January 2024 - 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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GHC proposals not rendering
by Simon Peyton Jones 22 Mar '24

22 Mar '24

If you go to https://github.com/ghc-proposals/ghc-proposals/pull/448, and click on the "rendered" link, namely https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0448-t…, Github is now displaying the .rst source, not the rendered proposal. This has started happening (consistently across other proposals) today. Does anyone know why? How can I fix this? Simon

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GHC 9.10 release
by Ben Gamari 02 Feb '24

02 Feb '24

Hi all, I am currently in the midst of planning the GHC 9.10 release [1], which we hope to fork on 23 Feb 2024. If you have a major patch that you would like to see included in 9.10, please be in touch. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/-/milestones/380

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Upcoming GHC 9.8.2 release
by Zubin Duggal 29 Jan '24

29 Jan '24

Hello all, We are currently in the process of preparing the 9.8.2 release, which should be released mid-February. If you would like any patches to be considered for inclusion into this release, please ensure that the corresponding MRs are marked with the ~"backport needed:9.8" label. Any submodule bumps that have made it into previous releases like 9.6.4 will also be backported to 9.8.2. Requests for additional submodule updates should be made on the release tracking ticket: https://gitlab.haskell.org/ghc/ghc/-/issues/24079 Cheers, Zubin

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GHC Steering Committee Call for Nominations
by Adam Gundry 28 Jan '24

28 Jan '24

Dear Haskellers, The GHC Steering Committee is seeking nominations for new members. The committee scrutinizes, debates and eventually decides to accept or reject proposals to change the language or major features supported by GHC. Our processes are described in the GitHub repository where proposals are submitted (https://github.com/ghc-proposals/ghc-proposals) In particular, please have a look at the committee bylaws (https://github.com/ghc-proposals/ghc-proposals/blob/master/committee.rst) We are looking for members who have the ability to: - understand GHC proposals (e.g. for new language extensions), - find holes and missing corner cases in the specifications, - foresee the interaction with other language or compiler features, - comment constructively and improve proposals through engagement with others, - judge the cost/benefit ratio of changes, and - come to a justifiable conclusion. Ideally, committee members should: - have substantial experience of writing or teaching Haskell; - have a track record of active contributions to the Haskell community; or - have expertise in language design and implementation, in either Haskell or related languages. It is our aim that this committee be diverse; by representing different viewpoints, we will make decisions that benefit larger segments of our community. Even if you are uncertain whether your background qualifies you for the role, you are warmly encouraged to apply. The committee's work requires a small, but non-trivial amount of time, especially when you are assigned a proposal for shepherding. We estimate the workload to be around 2 hours per week, and our process works best if members usually respond to technical emails within 1-2 weeks (within days is even better). Please keep that in mind if your email inbox is already overflowing. To nominate yourself, please send an email to me (as the committee secretary) at adam(a)well-typed.com by February 16th 2024, briefly summarising your background and relevant experience. I will distribute the nominations among the committee, and we will keep the nominations and our deliberations private. Self-nominations are the norm. You can nominate someone else, but please obtain their explicit consent to do so. (We don't want to choose someone who turns out to be unable to serve.) On behalf of the committee, Adam Gundry -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 27 Old Gloucester Street, London WC1N 3AX, England

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How to disable the simple optimizer
by Facundo Domínguez 26 Jan '24

26 Jan '24

Dear devs, I noticed that GHC sometimes inlines bindings during desugaring, e.g foo = z where z = z1 + z2 z1 = 42 z2 = 1 is desugared to foo = 42 + 1 This inlining is problematic when using a plugin like Liquid Haskell, which wants to analyse in Core the local bindings provided by the user. Until ghc-9.6 LH didn't experience the inlining when using the GHC API. And this is despite the fact that using ghc from the command line would seem to always inline during desugaring, even when using -O0. When using ghc-9.8.1 though, even the output of the GHC API started having the bindings inlined. Some debugging with -ddump-ds and -ddump-ds-preopt shows that inlining must be happening in the simple optimizer. Is there anything one could do to disable the optimizations? Thanks in advance, Facundo -- 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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Help needed with optimization
by Tom Ellis 24 Jan '24

24 Jan '24

Hello GHC devs, I'm trying to understand why my code is not being optimized in the way I would expect. I'm completely stuck and I think I need the advice of an expert. I'm writing an effect system on top of transformers. The effect system wraps monad transformers in a newtype that encodes the composition structure of the transformers at the type level. Because it's a newtype all of the class members are inherited directly from the underlying type using coerce. When I implement something using this effect system I would expect to generate exactly the same code as if I had written it using transformers directly. However, it generates significantly worse code, even in a very simple case. Firstly, a case where I do get the same code. All of these compile to the constant 1. Hooray! https://github.com/tomjaguarpaw/ad/blob/cd0d876ddb448fe611515e8768dee66dc02… Secondly, a simple cases where I do not get the same code. `mySumMTL` and `mySumNewtype` yield the same code, as expected. After all, `mySumNewtype` does exactly the same thing as `mySumMTL`, it's just wrapped in some newtypes. However, `mySumEff` yields worse code, despite *also* being the same thing as `mySumMTL` just wrapped in some newtypes. https://github.com/tomjaguarpaw/ad/blob/cd0d876ddb448fe611515e8768dee66dc02… You can compare the generated loops at: https://github.com/tomjaguarpaw/ad/blob/cd0d876ddb448fe611515e8768dee66dc02… Does anyone have a clue what's going wrong in the optimizer here? I don't think the singleton that I pass around to access the type level index at runtime has anything to do with it. That seems to be optimized away by inlining. Is the simplifier confused by all the coercions? Thanks for any help anyone may be able to shed, Tom

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Google Summer of Code Ideas
by Aaron Allen 23 Jan '24

23 Jan '24

Hello, Haskell will be applying for the Google Summer of Code 2024 program and it would be great to have some project ideas involving GHC. If anything comes to mind, you can submit an idea proposal using the instructions found here: https://summer.haskell.org/ideas.html. Kind Regards, Aaron

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GHC Weekly call
by Simon Peyton Jones 23 Jan '24

23 Jan '24

Dear GHC developers Every week (2pm London time, Tuesdays) we hold a call for people to share what they are working on GHC-land. Would you like to come? The call is open to *any member of the GHC Team. *And *the GHC Team is inclusive*: if you are working on GHC you are a member of the Team: see here for details <https://gitlab.haskell.org/ghc/ghc-hq#2-the-ghc-team>. Why come? - It gives you a window into what is happening in GHC land - It gives you a chance to influence what is happening - It builds community -- you get face-to-face contact with others working on GHC If you'd like to come, drop a line to Matthew Pickering matthew(a)well-typed.com See you there! Simon

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Do you use stack to build hadrian?
by Bryan Richter 19 Jan '24

19 Jan '24

If nobody uses stack to build hadrian, (or even if a few people do), there's talk of removing that option because very few people do use it. Feel free to weigh in at https://gitlab.haskell.org/ghc/ghc/-/issues/24274#note_541913 -Bryan

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