- ghc-devs - Haskell.org

Representation of single-method classes
by Tom Ellis 11 Dec '25

11 Dec '25

Dear GHC friends, I would like to know whether the representation of FooD1: data Foo = MkFoo A B class FooD1 where foo :: Foo has a representation at least as efficient as FooD2: class FooD2 where fooA :: A fooB :: B in particular, that FooD1 doesn't occur an extra indirection. Now, according to https://gitlab.haskell.org/ghc/ghc/-/issues/20897 FooD1 used to be compiled as a newtype, so the answer was "yes". Then something called "Plan B" (described in that ticket) was enacted, and I believe the answer is therefore still "yes". Can someone please confirm that? For a teaser of why I want this: I find type classes very difficult to evolve in a way that satisfies my stability needs. Part of the reason for this is that type classes as typically used don't really permit any form of data abstraction: you list out all the methods explicitly in the class definition. There is no data hiding. To address this issue I am considering defining classes abstractly, like class MyClass where myClassImpl :: MyClassD for some abstract type MyClassD (i.e. whose implementation details are not exposed to users) and I want to know if there would be a performance cost. Thanks, Tom

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GHC 9.12.3-rc3 is now available
by Zubin Duggal 11 Dec '25

11 Dec '25

The GHC developers are very pleased to announce the availability of the third release candidate for GHC 9.12.3. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][] and via [GHCup](https://www.haskell.org/ghcup/). GHC 9.12.3 is a bug-fix release fixing several issues of a variety of severities and scopes. 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][]. This release candidate will have a two-week testing period. If all goes well the final release will be available the week of 22 December 2025. The changes from the first release candidate are - Fix use after free on windows due to a linker optimisation introduced in an earlier RC (#26613) - A fix for a bug in the configure script which result in the LLVM toolchain not being detected in certain situations (#26564) - Fixes for the WASM backend 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 that 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://gitlab.haskell.org/ghc/ghc/-/blob/ghc-9.12/docs/users_guide/9.12.3-… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.3-rc3 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

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GitLab read only/licensing.
by Andreas Klebinger 08 Dec '25

08 Dec '25

As many of you might have noticed GitLab forced our GitLab instance into read only mode today. We are currently working to resolve this. Without going into too much detail Ben has reach out to GitLab for a renewal of the current license over a month ago and multiple times since. Sadly GitLab hasn't been very responsive so far. I hope we can find a workaround in short order. Cheers Andreas

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Strange package error -- HELP!
by Simon Peyton Jones 02 Dec '25

02 Dec '25

Friends I'm getting this from my newly-built in-tree compiler ~/code/HEAD-11/_build/ghc-stage1 -c T24984.hs -O T24984.hs:1:1: error: [ ]8;;https://errors.haskell.org/messages/GHC-45102 \GHC-45102 ]8;; \] Ambiguous module name ‘Prelude’. * it was found in multiple packages: base-4.22.0.0 base-4.22.0.0* What a bonkers message. What is going wrong? I do see this: ls -d /home/simonpj/code/HEAD-11/_build/stage1/lib/x86_64-linux-ghc-9.15.20251119/base* /home/simonpj/code/HEAD-11/_build/stage1/lib/x86_64-linux-ghc-9.15.20251119/base-4.22.0.0-c7d2 /home/simonpj/code/HEAD-11/_build/stage1/lib/x86_64-linux-ghc-9.15.20251119/base-4.22.0.0-inplace simonpj@LHR-WD-22561:~/code/HEAD-11/testsuite/tests/perf/compiler$ So there are two base packages around somehow. I don't know if that's the reason. I'd love a bit of help.. it's super annoying Maybe it's some leftover not-clean stuff -- but how would I diagnose? How can I see what package directories it is looking in, which package files it is reading? THanks Simon

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Strange behaviors regarding plugin-related compiler options
by Wolfgang Jeltsch 02 Dec '25

02 Dec '25

Hi! While working with a GHC plugin, I accidentally found out the following behaviors: * The use of a plugin, at least one that is declared as pure, doesn’t make the compiled module considered unsafe even when `-fplugin-trustworthy` is missing. * Any option of the form `-fplugin-opt⟨some-garbage⟩` is accepted. Is any of these two behaviors intended? I’m using GHC 9.12.2 through Cabal. All the best, Wolfgang

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GHC 9.14.1-rc3 is now available
by Zubin Duggal 02 Dec '25

02 Dec '25

The previous email had an incorrect subject line, GHC 9.14.1-rc3 is what it was intended to say. The contents following this are the same. The GHC developers are very pleased to announce the availability of the third release candidate of GHC 9.14.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. The changes from the second release candidate are: - Fix RTS segfaults in certain situations (#26495) - Fix use after free on windows due to a linker optimisation introduced in an earlier RC (#26613) GHC 9.14 will bring 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 * SSE/AVX2 support in the x86 native code generator backend * A major update of the Windows toolchain and improved compatibility with macOS Tahoe * ... and many more A full accounting of changes can be found in the [release notes][]. Given the many specialisation improvements and their potential for regression, we would very much appreciate testing and performance characterisation on downstream workloads. Note that while this release makes many improvements in the specialisation optimisation, polymorphic specialisation will remain disabled by default in the final 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 that 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. [downloads.haskell.org] https://downloads.haskell.org/ghc/9.14.1-rc3 [release notes]: https://downloads.haskell.org/ghc/9.14.1-rc3/docs/users_guide/9.14.1-notes.… [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…

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GHC 9.12.3-rc3 is now available
by Zubin Duggal 02 Dec '25

02 Dec '25

The GHC developers are very pleased to announce the availability of the third release candidate of GHC 9.14.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. The changes from the second release candidate are: - Fix RTS segfaults in certain situations (#26495) - Fix use after free on windows due to a linker optimisation introduced in an earlier RC (#26613) GHC 9.14 will bring 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 * SSE/AVX2 support in the x86 native code generator backend * A major update of the Windows toolchain and improved compatibility with macOS Tahoe * ... and many more A full accounting of changes can be found in the [release notes][]. Given the many specialisation improvements and their potential for regression, we would very much appreciate testing and performance characterisation on downstream workloads. Note that while this release makes many improvements in the specialisation optimisation, polymorphic specialisation will remain disabled by default in the final 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 that 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. [downloads.haskell.org] https://downloads.haskell.org/ghc/9.14.1-rc3 [release notes]: https://downloads.haskell.org/ghc/9.14.1-rc3/docs/users_guide/9.14.1-notes.… [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…

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Reinstallable Base next steps
by Matthew Pickering 25 Nov '25

25 Nov '25

Hi all, I'm trying to establish consensus around the next stages of the reinstallable base project. It seems to me the next step is to move the `base` package out of the GHC tree into a separate repo so it can be maintained like a normal haskell package. If you have an opinion then please leave your feedback on this email thread. The issue is discussed here: https://github.com/haskell/core-libraries-committee/issues/375 and also on discourse https://discourse.haskell.org/t/what-are-the-next-steps-for-reinstallable-b… Cheers, Matt

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Let's fix static pointers (#26556)
by Simon Peyton Jones 19 Nov '25

19 Nov '25

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. > <#m_5415477730196602759_discussion>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 > > <#m_5415477730196602759_proposal>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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GHC call today: alternative meeting link
by Sam Derbyshire 18 Nov '25

18 Nov '25

Jitsi is suffering from today's CloudFlare outage, so here's an alternate meeting link for today's GHC call: https://meet.google.com/vcq-hwba-ppo?hs=224

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