September 2023 - Haskell-Cafe

Best way to implement "scoped exceptions"?
by Tom Ellis 28 May '25

28 May '25

I'd like to implement "scoped exceptions"[1], that is, a combinator of type withScopedException :: ((forall a. e -> IO a) -> IO r) -> IO (Either e r) so I can use exceptions in a convenient way, without having to set up fragile "action at a distance" based on the type of the thing I'm throwing and catching, for example: scopedExceptionExample :: IO (Either String (Either Int Void)) scopedExceptionExample = do withScopedException $ \throw1 -> withScopedException $ \throw2 -> if (1 :: Int) < 0 then throw1 "Hello" else throw2 1234 -- ghci> scopedExceptionExample -- Right (Left 1234) Here's a hacky way of doing it, based on tagging each exception with a unique value, and then filtering when handling: data MyException where MyException :: e -> Data.Unique.Unique -> MyException instance Show MyException where show _ = "<MyException>" instance Exception MyException withScopedException :: ((forall a. e -> IO a) -> IO r) -> IO (Either e r) wiathScopedException f = do fresh <- Data.Unique.newUnique flip tryJust (f (\e -> throwIO (MyException e fresh))) $ \case MyException e tag -> -- unsafeCoerce is very unpleasant if tag == fresh then Just (unsafeCoerce e) else Nothing This is the approach taken by the effectful library[2]. But is there a better way? Can I persuade GHC's RTS to work like this directly? Tom [1] I don't know if these already have a name [2] https://hackage.haskell.org/package/effectful-core-2.2.1.0/docs/src/Effectf…

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CFP, PEPM 2024 ACM SIGPLAN Workshop on Partial Evaluation and Program Manipulation
by Meng Wang 04 Oct '23

04 Oct '23

Dear Haskellers, Gabriele and I are organising PEPM this year. Over the years, PEPM has grown into a conference of general PL topics and Haskell/FP is strongly represented. We look forward to receiving your submissions. Best regards, Gabriele Keller (Utrecht University, Netherlands) Meng Wang (University of Bristol, UK) ---------------------------------------------------------------------------------------------------------- ** ** CALL FOR PAPERS ** ** PEPM at POPL 2024 ** Workshop on PARTIAL EVALUATION AND PROGRAM MANIPULATION ** 16th of January 2024, London, United Kingdom ** ** Submission Deadline: ** 18 October 2023 ** ** https://popl24.sigplan.org/home/pepm-2024 ** https://easychair.org/conferences/?conf=pepm24 ** ----------------------------------------------------------------------------------------------------------- ACM SIGPLAN Workshop on PARTIAL EVALUATION AND PROGRAM MANIPULATION (PEPM) 2024 =============================================================================== * Website : https://popl24.sigplan.org/home/pepm-2024 * Time : 16th January 2024 * Place : London, United Kingdom (co-located with POPL 2024) The ACM SIGPLAN Workshop on Partial Evaluation and Program Manipulation (PEPM) has a history going back to 1991 and has been co-located with POPL every year since 2006. It originated with the discoveries of useful automated techniques for evaluating programs with only partial input. Over the years, the scope of PEPM has expanded to include a variety of research areas centred around the theme of semantics-based program manipulation — the systematic exploitation of treating programs not only as subjects to black-box execution but also as data structures that can be generated, analysed, and transformed while establishing or maintaining important semantic properties. Scope ----- In addition to the traditional PEPM topics (see below), PEPM 2024 welcomes submissions in new domains, in particular: * Semantics based and machine-learning based program synthesis and program optimisation. * Modelling, analysis, and transformation techniques for distributed and concurrent protocols and programs, such as session types, linear types, and contract specifications. More generally, topics of interest for PEPM 2024 include, but are not limited to: * Program and model manipulation techniques such as: supercompilation, partial evaluation, fusion, on-the-fly program adaptation, active libraries, program inversion, slicing, symbolic execution, refactoring, decompilation, and obfuscation. * Techniques that treat programs/models as data objects including metaprogramming, generative programming, embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. * Program analysis techniques that are used to drive program/model manipulation such as: abstract interpretation, termination checking, binding-time analysis, constraint solving, type systems, automated testing and test case generation. * Application of the above techniques including case studies of program manipulation in real-world (industrial, open-source) projects and software development processes, descriptions of robust tools capable of effectively handling realistic applications, benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, embedded and resource-limited computation, and security. This list of categories is not exhaustive, and we encourage submissions describing new theories and applications related to semantics-based program manipulation in general. If you have a question as to whether a potential submission is within the scope of the workshop, please contact the programme co-chairs, Gabriele Keller (g.k.keller(a)uu.nl<mailto:g.k.keller@uu.nl>) and Meng Wang (meng.wang(a)bristol.ac.uk<mailto:meng.wang@bristol.ac.uk>). Submission categories and guidelines ------------------------------------ Three kinds of submissions will be accepted: * Regular Research Papers should describe new results, and will be judged on originality, correctness, significance, and clarity. Regular research papers must not exceed 12 pages. * Short Papers may include tool demonstrations and presentations of exciting if not fully polished research, and of interesting academic, industrial, and open-source applications that are new or unfamiliar. Short papers must not exceed 6 pages. * Talk Proposals may propose lectures about topics of interest for PEPM, existing work representing relevant contributions, or promising contributions that are not mature enough to be proposed as papers of the other categories. Talk Proposals must not exceed 2 pages. References and appendices are not included in page limits. Appendices may not necessarily be read by reviewers. Both kinds of submissions should be typeset using the two-column ‘sigplan’ sub-format of the new ‘acmart’ format available at: http://sigplan.org/Resources/Author/ and submitted electronically via EasyChair: https://easychair.org/conferences/?conf=pepm24 Reviewing will be single-blind. Submissions are welcome from PC members (except the two co-chairs). Accepted regular research papers will appear in formal proceedings published by ACM, and be included in the ACM Digital Library. Accepted short papers do not constitute formal publications and will not appear in the proceedings. At least one author of each accepted contribution must attend the workshop (physically or virtually) and present the work. In the case of tool demonstration papers, a live demonstration of the described tool is expected. Important dates --------------- * Paper submission deadline : **Wednesday 18th October 2023 (AoE)** * Author notification : **Wednesday 15th November 2023 (AoE)** * Workshop : **Tuesday 16th January 2024** Best paper award ---------------- PEPM 2024 continues the tradition of a Best Paper award. The winner will be announced at the workshop. Programme committee ------------------- * Chairs: Gabriele Keller (Utrecht University, Netherlands) Meng Wang (University of Bristol, UK)

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two HLS processes with same name
by Dennis Raddle 03 Oct '23

03 Oct '23

I'm running the HLS plugin in VS Code on MacOS, on an M2 Apple Silicon MacBook Pro. My computer is running low on RAM and the HLS process seems to be fairly large for me (over 1 GB). When I first run VS Code on a Haskell workspace, there is one process called "haskell-language-server-9.2.8". Eventually another process, taking an additional 1 GB, joins it. The two processes have the same name. Is it supposed to work like this? Note about Apple Silicon FYI: I bought this computer in June with 16 GB Integrated RAM. RAM is very expensive in a SoC. I'm discovering that a lot of programs are memory hogs, not sure if it's the latest MacOS or M2, but between running software development stuff and music-related apps, I'm running out of RAM. I just bought a similar MacBook but with 32 GB RAM, and will sell the current one. So eventually I won't worry about an extra GB process or two. Dennis

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Taking over maintainership of `fec`
by Jean-Paul Calderone 02 Oct '23

02 Oct '23

Hello all, I would like to take over maintainership of the `fec` package ( https://hackage.haskell.org/package/fec) fec is a wrapper around a C library for a certain forward erasure code scheme. Neither the C library nor the Haskell bindings have changed much since the package was last published on Hackage but recently I've fixed a multithreading bug that leads to corrupt results and an integer overflow bug that leads to memory corruption and typically a segfault. I would love to make these improvements available via Hackage. I don't think `fec` has much of a user base but I've recently published several cryptographic libraries on Hackage which depend on it and suffer from the two issues mentioned above. I'm also an upstream maintainer of the project (https://github.com/tahoe-lafs/zfec) I have emailed the current maintainer and original author of the Haskell bindings, Adam Langley (cced), about becoming a maintainer but I haven't yet heard back. Thanks, Jean-Paul

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Using deriving with indexed data types
by Tom Ellis 01 Oct '23

01 Oct '23

How are Haskell folks using indexed types (indexed by types of DataKinds) whilst still being able to take advantage of GHC's deriving? I wrote up my approach into a small library: https://github.com/tomjaguarpaw/ad/blob/589926448865700fa616e8c6f6e4a011384… Here's an example of using my approach: https://github.com/tomjaguarpaw/ad/blob/master/indexed-types/src/IndexedTyp… and it shows you can still use deriving for all your instances: https://github.com/tomjaguarpaw/ad/blob/master/indexed-types/src/IndexedTyp… I'd be interested to know what other approaches people are using in practice. Thanks, Tom

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[ANNOUNCE] GHC 9.8.1-rc1 is now available
by Ben Gamari 30 Sep '23

30 Sep '23

The GHC developers are very pleased to announce the availability of the release candidate of GHC 9.8.1. Binary distributions, source distributions, and documentation are available at https://downloads.haskell.org/ghc/9.8.1-rc1 GHC 9.8 will bring 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]. This candidate includes roughly 20 new commits relative to alpha 4, including what we believe should be nearly the last changes to GHC's boot libraries. As always, GHC's release status 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. As always, do give this release a try and open a [ticket] if you see anything amiss. 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-alpha4/docs/users_guide/9.8.1-notes… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status

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[Haskell] [ANNOUNCE] Haskell Language Server 2.3.0.0 released
by Zubin Duggal 29 Sep '23

29 Sep '23

Binaries for this release are available at https://downloads.haskell.org/~hls/haskell-language-server-2.3.0.0/. These binaries can be installed using [GHCup](https://www.haskell.org/ghcup/) or the Haskell VSCode extension. All of these tarballs have associated GPG signatures. The signature should be from `Zubin Duggal <zubin(a)well-typed.com>` (key ID [588764FBE22D19C4](https://keys.openpgp.org/search?q=588764FBE22D19C4)) The prebuilt binaries in this release support the following GHC versions: - 9.0.2 - 9.2.8 - 9.4.6 - 9.6.2 - 9.6.3 ## Changelog * Binaries for GHC 9.6.3 * Drop support for GHC 8.10 * Remove `hls-haddock-comments-plugin`, `hls-stan-plugin`, and `hls-tactics-plugin` * Don't suggest bogus modules names in `hls-module-name-plugin` (#3784) * Add support for external Ormolu (#3771) * Improve refine imports behaviour for qualified imports (#3806) Happy editing! - Zubin

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How can I get in touch with Stan maintainers?
by Tom Ellis 28 Sep '23

28 Sep '23

Hello, Does anyone know how I can get in touch with the Stan maintainers? Stan is a static analysis tool for Haskell. Its maintainers seem to have stopped supporting it after the GHC 8.x series because the GHC API changed too frequently for them to want to keep making adjustments. Last year I did quite a lot of work to bring Stan and its dependencies up to 9.4. I made a PR to the Stan Github repo last month: https://github.com/kowainik/stan/pull/506 I haven't heard anything from the maintainers. The only maintainer I know of is Veronika Romashkina. I've sent her a private email about this but had no response. Does anyone know how I can get in contact with Veronika or another Stan maintainer, if there is one? I would be willing to be a comaintainer for Stan and the rest of the Kowainik ecosystem (if only to make sure they continue to work with future GHCs). Thanks, Tom

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[ANNOUNCE] GHC 9.6.3 is now available
by Bryan Richter 26 Sep '23

26 Sep '23

The GHC developers are happy to announce the availability of GHC 9.6.3. Binary distributions, source distributions, and documentation are available on the release page. https://www.haskell.org/ghc/download_ghc_9_6_3.html This release is primarily a bugfix release addressing a few issues found in the 9.6 series. These include: * Disable Polymorphic Specialisation (a performance optimisation) by default. It was discovered that Polymorphic Specialisation as currently implemented in GHC can lead to hard to diagnose bugs resulting in incorrect runtime results. Users wishing to use this optimisation despite the caveats will now have to explicitly enable the new `-fpolymorphic-specialisation` flag. For more details see #23469 as well as #23109, #21229, #23445. * Improve compile time and code generation performance when `-finfo-table-map` is enabled (#23103). * Make the recompilation check more robust when code generation flags are changed (#23369). * Addition of memory barriers that improve soundness on platforms with weak memory ordering. * And dozens of other fixes. A full accounting of changes can be found in the [release notes]. As some of the fixed issues do affect correctness users are encouraged to upgrade promptly. We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake pool, Well-Typed, Tweag I/O, Serokell, Equinix, SimSpace, 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. As always, do give this release a try and open a [GHC ticket] if you see anything amiss. Enjoy! -Bryan [release notes]: https://downloads.haskell.org/~ghc/9.6.3/docs/users_guide/9.6.3-notes.html [GHC ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

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[ANNOUNCE] GHC 9.8.1-alpha4 is now available
by Ben Gamari 24 Sep '23

24 Sep '23

The GHC developers are very pleased to announce the availability of the third alpha prerelease of GHC 9.8.1. Binary distributions, source distributions, and documentation are available at https://downloads.haskell.org/ghc/9.8.1-alpha4 GHC 9.8 will bring 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]. This alpha includes roughly 40 new commits relative to alpha 3, including what we believe should be nearly the last changes to GHC's boot libraries. 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. As always, do give this release a try and open a [ticket] if you see anything amiss. 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-alpha4/docs/users_guide/9.8.1-notes…

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