October 2023 - ghc-devs

Reinstallable - base
by Simon Peyton Jones 07 Aug '24

07 Aug '24

Dear GHC devs Given the now-agreed split between ghc-internal and base <https://github.com/haskellfoundation/tech-proposals/pull/51>, what stands in the way of a "reinstallable base"? Specifically, suppose that - GHC 9.8 comes out with base-4.9 - The CLC decides to make some change to `base`, so we get base-4.10 - Then GHC 9.10 comes out with base-4.10 I think we'd all like it if someone could use GHC 9.10 to compile a library L that depends on base-4.9 and either L doesn't work at all with base-4.10, or L's dependency bounds have not yet been adjusted to allow base-4.10. We'd like to have a version of `base`, say `base-4.9.1` that has the exact same API as `base-4.9` but works with GHC 9.10. Today, GHC 9.10 comes with a specific version of base, *and you can't change it*. The original reason for that was, I recall, that GHC knows the precise place where (say) the type Int is declared, and it'll get very confused if that data type definition moves around. But now we have `ghc-internal`, all these "things that GHC magically knows" are in `ghc-internal`, not `base`. *Hence my question: what (now) stops us making `base` behave like any other library*? That would be a big step forward, because it would mean that a newer GHC could compile old libraries against their old dependencies. (Some changes would still be difficult. If, for example, we removed Monad and replaced it with classes Mo1 and Mo2, it might be hard to simulate the old `base` with a shim. But getting 99% of the way there would still be fantastic.) Simon

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Upcoming GHC 9.4.8 release
by Zubin Duggal 10 Jan '24

10 Jan '24

Hi all, We hope to prepare the 9.4.8 release in the coming weeks. This will likely be the final release in the 9.4 series. The major patches currently slated for inclusion into the 9.4.8 are: 1. A process submodule bump to 1.6.17.0 avoiding segfaults on Darwin (https://github.com/haskell/process/issues/295) 2. A fix for a recompilation checking bug where GHC may miss changes in transitive dependencies when deciding to relink a program (https://gitlab.haskell.org/ghc/ghc/-/merge_requests/11178) 3. A fix for a packaging bug so that we can ensure all released binaries and bundled libraries are built with `-split-sections` enabled (https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9492) If you would like any patches to be considered for inclusion in this release please ensure that the corresponding Merge Requests are marked with the ~"backport needed:9.4" by **the end of this week**. Please note that we will be quite conservative with the changes we accept for this branch at this point in its lifecycle. The current set of all MRs being considered for inclusion can be viewed at https://gitlab.haskell.org/ghc/ghc/-/merge_requests?scope=all&state=all&lab… Thanks, Zubin

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Type-level sized Word literals???
by Viktor Dukhovni 31 Oct '23

31 Oct '23

I am working on some code where it is useful to have types indexed by a 16-bit unsigned value. Presently, I am using type-level naturals and having to now and then provide witnesses that a 'Nat' value I am working with is at most 65535. Also, perhaps there's some inefficiency here, as Naturals have two constructors, and so a more complex representation with (AFAIK) no unboxed forms. I was wondering what it would take to have type-level fixed-size Words (Word8, Word16, Word32, Word64) to go along with the Nats? It looks like some of the machinery (KnownWord16, SomeWord16, wordVal16, etc.) can be copied straight out of GHC.TypeNats with minor changes, and that much works, but the three major things that are't easily done seem to be: - There are it seems no TypeReps for types of Kind Word16, so one can't have (Typeable (Foo w)) with (w :: Word16). - There are no literals of a promoted Word16 Kind. type Foo :: Word16 -> Type data Foo w = MkFoo Int -- 1 has Kind 'Natural' (a.k.a. Nat) x = MkFoo 13 :: Foo 1 -- Rejected, -- The new ExtendedLiterals syntax does not help -- x = MkFoo 13 :: Foo (W16# 1#Word16) -- Syntax error! - There are unsurprisingly also no built-in 'KnownWord16' instances for any hypothetical type-level literals of Kind Word16. Likely the use case for type-level fixed-size words is too specialised to rush to shoehorn into GHC, but is there something on the not too distant horizon that would make it easier and reasonable to have fixed-size unsigned integral type literals available? [ I don't see a use-case for unsigned versions, they can trivially be represented by the unsigned value of the same width. ] With some inconvenience, in many cases I can perhaps synthesise Proxies for types of Kind Word16, and just never use literals directly. -- Viktor.

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"Merge-buddy" request
by Alan & Kim Zimmerman 28 Oct '23

28 Oct '23

Hi all I have been landing a series of MRs to simplify the exact print annotations. They are split into pieces so each change is not too big. I still have quite a number of them to land, but am finding the process slow, as I - make the MR - wait for CI to go green - wait for a review, or ping people on chat to do the review - once approved, land it. I don't want to loudly shout for reviewers each time, and become a general irritant. Is there anyone (ideally with an interest in the exact print annotations) that would like to be my "merge buddy" that I can easily ask to do the reviews? I am happy for each one to take a couple of days, so it is not a high pressure thing, I just want to get into a cadence on them. My current one is https://gitlab.haskell.org/ghc/ghc/-/merge_requests/11496. Admittedly only one reviewer requested, and he is always busy, and I do not want to make him even more so. Alan

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PMC: addConCt and newtypes bottom info
by Rodrigo Mesquita 27 Oct '23

27 Oct '23

Dear Sebastian and GHC devs, Regarding this bit from the function addConCt in the GHC.HsToCore.Pmc.Solver module, Nothing -> do let pos' = PACA alt tvs args : pos let nabla_with bot' = nabla{ nabla_tm_st = ts{ts_facts = addToUSDFM env x (vi{vi_pos = pos', vi_bot = bot'})} } -- Do (2) in Note [Coverage checking Newtype matches] case (alt, args) of (PmAltConLike (RealDataCon dc), [y]) | isNewDataCon dc -> case bot of MaybeBot -> pure (nabla_with MaybeBot) IsBot -> addBotCt (nabla_with MaybeBot) y IsNotBot -> addNotBotCt (nabla_with MaybeBot) y _ -> assert (isPmAltConMatchStrict alt ) pure (nabla_with IsNotBot) -- strict match ==> not ⊥ My understanding is that given some x which we know e.g. cannot be bottom, if we learn that x ~ N y, where N is a newtype (NT), we move our knowledge of x not being bottom to the underlying NT Id y, since forcing the newtype in a pattern is equivalent to forcing the underlying NT Id. Additionally, we set x’s BottomInfo to MaybeBot — However, I don’t understand why we must reset x’s BotInfo to MaybeBot — couldn’t we keep it as it is while setting y’s BotInfo to the same info? An example where resetting this info on the newtype-match is important/necessary would be excellent. FWIW, I built and tested the PMC of ghc devel2 with MaybeBot -> pure (nabla_with MaybeBot) IsBot -> addBotCt (nabla_with IsBot) y IsNotBot -> addNotBotCt (nabla_with IsNotBot) y And it worked without warnings or errors… Thanks in advance! Rodrigo

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How do you keep tabs on commits that fix issues?
by Bryan Richter 16 Oct '23

16 Oct '23

I am not sure of the best ways for checking if a certain issue has been fixed on a certain release. My past ways of using git run into certain problems: The commit (or commits!) that fix an issue get rewritten once by Marge as they are rebased onto master, and then potentially a second time as they are cherry-picked onto release branches. So just following the original commits doesn't work. If a commit mentions the issue it fixes, you might get some clues as to where it has ended up from GitLab. But those clues are often drowning in irrelevant mentions: each failed Marge batch, for instance, of which there can be many. The only other thing I can think to do is look at the original merge request, pluck out the commit messages, and use git to search for commits by commit message and check each one for which branches contain it. But then I also need to know the context of the fix to know whether I should also be looking for other, logically related commits, and repeat the dance. (Sometimes fixes are only partially applied to certain releases, exacerbating the need for knowing the context.) This seems like a mechanism that can't rely on trusting the author of the original set of patches (which may be your past self) and instead requires a deep understanding to be brought to bear every time you would want to double check the situation. So it's not very scalable and I wouldn't expect many people to be able to do it. Are there better mechanisms already available? As I've said before, I am used to a different git workflow and I'm still learning how to use the one used by GHC. I'd like to know how others handle it. Thanks! -Bryan

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Proposal for communication with GHC API users
by Facundo Domínguez 13 Oct '23

13 Oct '23

Dear GHC devs, Your feedback is appreciated in a new proposal to streamline communication with the authors of the GHC API as GHC evolves [1]. The proposal aims to set a process to collect detailed information on how users depend on the GHC API, and how GHC developers think these dependencies should be adjusted when there are breaking changes. This proposal is related to the endeavor to stabilize the GHC API [2]. Although, rather than trying to define a stable API, it sets a process to deal with the current state of affairs where users depend on many unstable parts. Looking forward to your thoughts! Facundo [1] https://gitlab.haskell.org/ghc/ghc/-/issues/24096 [2] https://discourse.haskell.org/t/charting-a-course-toward-a-stable-api-for-g… -- All views and opinions expressed in this email message are the personal opinions of the author and do not represent those of the organization or its customers.

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[ANNOUNCE] GHC 9.8.1 is now available
by Ben Gamari 09 Oct '23

09 Oct '23

The GHC developers are very pleased to announce the availability of the final release of GHC 9.8.1. Binary distributions, source distributions, and documentation are available at downloads.haskell.org: https://downloads.haskell.org/ghc/9.8.1 GHC 9.8 brings a number of new features and improvements, including: * Preliminary support the `TypeAbstractions` language extension, allowing types to be bound in type declarations [TypeAbstractions]. * 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][]. Plans for GHC's future releases can be found on the [GHC Wiki][status]. 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. Happy compiling, ~ Ben [TypeAbstractions]: 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/docs/users_guide/9.8.1-notes.html [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status

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Darwin runners fixed on master
by Bryan Richter 05 Oct '23

05 Oct '23

Hi all, A reinstall of Darwin runners exposed https://gitlab.haskell.org/ghc/ghc/-/issues/24055, which shows up as a failure to fetch perf notes. The fix has landed on master so you may want to rebase to pick it up. -Bryan

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Migrating GHC's real-time communications to Matrix? (#24010)
by Ben Gamari 02 Oct '23

02 Oct '23

Hello all, GHC's user and contributor documentation currently directs users to #ghc on irc.libera.chat for discussion of GHC development. However, for a variety of reasons the center-of-mass of discussion has been gradually shifting towards Matrix (#GHC:matrix.org). Given that Matrix appears to be both more active and more accessible to newcomers, I suggest that we recognize this shift in our documentation. If you agree or disagree with this proposal please leave a :thumbsup: or :thumbsdown: on [#24010] and feel free to leave a comment explaining your feelings. Cheers, - Ben [#24010]: https://gitlab.haskell.org/ghc/ghc/-/issues/24010

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