July 2024 - ghc-devs - Haskell.org

How to implement type-level list concatenation as a GHC primitive type family
by Hécate 10 Aug '24

10 Aug '24

Hi GHC devs, I was wondering recently, considering that type family evaluation is notoriously slow, how one would implement type-level list concatenation directly within GHC in a way that is much less expensive to run. So I am looking for pointers, as this part of GHC is fairly unknown to me. Thinking about it, I'm actually unsure where the tyfam evaluation is happening. Any advice would be appreciated. Cheers, Hécate -- Hécate ✨ 🐦: @TechnoEmpress IRC: Hecate WWW: https://glitchbra.in RUN: BSD

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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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Question about profiling reports
by Celeste Hollenbeck 05 Aug '24

05 Aug '24

Hi GHC Team, I have a question for a research project. I'm looking at GHC's profiler, and the documentation says a profiling report displays "a break-down by cost centre of the most costly functions in the program". Here's an example of the report that I'm talking about: MAIN MAIN 102 0 0.0 0.0 100.0 100.0 CAF GHC.IO.Handle.FD 128 0 0.0 0.0 0.0 0.0 CAF GHC.IO.Encoding.Iconv 120 0 0.0 0.0 0.0 0.0 CAF GHC.Conc.Signal 110 0 0.0 0.0 0.0 0.0 CAF Main 108 0 0.0 0.0 100.0 100.0 main Main 204 1 0.0 0.0 100.0 100.0 fib Main 205 2692537 100.0 100.0 100.0 100.0 This example is under Section 8.1 of the GHC User's Guide https://downloads.haskell.org/ghc/latest/docs/users_guide/profiling.html#:~…'s%20profiling%20system%20assigns%20costs,to%20the%20enclosing%20cost%20centre. It looks like the numbers often add up to less than 100% for the %time, but I don't see any documentation on a threshold for what makes a cost centre "costly"—so I assume that "costly" means that it takes up any time whatsoever, and any cost centres that take up any time at all are included in the report? So perhaps the numbers under %time don't add up to 100% all the time because of rounding error or perhaps garbage collection? Or something else that is not profiled? Is that correct? Thanks, Celeste

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Acknowledgement of CI instability
by Sam Derbyshire 05 Aug '24

05 Aug '24

Hi all, The GHC team would like to acknowledge some ongoing CI instabilities in the GHC project: - regular failures of the i386 job, possibly related to the bump to debian 12 <https://gitlab.haskell.org/ghc/ghc/-/commit/203830065b81fe29003c1640a354f11…> (although the job still failed occasionally before this), - flakiness of the MultiLayerModulesDefsGhciReload test causing the fedora33-release job to fail, - lack of availability of darwin runners, causing aarch64-darwin and x86_64-darwin jobs to time out. These issues are currently causing a string of marge batch failures, holding up several MRs. We are currently short on resources for addressing problems with CI, so please bear with us while we sort the situation out. In the meantime, feel free to let us know of any other CI issues that are impacting your work on GHC. Best, Sam

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Haskell Implementors' Workshop - Change of date
by Matthew Pickering 29 Jul '24

29 Jul '24

This year the decision has been taken to combine together Haskell Implementors' Workshop and Haskell Symposium. Therefore the workshop will now take place on the dates September 6/7 (the dates of the Haskell Symposium). Please email me if you have any questions. Cheers, Matt

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questions about head.hackage
by jappie klooster 24 Jul '24

24 Jul '24

Hello everyone, For the stability working group I'm trying to understand how many packages head.hackage allows you to build. Does anyone know the answer to this or have an idea how I can find out? For context, the stability working group is a group of interested people who try and allow ghc and haskell system to innovate, while reducing the amount of friction caused to the wider Haskell community. Current efforts include the marking language proposals as stable, and my work of investigating historic breakage. Investigating this breakage will allow us to make better informed decisions on which pain points to prioritize. -- With regards, Jappie Klooster e: hi(a)jappie.me w: https://jappie.me m: +31644237437

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Gitlab upgrade tomorrow
by Andreas Klebinger 18 Jul '24

18 Jul '24

Hi all, Gitlab will be temporarily unavailable tomorrow around 9:00 AM CEST while it's updated. This should not take long, but a short downtime is expected. Cheers, - Andreas

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Cabal and GHC: the big picture
by Simon Peyton Jones 18 Jul '24

18 Jul '24

Friends You may remember a recent thread on ghc-devs about GHC and Cabal <https://mail.haskell.org/pipermail/ghc-devs/2024-July/021678.html>. In it I say how I feel I lack the "big picture" of how GHC and Cabal interact, and that my mental model is probably faulty. Tom Ellis took pity on me, and together we wrote this big-picture overview about how GHC and Cabal interact <https://docs.google.com/document/d/1mQEpV3fYz1pHi64KTnlv8gifh9ONQ-jytk5sIHq…>. Would you like to: - Read it as a consumer. - Does it tell you stuff that is useful? - What else would you like to know? - What is un-clear or missing? - Read it as an expert. - Is it accurate? - Are any bits misleading? - Do the links go to appropriate places? - What other links or resource would be helpful. It is not intended as a replacement for the GHC user guide, nor the Cabal user guide; rather it is littered with links to those guides which give much fuller details. Rather, it is intended to put you (well, me for one!) in a position where you can more easily make sense of those documents. We'd love to have your help in improving it. Simon

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submodules update warning?
by Brandon Allbery 12 Jul '24

12 Jul '24

I got several of these while updating my local ghc checkout: ``` Fetching submodule libraries/binary remote: error: MIDX does not contain the BTMP chunk remote: warning: unable to load pack: 'pack-1d2d40f15dad726ad220801f9d5bd5b527bc7493.idx', disabling pack-reuse >From https://gitlab.haskell.org/ghc/packages/binary * [new branch] wip/edsko -> origin/wip/edsko ``` Is this something I need to worry about? -- brandon s allbery kf8nh allbery.b(a)gmail.com

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CI stuck?
by Simon Peyton Jones 12 Jul '24

12 Jul '24

Dear GHC devs Is GHC's CI stuck in some way? My !12928 has been scheduled by Marge over 10 times now, and each time the commit has failed. Ten seems... a lot. Thanks Simon

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