupled to the TH support library, GHC-as-a-library needs to ship with its own internal version of the library. So the code that tries to process the results of these quotes is using the *target* GHC’s TH definitions. And that leads to a conflict: code like leftName :: Namel leftName = ‘Left is now a type mismatch between the type of `’Left` being template-haskell-2.19.0.0:Language.Haskell.TH.Syntax.Name (example when using GHC 9.4.5 as the host) and the type of `leftName` being ghc-lib-9.9.20230712:Language.Haskell.TH.Syntax.Name (example when the target version is built from recent `master`). Currently, `ghc-lib-gen` has a pre-processing step on the GHC source tree that replaces these quotations with applications containing direct references to the target TH constructors: https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd… but I am worried that this is very fragile. So any ideas on how to tackle this situation better? Thanks, Gergo

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Can't build nofib
by Simon Peyton Jones 12 Jul '23

12 Jul '23

Friends With a clean HEAD I can't build nofib. See below. What should I do? Thanks Simon (cd nofib; cabal v2-run -- nofib-run --compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`) Resolving dependencies... Error: cabal: Could not resolve dependencies: [__0] trying: nofib-0.1.0.0 (user goal) [__1] next goal: base (dependency of nofib) [__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib => base>=4.5 && <4.17) [__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same characteristics that caused the previous version to fail: excluded by constraint '>=4.5 && <4.17' from 'nofib') [__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0, base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0, base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0, base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0, base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1, base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0, base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2, base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2, base-3.0.3.1 (constraint from non-upgradeable package requires installed instance) [__1] fail (backjumping, conflict set: base, nofib) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: base, nofib make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1

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