- ghc-devs - Haskell.org

Critical code generation bug with GHC 9.12.3
by Zubin Duggal 08 Jan '26

08 Jan '26

Hello all, Unfortunately we discovered a major code generation regression in GHC 9.12.3 that results in incorrect runtime results for certain operations involving sub-word divisions. We recommend avoiding upgrading to this minor release at this time. See https://gitlab.haskell.org/ghc/ghc/-/issues/26711 for more details. This is the same bug that affected GHC 9.12.1. A patch re-introducing this bug was accidently backported onto the 9.12.3 branch and went unnoticed throughout the testing period. We are taking steps to ensure similar situations are less likely to occur again, including looking at ways to improve our testing procedure for minor GHC release, potentially using the Stackage infrastructure to run the testsuites of a broad range of packages to detect such issues. We are also starting to prepare GHC 9.12.4 with a fix, which we hope to release soon. However, a significant amount of time has passed since the initial set of backports to GHC 9.12.3 were completed, which means that a number of merge requests addressing issues of various severities has been marked for backporting to the GHC 9.12 branch. We are in the process of triaging this list and preparing a robust and reliable GHC 9.12.4 release. Thanks, Zubin.

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Static forms deprecation warning
by Simon Peyton Jones 03 Jan '26

03 Jan '26

Zubin The accepted GHC proposal 732: Simplify static forms <https://github.com/ghc-proposals/ghc-proposals/pull/732> needs a deprecation warning added to GHC 9.14, to warn about static forms that mention nested free variables. Ticket #26718 <https://gitlab.haskell.org/ghc/ghc/-/issues/26718> tracks the warning. I have implemented that warning in branch wip/T26178 in the main GHC repo. It's based on the ghc-9.14 branch. I'm not sure how to run CI on it. Can I hand it off to you now? It should be part of the 9.14.2 release as agreed by the GHC steering committee. RSVP Thanks Simon

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GitLab account no longer working and unable to recover?
by Iustin Pop 28 Dec '25

28 Dec '25

Hello, Sorry, from https://gitlab.haskell.org/ghc/ghc/-/wikis/report-a-bug it's not clear if there's a specific list for gitlab issues or not. I wanted to report a bug surfaced by HLS, but it looks like my account or password no longer works, and I can't recover the password. The account page is still up, with my correct email address: https://gitlab.haskell.org/iustin, but the recovery option doesn't work. I checked and the recovery emails don't land in spam. Any hints? thanks! iustin

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GHC 9.14.1 is now available
by Zubin Duggal 27 Dec '25

27 Dec '25

The GHC developers are very pleased to announce the release of GHC 9.14.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. GHC 9.14 brings a number of new features and improvements, including: * Significant improvements in specialisation: * The `SPECIALISE` pragma now allows use of type application syntax * The `SPECIALISE` pragma can be used to specialise for expression arguments as well as type arguments. * Specialisation is now considerably more reliable in the presence of `newtype`s * Significant GHCi improvements including: * Correctness and performance improvements in the bytecode interpreter * Features in the GHCi debugger * Support for multiple home units in GHCi * Implementation of the [Explicit Level Imports proposal][levels] * `RequiredTypeArguments` can now be used in more contexts * Greatly improved [SSE/AVX2 support][sse] in the x86 native code generator backend * Initial native code generator support for LoongArch * The WebAssembly backend now supports [evaluation via the interpreter][wasm-ghci], allowing GHCi and TemplateHaskell evaluation, including `foreign import javascript` usage from within the browser * A new primop [`annotateStack#`][annotate-stack] for pushing arbitrary data onto the call stack for later extraction when decoding stack traces * [`-Wincomplete-record-selectors`][incomplete-selectors] is now part of `-Wall`. Libraries compiled with `-Werror` may need adjustment. * A major update of the Windows toolchain * Improved compatibility with [macOS Tahoe][macos] * ... and many more A full accounting of changes can be found in the [release notes][]. See the [migration guide][] for guidance on migrating programs to this release. Note that while this release makes many improvements in the specialisation optimisation, polymorphic specialisation remains disabled by default in the release due to concern over regressions of the sort identified in [#26329][T26329]. Users needing more aggressive specialisation can explicitly enable this feature with the `-fpolymorphic-specialisation` flag. Depending upon our experience with 9.14.1, we may enable this feature by default in a later minor release. GHC development is sponsored by: * [Juspay](https://juspay.com/) * [QBayLogic](https://qbaylogic.com/) * [Channable](https://www.channable.com/) * [Haskell Foundation](https://haskell.foundation/) * [Serokell](https://serokell.io/) * [Well-Typed](https://well-typed.com/) * [Tweag](https://www.tweag.io/) * [Dotcom-Monitor](https://www.dotcom-monitor.com/) * [LoadView](https://www.loadview-testing.com/) * [Web Hosting Buddy](https://webhostingbuddy.com/) * [Find My Electric](https://www.findmyelectric.com/) * [Standard Chartered](https://www.sc.com) * [UpCloud](https://upcloud.com) We would like to thank these sponsors 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 comprises this release. As always, do give this release a try and open a [ticket][] if you see anything amiss. [downloads.haskell.org] https://downloads.haskell.org/ghc/9.14.1 [release notes]: https://downloads.haskell.org/ghc/9.14.1/docs/users_guide/9.14.1-notes.html [ticket]: https://gitlab.haskell.org/ghc/homepage/-/issues/new [macos]: https://gitlab.haskell.org/ghc/ghc/-/issues/26166 [T26329]: https://gitlab.haskell.org/ghc/ghc/-/issues/26329 [levels]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0682-e… [wasm-ghci]: https://www.tweag.io/blog/2025-04-17-wasm-ghci-browser/ [incomplete-selectors]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0516-i… [annotate-stack]: https://gitlab.haskell.org/ghc/ghc/-/issues/26218 [migration guide]: https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.14 [sse]: https://gitlab.haskell.org/ghc/ghc/-/issues/25030

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GHC 9.12.3 is now available
by Zubin Duggal 27 Dec '25

27 Dec '25

The GHC developers are very pleased to announce the release of GHC 9.12.3. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. GHC 9.12.3 is a bug-fix release fixing many issues of a variety of severities and scopes, including: * Fix a number of crashes and miscompilations in the compiler frontend (#25004, #25960, #26256) * Improvements to efficiency of the runtime linker (#26009) * Fixes for several bugs in bytecode generation and the bytecode interpreter (#23210, #25975, #25750) * Fixes for bugs in the handling of WHITEHOLEs in the RTS (#26204, #26205) * Fix incorrect code generation for SSE vector operations (#25859) * Fix a use-after-free in the Windows runtime linker (#26613) * Support for synchronous JSFFI exports for the wasm backend * And many more! A full accounting of these fixes can be found in the [release notes][]. As always, GHC's release status, including planned future releases, can be found on the GHC Wiki [status][]. GHC development is sponsored by: * [Juspay](https://juspay.com/) * [QBayLogic](https://qbaylogic.com/) * [Channable](https://www.channable.com/) * [Haskell Foundation](https://haskell.foundation/) * [Serokell](https://serokell.io/) * [Well-Typed](https://well-typed.com/) * [Tweag](https://www.tweag.io/) * [Dotcom-Monitor](https://www.dotcom-monitor.com/) * [LoadView](https://www.loadview-testing.com/) * [Web Hosting Buddy](https://webhostingbuddy.com/) * [Find My Electric](https://www.findmyelectric.com/) * [Standard Chartered](https://www.sc.com) * [UpCloud](https://upcloud.com) * [Mercury](https://mercury.com) We would like to thank these sponsors 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. [release notes]: https://downloads.haskell.org/~ghc/9.12.3/docs/users_guide/9.12.3-notes.html [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.3 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

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Missing instances in `dump-decls` output
by Wolfgang Jeltsch 25 Dec '25

25 Dec '25

Hi! It seems that `dump-decls` of GHC 9.12.2 misses instances. For example, I cannot find the `Functor` instance of `Data.Functor.Product.Product` in `tests/interface-stability/base-exports.stdout`. In the `HEAD` version it is present, though. Is this a known problem? Which `base-exports.stdout` files are reliable? The one of GHC 9.14.1? The one of `HEAD`? All the best, Wolfgang

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Old feature branches
by Wolfgang Jeltsch 22 Dec '25

22 Dec '25

Hi! Is it actually intended that references to feature branches that have already been merged into `master` stay in the GHC repository? There is a lot of branch references in this repository, which sometimes makes it a bit hard to find a particular one that you’re looking for. Should I delete my own obsolete feature branch references? All the best, Wolfgang

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Devops related changes and announcement of downtime on 2025/01/02 17:00UTC
by Magnus Viernickel 21 Dec '25

21 Dec '25

Dear GHC developers and maintainers, Now that GHC 9.14 has landed, we want to inform you about a couple of devops related changes. First of all, Debian 9 and Debian 10 have gone out of LTS support 2022 and 2024, respectively. We will stop testing these in our nightly release pipelines. In addition to our testing on Debian 11 and Debian 12, we will start testing Debian 13, as well. If you have interest in us continuing to test releases 9 and 10, please reach out to Magnus. (magnus(a)well-typed.com) Second, we will have a maintenance downtime of our Gitlab deployment on Friday, 2nd January 2026, 17:00 UTC. We are planning to have the downtime last at most two hours until 19:00 UTC but if there are any hiccups, it is possible we will have to extend this downtime, make sure you have your checkouts of GHC and the boot libraries ready before then. Last, but not least, we will make a couple of changes in the way new users are handled; new users will not be able to create /any/ repositories after they join, they will have to ask an admin for permission to create repositories. This includes forking the compiler repository. Mind that new users can still create issues on the GHC bugtracker. To make it easy for newcomers to contribute, I will make according changes in the contributing guide and I will create a new issue template for allowing to create repos. This will hopefully reduce the number of spam and advertisement repositories created. Please send a mail to Magnus (magnus(a)well-typed.com) if you have any reasonable objections to these plans. Warm regards and happy holidays (if applicable) Magnus

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Naïve question: Compound instance definitions???
by Viktor Dukhovni 20 Dec '25

20 Dec '25

An existing widely-used typeclass such as `Num` cannot at present be compatibly refined to abstract a subset of its methods into more general superclasses:: class => AdditiveSemigroup a where (+) :: a -> a -> a class AdditiveSemigroup a => AdditiveMonoid a additiveIdentity :: a class AdditiveMonoid a => AdditiveGroup a negate :: a -> a (-) :: a -> a -> a class AdditiveMonoid a => CommutativeRing a where (*) :: a -> a -> a class CommutativeRing a => Num a where abs :: a -> a signum :: a -> a -- Possibly yet another class for this method. fromInteger :: Integer -> a because then all existing definitions of `Num` instances become invalid by specifying methods that no longer belong to `Num`, and failing to speicify the requisite superclass instances. As a result we can't have a satisfactory definition of addition for `Complex Integer`, because the `Num` instance has no sensible `abs` method (the square of the absolute value though integral, fails to satisfy the trianle inequality and is not compatible with abs on the corresponding complex floats). But I'm wondering whether this barrier could be overcome by allowing a single instance definitions to **implicitly** also define any requisite superclass instances by including definitions of the appropriate methods? Thus tbe below: instance Num ... where (+) :: ... additiveIdentity :: ... negate :: ... (-) :: ... (*) : ... abs :: ... signum :: ... fromInteger :: ... would be legal, and would produce implicit instances of the associated superclasses. One might then be able to define more complex `Complex` integral types: instance CommutativeRing Complex Integer where additiveIdentity = 0 :+ 0 (a :+ b) + (c :+ d) = (a + c) :+ (b + d) negate (a :+ b) = (negate a :+ negate b) (a :+ b) - (c :+ d) = (a - c) :+ (b - d) (a :+ b) * (c :+ d) = (a*c - b*d) :+ (a*d + b*c) which is not a `Num`, but is still a commutative ring. -- Viktor. 🇺🇦 Слава Україні!

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A future for fusion
by Jaro Reinders 11 Dec '25

11 Dec '25

Dear Simon Peyton Jones and other interested GHC devs, In a recent thread on the issue tracker of the vector package, I was asked for my thoughts on a specific improvement to their fusion system [1]. I replied I was a bit disillusioned with GHC's approach to fusion because it is not reliable enough for the average user. Simon pointed me to a GHC plugin by Harendra Kumar which acts as a lubricant for fusion, telling GHC to inline more than usual [2]. I suggested we could integrate this into GHC by giving an inlining discount to any function which contains function that is also mentioned in a rewrite rule. However, I also noted I think this will have a large impact on code size and compilation time in larger code bases. Simon agreed, but noted that Harendra's plugin is much more selective than what I suggested. Since I think this discussion is not that relevant to the particular issue in the vector package, I'd like to continue this discussion here. To figure out what Harendra's plugin does, I skimmed the blog post and found this explanation [2]: > fusion-plugin gives programmers control through fusion annotations. > > {-# ANN type Step Fuse #-} > data Step s a = Yield a s | Skip s | Stop > > This tells the compiler: > > Any binding that scrutinizes or constructs Step must be inlined, > no matter its size. > > The plugin scans bindings during the simplifier pass: > > * If a fusible type (Step) is involved → mark it INLINE. > * Run another simplifier pass → constructors eliminated → fusion restored. Translating this to the fold/build fusion system in GHC today, we would force inlining of any function that calls `foldr` or `build` (or `augment`). Essentially, we would want to inline any binding that refers to a function that partakes in the rewrite rules for fusion. I'll admit we probably don't have to generalise this to all rewrite rules, but even with this restriction it seems like a rather aggressive optimisation strategy to me. Please correct me if I have misinterpreted anything. Cheers, Jaro [1] https://github.com/haskell/vector/issues/156#issuecomment-3623339336 [2] https://blog.composewell.com/versions/v2/posts/streamly-fusion-3.html

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