- ghc-devs - Haskell.org

Google Summer of Code Project Ideas
by Aaron Allen 20 Jan '26

20 Jan '26

Dear GHC Devs, Haskell will be applying for the Google Summer of Code 2026 program and it would be great to have some project ideas involving GHC. If anything comes to mind, please submit an idea proposal using the instructions found here: https://summer.haskell.org/ideas.html. We are hoping to have ideas submitted prior to February 1st.

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Test output depending on the I/O subsystem
by Wolfgang Jeltsch 20 Jan '26

20 Jan '26

Hi! I have a `base` test that I’d like to be run on all platforms and, in the case of Windows, with both I/O subsystems: the POSIX one and the (native) Windows one. However, the output of this test depends on the choice of I/O subsystem. How do I implement such a test with GHC’s testing framework? Can I provide an extra golden file with a name of the pattern `*.stdout-winio` or similar, or do I have to create two tests, which would then use the exact same Haskell code? All the best, Wolfgang

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New contributor, interested in testing infrastructure
by Michael Alan Dorman 14 Jan '26

14 Jan '26

Hi, I’m a decade-plus Haskell dabbler who’s now in a position to contribute to GHC, and I’ve been considering how I might do so most effectively. In reading the recent bug report about the contribution experience <https://gitlab.haskell.org/ghc/ghc/-/issues/26728>, one takeaway I had was that improving the testing story could make a big difference—making sure tests are reliable, trying to make the test suite fail as fast as possible, etc. Since most of my $JOB for the last dozen years was devoted to building bespoke testing infrastructure and working with engineers to come up with effective strategies for optimizing fail-fast—and working in a codebase that, itself, started with an unreliable test suite—this seems like an issue where I could help. Unfortunately, I couldn’t find any documentation on whether there is a person or a group that is primarily responsible for the testing infrastructure—no doubt the information is out there somewhere, and I just didn’t look in the right place—and I would appreciate any pointers as to who best to talk to in order to get involved. Thanks, Mike.

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on-boarding experience survey
by Magnus Viernickel 14 Jan '26

14 Jan '26

Dear GHC Hackers, we are trying to improve the on-boarding experience for new GHC contributors. We are wary of the fact that long-term GHC contributors have a hard time estimating the amount of resistance a new contributor goes through. If you had a hard time getting started with GHC in the past, now there's a chance for you to lend us a bit of your time to improve the experience for everyone. Please go to https://gitlab.haskell.org/ghc/ghc and tell us: - Did you find the contributing page (https://gitlab.haskell.org/ghc/ghc/-/blob/master/HACKING.md) - Did you find the instructions there helpful? - Which of the instructions did not help you and you find redundant? - If you have been contributing, which instructions were missing/ you had to discover through other means? - Which other recommendations do you have to improve the developer documentation? After following the instructions, which other hurdles are still there, i.e. which other parts of the contribution experience were lacking? Have you been able to solve your problems? Did you use any communication platforms to contact GHC devs? If so, which? Out of the following, which communication platform are you/were you comfortable with using for GHC purposes? 1. The GHC Matrix 2. The ghc-devs mailing list 3. The GHC IRC 4. The Haskell Discourse 5. One of the other Haskell forums (please specify) 6. Other (please specify) You may answer in response to this mail to the ghc-devs mailing list or on this GHC issue: https://gitlab.haskell.org/ghc/ghc/-/issues/26771. Your answers and thoughts are greatly appreciated. Thank you so much. Best Magnus

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Re: Let's fix static pointers (#26556)
by Simon Peyton Jones 09 Jan '26

09 Jan '26

Great thanks Edsko -- v helpful. By the way, I am not entirely sure why the proposal mentions the IsStatic class > explicitly, but I just wanted to mention that it is really quite useful; I > declare two instances in my Unfolder episode :) The proposal mentions IsStatic only because I wanted to give the expansion that GHC performs. I agree that it's useful, but it's really orthogonal to the proposal. I'll look out for the Unfoldr episode -- if you remember do send me a link Thanks again Simon On Fri, 9 Jan 2026 at 12:25, Edsko de Vries <edsko(a)well-typed.com> wrote: > Hi Simon, > > Apologies for my slow reply. It's been nearly 15 years (!) since I wrote > distributed-process and distributed-static (fun fact: it was my first > task when I joined Well-Typed) and I felt I needed to set aside some time > to properly page everything back in, as I haven't really used static > pointers ever since (somewhat sadly..). I realize that the proposal how now > been written and accepted, so this email comes too late in a way, but I > finally had some time now to look at this properly. In hindsight, the > proposal is simple enough, but I didn't realize that at the time. > > Anyway. The proposal is fine with me :) I seems to me that it's no more > than a minor inconvenience to the programmer. I have now prepared a Haskell > Unfolder episode (which will likely be the next episode we air, episode > #53) on static pointers, where I discuss the primitive building blocks > provided by ghc, and show how we can implement some compositional > infrastructure around that (including some thoughts on type classes, which > I believe you and I discussed a very long time ago). I'll also mention the > proposal and the upcoming changes in 9.14.2 (warning) and 9.16 (change > implemented). I tried building it with your branch (wip/T26718) and that > all worked fine. I also added one module in which your branch issues the > warning expect, and that seemed good also. > > By the way, I am not entirely sure why the proposal mentions the IsStatic > class explicitly, but I just wanted to mention that it is really quite > useful; I declare two instances in my Unfolder episode :) > > -Edsko > On 11/19/25 13:02, Simon Peyton Jones wrote: > > Thanks Laurent and Mathieu > > *Edsko, Duncan, Facundo*: do you have any views? > > I have now written a GHC proposal > <https://github.com/ghc-proposals/ghc-proposals/blob/wip/spj-static/proposal…>. > Could you add your thoughts to it? I think it's a no-brainer myself, but > we should go through the proper process. > > Do any of you know people I should consult? Eg authors of libraries that > use StaticPtrs? > > Thanks > > Simon > > > > On Fri, 7 Nov 2025 at 21:18, Simon Peyton Jones < > simon.peytonjones(a)gmail.com> wrote: > >> Dear Laurent, Duncan, Mathieu, Facundo, Edsko >> >> I have spent a little while digging into *static pointers* recently. >> See my post below. I wonder if you have any comments on my proposal? >> >> Do you know anyone else I should consult? >> >> Thanks! >> >> Simon >> >> On Fri, 7 Nov 2025 at 18:13, Simon Peyton Jones (@simonpj) < >> gitlab(a)gitlab.haskell.org> wrote: >> >>> Simon Peyton Jones <https://gitlab.haskell.org/simonpj> created an >>> issue: #26556 <https://gitlab.haskell.org/ghc/ghc/-/issues/26556> >>> >>> Static pointers are not properly implemented. For example: >>> >>> - #26545 <https://gitlab.haskell.org/ghc/ghc/-/issues/26545> >>> - #24464 <https://gitlab.haskell.org/ghc/ghc/-/issues/24464> >>> - #24773 <https://gitlab.haskell.org/ghc/ghc/-/issues/24773> >>> >>> among others. Moreover, the implementation is very messy, scattered >>> about in FloatOut and elsewhere. >>> >>> Let's fix it. >>> Discussion >>> >>> I embarked on what I thought would be a simple refactor to >>> >>> - Identify static bindings in the type checker >>> - Promote them to top level desugarer >>> >>> thereby avoiding all the terribly painful static-form-floating stuff >>> that been an ongoing source of breakage and irritation. >>> >>> Sadly it was not as simple as I had thought. Merge request !14994 >>> <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14994> is my work >>> in progress >>> >>> - >>> >>> At first it seems simple: given static e >>> - When typechecking e ensure that all its free variables are >>> top-level defined >>> - When desugaring, move e to top level >>> >>> Apparently simple! >>> - >>> >>> *Complication 1*. e might generate constraints. We don't want to >>> solve those from locally-bound Givens, because they'll be out of scope when >>> we promote to top level. >>> >>> Solution: wrap the constraints in an implication with SkolInfo of >>> StaticFormSkol; and in the constraint solver zap all Givens when >>> walking inside such an implication. That was done in >>> >>> commit 39d4a24beaa7874a69ffdc1528ca160818829169Author: Simon Peyton Jones <simon.peytonjones(a)gmail.com>Date: Tue Sep 30 23:11:19 2025 +0100 Build implication for constraints from (static e) This commit addresses #26466, by buiding an implication for the constraints arising from a (static e) form. The implication has a special ic_info field of StaticFormSkol, which tells the constraint solver to use an empty set of Givens. >>> >>> So that complication wasn't at all bad. >>> - >>> >>> *Complication 2*. What if we have >>> >>> f x = let y = reverse "hello" in ...(static (y++y))... >>> >>> The free vars of the static are just {y}, and y is >>> morally-top-level. It in turn has no free variables. >>> >>> Sadly (as it turns out) GHC tries to accept this case. When looking >>> at the defn of y (with no static in sight yet) the typechecker marks >>> it at a "static binding", meaning that it too can (and indeed must) be >>> floated to top level. >>> >>> So if the desugarer moves the static to the top level, it must move y >>> too. And that means it must mark the typechecked binding in some way, so >>> the desugarer can identify it. Not so hard, but there is quite a bit of new >>> plumbing. >>> - >>> >>> *Complication 3*. But what if y's RHS generates constraints, which >>> use Givens (or solved dictionaries, which are very similar) from its >>> context. E.g. >>> >>> f x = let p = x+1::Int; y = 2+3::Int in ... >>> >>> Now there may be a d :: Num Int lying around from dealing with p, >>> and y may use it. Oh no! Now that'll be out of scope if we move y to >>> top level. >>> >>> Plausible solution: use them same mechanism for static bindings as >>> we did for static e expressions. That is, build an implication >>> constraint whose SkolInfo says "zap Givens". This turned out to be >>> considerably harder to implement than it was for Complication 1. >>> - >>> >>> *Complication 4*. What if y is not generalised, perhaps because of >>> the Monomorphism Restriction? e.g. >>> >>> f :: Num a => a -> blahf x = let y = 3+3 in (x+y, static( ..y.. )) >>> >>> Now y is monomorphic and really does use the dictionary passed to f. >>> So it really cannot appear in the static. Somehow y really isn't >>> static after all. We must reject this program. Not only is it an >>> implementation mess (Complications 1,2,3 are already imposing quite a >>> signficant implemenation burden) but it becomes pretty hard to explain to >>> the programmer just which uses of static are OK and which are not. >>> >>> What a swamp. At this point I threw up my hands and wrote this >>> summary >>> >>> Proposal >>> >>> To me the solution is clear: the rule should be >>> >>> - *in static e, all the free vars of e should be bound at top level* >>> >>> That is a nice simple rule; it is easy to explain and easy to implement. >>> It is also what the user manual says! >>> >>> In retrospect, by addressing Complication 2 I was trying too hard! (And >>> this extra feature is entirely undocumented.) I thought that I could deal >>> with Complication 2 using the same mechanism as the one that deals with >>> MonoLocalBinds. But I was wrong. >>> >>> Making this change could perhaps break some programs. They would all be >>> easy to fix, by moving bindings for any free variables to top level. But >>> note that the status quo is not stable: it has bugs e.g #24464 >>> <https://gitlab.haskell.org/ghc/ghc/-/issues/24464>, #26545 >>> <https://gitlab.haskell.org/ghc/ghc/-/issues/26545>. What we have is at >>> attempt to be clever that is simply wrong. >>> >>> — >>> View it on GitLab <https://gitlab.haskell.org/ghc/ghc/-/issues/26556>. >>> You're receiving this email because of your activity on >>> gitlab.haskell.org. Unsubscribe >>> <https://gitlab.haskell.org/-/sent_notifications/4b29fc65ccdc21e95267b66fdfb…> >>> from this thread · Manage all notifications >>> <https://gitlab.haskell.org/-/profile/notifications> · Help >>> <https://gitlab.haskell.org/help> >>> >>

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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 04 Jan '26

04 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 29 Dec '25

29 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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