January 2021 - ghc-devs

Lexing / Parsing and final token
by Alan & Kim Zimmerman 19 Jan '21

19 Jan '21

I am (still) working on !2418 to bring the API Annotations into the GHC ParsedSource, and making good progress. I am currently making a rough port of ghc-exactprint, to ensure I can get all the tests around modifying the AST to work. One of the last pieces is being able to capture the spacing from the last token in the file to the EOF. I guess technically it is the second last token. Empirically (calling getTokenStream), it seems this is always ITsemi. I am not sure how this comes about, as the `module` parsing rule in Parser.y ends with body or body2, and those both finish with an actual or virtual '}'. Can I rely on the token before ITEof always being ITsemi? Alan

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Large Haddock submodule merge landing soon
by Ben Gamari 19 Jan '21

19 Jan '21

Hi all, I have a very large Haddock change (merging Haddock's development branch, `ghc-8.10`, into the `ghc-head` branch) pending in !4819. I will try merge it as soon as Marge's next batch finishes. If you have a Haddock change outstanding you will need to perform a rather significant rebase after this change goes in. However, I am available to help with rebase if needed; just send me the MR number of your merge request and I can take of the rest. Cheers, - Ben

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presentation: Next-gen Haskell Compilation Techniques
by Csaba Hruska 11 Jan '21

11 Jan '21

Hello, I did an online presentation about Haskell related (futuristic) compilation techniques. The application of these methods is also the main motivation of my work with the grin compiler project and ghc-wpc. video: https://www.youtube.com/watch?v=jyaR8E325ok slides: https://docs.google.com/presentation/d/1g_-bHgeD7lV4AYybnvjgkWa9GKuP6QFUyd2… Regards, Csaba

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How to get dwarf stack trace of all live threads (HECs) on demand (e.g. SIGQUIT)?
by YueCompl 07 Jan '21

07 Jan '21

Dear GHC devs, I recently encountered 100% ~ 400% CPU hogging situations to trouble-shoot, I managed to build and use a dwarf enabled GHC-8.10.3 on Ubuntu 18.04 and 20.04 for that, then I find it seemingly dumps only the main thread's stack trace when I press Ctrl+\, but my program is a concurrent language server that will fork a new thread to handle each LSP request, so I really need to see what is going on those threads when CPU get hogged, as the situation is not trivially reproducible, only occurs occasionally after some time of active IDE activity. And I can't figure out how to do that from the user guide, so help please! Thanks & best regards, Compl

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Title index
by Simon Peyton Jones 07 Jan '21

07 Jan '21

Ben On GHC's wiki home page<https://gitlab.haskell.org/ghc/ghc/-/wikis/home> we have Title Index<https://gitlab.haskell.org/ghc/ghc/wikis/index> over in the right margin. I thought the title index was auto-generated. But it doesn't contain this page<https://gitlab.haskell.org/ghc/ghc/-/wikis/Functional-dependencies-in-GHC>. What's up? Thanks Simon

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Debugging annotation machinery
by Phyx 04 Jan '21

04 Jan '21

Hi, I am changing the hashing algorithm used to create Fingerprints but somehow the annrun01.hs test has started failing and I don't quite understand why. >From what I can tell the failure happens during deserialization but the module itself is still found. The calls findGlobalAnns deserializeWithData target seem to return nothing. Calling ~/ghc/inplace/bin/ghc-stage2.exe --show-iface Annrun01_Help.hi shows that GHC itself understands the module fine though. Does someone have an idea why deserialization might be failing? but also how does one debug this? Almost none of the structures have a Show or Outputable instance... Thanks, Tamar

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Nominations close Jan 11th for the Haskell Foundation Board
by Simon Peyton Jones 04 Jan '21

04 Jan '21

Friends Happy new year! The closing date for self-nominations for membership of the Board of the Haskell Foundation<https://haskell.foundation/> is in just under a week: Monday January 11th 2021 The Haskell Foundation is a new non-profit organisation that seeks to articulate the benefits of functional programming to a broader audience, to erase barriers to entry, and to support Haskell as a solidly reliable basis for mission-critical applications. The Board provides the strategic leadership for the Foundation, including its goals, governance, finances, and staff. Membership of the Board is a key leadership role, not an honorary appointment. The Call for Nominations<https://haskell.foundation/board-nominations/> gives more details. Please do consider nominating yourself, or encouraging a suitable (but perhaps modest) colleague to do so. The HF needs a strong Board, and that means strong nominations. Thanks! Please do forward this message on social media or elsewhere. Simon Peyton Jones

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Interruptible foreign calls, part 2
by David Feuer 04 Jan '21

04 Jan '21

Another issue is that (as the recently expanded documentation shows), an `interruptible` foreign call is not actually an "interruptible operation" in the sense used in Control.Exception. To make it so, the caller has to manually call `allowInterrupt`. Or, in the single-threaded runtime, has to call `interruptibleYield`. So ... they have to make a foreign call to `rts_isThreaded` and then use the result to figure out how to deliver a potential exception. What a terrible user experience! Why not bake the "deliver exceptions" part into the FFI code for `interruptible` calls? This stuff has been documented so very recently (not in any released GHC) that I don't imagine much will break if we make these changes.

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Interruptible foreign calls and uninterruptibleMask
by David Feuer 04 Jan '21

04 Jan '21

As I understand it, `interruptible` foreign calls aren't affected by the masking state at all. It seems to me that they really should respect `uninterruptibleMask`. In particular, within the scope of `uninterruptibleMask`, if an exception arrives and successfully interrupts the foreign call, then the `allowInterrupt` call will not actually deliver the exception, and the failure/EINTR loop (e.g., throwIfMinus1Retry) will just perform the interrupted call again. How I think it should work is that an `interruptible` foreign call should act exactly like a `safe` foreign call in the scope of `uninterruptibleMask`. Does this make sense? If so, does it need a GHC proposal?

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ghcs-nix: An archive of GHC derivations
by Ben Gamari 03 Jan '21

03 Jan '21

Hi all, As a NixOS user, I have in the past found that I need an older GHC release that is no longer available (or not yet available) in nixpkgs. While mpickering's old-ghc-nix [1] conveniently allows standard GHC binary distributions to be adapted for use under NixOS, I have found that it is less than reliable when building later projects [2]. For this reason, I have started collecting a set of nix expressions for building GHC versions all the way back to GHC 8.4.1 and extending up to GHC 9.0.1-rc1. In addition, there is a binary cache with pre-built binaries (complements of Cachix) here [3]. I intend on continuing to add new releases to this as needed. I hope others also find this helpful. Cheers, - Ben [1] https://github.com/mpickering/old-ghc-nix [2] https://gitlab.haskell.org/ghc/ghc/-/issues/19144 [3] https://app.cachix.org/cache/ghc

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