January 2022 - ghc-devs

Question about ambiguous predicates in pattern bindings
by Benjamin Redelings 16 Aug '22

16 Aug '22

Hi, 1. I'm reading "A Static semantics for Haskell" and trying to code it up. I came across some odd behavior with pattern bindings, and I was wondering if someone could explain or point me in the right direction. Suppose you have the declaration (x,y) = ('a',2) My current code is yielding: x :: Num a => Char y :: Num a => a However, I notice that ghci gives x the type Char, with no constraint, which is what I would expect. It also gives y the type 'Num b => b', so I don't think it is defaulting a to Int here. The weird results from my code stem from rule BIND-PRED in Figure 15 of https://homepages.inf.ed.ac.uk/wadler/papers/staticsemantics/static-semanti… E |- bind ~~> \dicts : theta -> monobinds in bind : (LIE_{encl}, theta => LVE) Here theta = ( Num a ) and LVE = { x :: Char, y :: a }. So, theta => LVE is { x :: Num a => Char, y :: Num a => a } The obvious thing to do is avoid changing a type T to Num a => T if T does not contain a. Also I'm not totally sure if that trick gets to the bottom of the issue. However, the paper doesn't mention define theta => LVE that way. Is there something else I should read on this? 2. If we just chop out predicates which mention variables not in the type ( == ambiguous predicates?) then I'm not totally sure how to create code for this. I would imagine that we have something like tup dn = ('a', fromInteger dn 2) x = case (tup dn) of (x,y) -> x y dn case (tup dn) of (x,y) -> y In this case its not clear where to get the `dn` argument of `tup` from, in the definition of `x`. Can we pass in `undefined`? Should we do something else? If anyone can shed light on this, I would be grateful :-) -BenRI

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9.2.2 release schedule?
by Shayne Fletcher 03 Feb '22

03 Feb '22

Anyone know of any scheduling plans for ghc-9.2.2? Happy holidays y'all :) -- Shayne Fletcher

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use Hadrian to see if compiler compiles?
by Norman Ramsey 02 Feb '22

02 Feb '22

I'm currently doing some refactoring of the GHC sources, which involves moving definitions between modules. As a sanity check I want to compile often, so if I've broken anything (or have inadvertently created circular imports) I can find out quickly. I'm currently using ./hadrian/build -j but I really don't need to build libraries or a stage 2 compiler. I tried looking over `./hadrian/build --help`, but I'm not confident that I understand what's there. My best guess is ./hadrian/build -j stage0:lib:ghc Would that accomplish my goal? Norman

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downloading ghc head version
by Harendra Kumar 31 Jan '22

31 Jan '22

Hi, I am trying to set up a CI for ghc head version. I am not sure what is the official supported method to install a nightly/head version of GHC. haskell/actions/setup on github seems to support it via ghcup. But it almost always fails, I saw it succeeding once till now. It tries to download it from the following URL, but fails with 404 not found: downloading: https://gitlab.haskell.org/ghc/ghc/-/jobs/artifacts/master/raw/ghc-x86_64-d… I tried this link in the browser and I get the same error. Is that the right way to install it? If not, can someone suggest a reliable way to get the head version, other than building it myself? Thanks, Harendra

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Request for Nominations to the GHC Steering Committee
by Joachim Breitner 29 Jan '22

29 Jan '22

Dear Haskell community, the GHC Steering committee is seeking nominations for at least two new members. The committee scrutinizes, nitpicks, improves, weights and eventually accepts or rejects proposals that extend or change the language supported by GHC and other (public-facing) aspects of GHC. Our processes are described at https://github.com/ghc-proposals/ghc-proposals which is also the GitHub repository where proposals are proposed. In particular, please have a look at the bylaws at https://github.com/ghc-proposals/ghc-proposals/blob/master/committee.rst We are looking for a member who has the ability * to understand such language extension proposals, * to find holes and missing corner cases in the specifications, * foresee the interaction with other language features and specifications, * comment constructively and improve the proposals, * judge the cost/benefit ratio and * finally come to a justifiable conclusion. We look for committee members who have some of these properties: * have substantial experience in writing Haskell applications or libraries, which they can use to inform judgements about the utility or otherwise of proposed features, * have made active contributions to the Haskell community, for some time, * have expertise in language design and implementation, in either Haskell or related languages, which they can share with us. 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. There is no shortage of people who are eager to get fancy new features into the language, both in the committee and the wider community. But each new feature imposes a cost, to implement, to learn, (particularly) through its unexpected interaction with other features. We need to strike a balance, one that encourages innovation (as GHC always has) while still making Haskell attractive for real-world production use and for teaching. We therefore explicitly invite “conservative” members of the community to join the committee. To make a nomination, please send an email to me (as the committee secretary) at mail(a)joachim-breitner.de until February 11th. I will distribute the nominations among the committee, and we will keep the nominations and our deliberations private. We explicitly encourage self-nominations. You can nominate others, 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, Joachim Breitner -- Joachim Breitner mail(a)joachim-breitner.de http://www.joachim-breitner.de/

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meaning of title of merge request on gitlab?
by Norman Ramsey 28 Jan '22

28 Jan '22

I've opened a very, very preliminary MR. The Commentary instructs me to tag it "WIP: <title>", but gitlab seems to suggest that in order to prevent it from being merged, I need "Draft: <title>". When a merge request is both WIP and Draft, how should it be titled? (The MR in question is https://gitlab.haskell.org/ghc/ghc/-/merge_requests/7442) Norman

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trouble with formatted Haddock documentation for new module
by Norman Ramsey 27 Jan '22

27 Jan '22

In a new module I'm writing for GHC, I define a record type that has a ton of named fields. Two of the fields have rather large types. Unfortunately, at least with default settings, the Haddock documentation that is well-nigh unreadable (image attached). Does anyone have a workaround? Hécate? (If it's useful, a draft of the offending module can be found at https://gitlab.haskell.org/ghc/ghc/-/blob/wip/backend-as-record/compiler/GH….) Norman

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Curiosity: What accounts for the growth in the RTS in 9.2?
by Norman Ramsey 26 Jan '22

26 Jan '22

I was chatting with a colleague who asked "How big is GHC's run-time system." In the spirit of "look and find out," I found this: nr@homedog ~/.ghcup> ls -hl **HSrts.a -rw-r--r-- 1 nr nr 8.7M Sep 27 16:13 ghc/8.10.7/lib/ghc-8.10.7/rts/libHSrts.a -rw-r--r-- 1 nr nr 8.8M Sep 27 16:36 ghc/9.0.1/lib/ghc-9.0.1/rts/libHSrts.a -rw-r--r-- 1 nr nr 11M Jan 24 15:06 ghc/9.2.1/lib/ghc-9.2.1/rts/libHSrts.a All these binaries were installed by ghcup using the default settings. Does anybody know what accounts for the big jump from 9.0 to 9.2? Norman

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any luck using "retrie" refactoring tool on GHC sources?
by Norman Ramsey 24 Jan '22

24 Jan '22

I'm trying to use the retrie refactoring tool, but I'm getting a mysterious error message: nr@homedog ~/a/g/compiler [1]> retrie --adhoc "forall x . not (backendNeedn'tLink x) == backendNeedsLink x" parseAdhocs:1:80: error: parse error on input ‘#-}’ retrie: user error (parse failed) The Tool was installed using cabal install; its github page is https://github.com/facebookincubator/retrie/#readme. Has anybody had any success using this tool on the GHC sources? I'd love a workaround, or just advice. Norman

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Not all splices are the same
by David Feuer 23 Jan '22

23 Jan '22

I've been a bit upset by the challenges Template Haskell poses for type inference. For example, (3 :: Int) == $$(...) may typecheck when $$(...) == (3 :: Int) does not. I don't imagine this problem can be solved in general, but I'm rather curious whether it might be possible to solve for "pure" splices, with types that look like forall m. Quote m => Code m a Would it be possible to get full bidirectional inference for these if they were somehow marked specially by the user? For instance, if I wrote $$$e that could mean e should be interpreted as having some type PCode a where newtype PCode a = PCode (forall m. Quote m => a)

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