November 2024 - Haskell-Cafe

Best way to implement "scoped exceptions"?
by Tom Ellis 27 May '25

27 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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[ANN] Darcs 2.18.4 release
by Ben Franksen 09 Jan '25

09 Jan '25

Hello Everyone On behalf of the Darcs [1] team, I would like to announce the release of Darcs 2.18.4 [2]. See [3] for the full release notes. The main reason for this release was to make it build with stack against stackage lts-22.34 on Linux. This in turn should hopefully make it re-appear in the Debian and Ubuntu GNU/Linux distros. Cheers Ben [1] https://darcs.net [2] https://hackage.haskell.org/package/darcs-2.18.4 [3] https://hackage.haskell.org/package/darcs-2.18.4/changelog

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Re: [Haskell-cafe] Configuring cabal to tell HPC to ignore certain functions
by Ryan Scott 29 Dec '24

29 Dec '24

Hi Ivan, Short answer: I'm not aware of a way to automate the process of excluding certain definitions from cabal-generated HPC coverage reports. Long answer: there is a way to manually modify HPC coverage reports using the hpc overlay command. Sadly, most the existing documentation on this command [1] is pretty sparse, so the only way I was able to discover this was by reading the original HPC paper [2] (in that paper, `hpc overlay` is called `hpc-makemtix`) and by finding an example of how to use `hpc overlay` in GHC's test suite [3]. To translate that example into prose: suppose you have the following program (hpc001.hs): main = print (const "Hello" "World") Here, the expression "World" is never evaluated, so by default HPC will claim that there is no coverage for it. This is pretty silly, however, so we'd like to instruct HPC not to warn about "World". To do so, first compile the example above with coverage enabled and run it: $ ghc -fhpc hpc001.hs $ ./hpc001 This should generate an hpc001.tix file. If you run `hpc markup hpc001.tix` and look at the resulting HTML report, you'll see that it warns about "World". Let's fix that. To do so, we have to define an HPC overlay file. The example given in GHC's test suite (sample_overlay.ovr) is: module "Main" { inside "main" { tick "\"World\"" on line 1; } } While I can't find a reference for the syntax that HPC overlay files use, this particular example is pretty self-explanatory: don't report about the expression "World" on line 1, which is found inside the `main` function in the `Main` module. In order to make use of this overlay file, we first have to convert it to a .tix file: $ hpc overlay sample_overlay.ovr > sample_overlay1.tix Then we have to combine it with our original .tix file: $ hpc combine hpc_sample.tix sample_overlay1.tix > total1.tix If we run `hpc markup total1.txt`, we now see that HPC no longer warns about "World", just as we'd hoped for. That is where my understanding of this feature ends, however. In particular, I'm not aware of a better way to generate overlay files (other than to write them by hand, which still feels somewhat laborious), and I'm not aware of any deeper integration with cabal so that these overlay files would be picked up and automatically applied (e.g., for reporting coverage on Hackage). Given that the HPC documentation for `hpc overlay` [1] labels this as an experimental feature, I'd be surprised if such tooling or integration existed. I agree that it would be cool if did, however. Best, Ryan ----- [1] https://hpc-bin.readthedocs.io/en/latest/hpc.html#hpc-overlay-and-hpc-draft [2] https://dl.acm.org/doi/abs/10.1145/1291201.1291203 [3] https://gitlab.haskell.org/ghc/ghc/-/blob/e39c8c993c1da534c5893ca418d1fa4cb…

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[ANNOUNCE] GHC 9.12.1-rc1 is now available
by Zubin Duggal 29 Nov '24

29 Nov '24

The GHC developers are very pleased to announce the availability of the release candidate for GHC 9.12.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. We hope to have this release available via ghcup shortly. GHC 9.12 will bring a number of new features and improvements, including: * The new language extension [OrPatterns] allowing you to combine multiple pattern clauses into one. * The [MultilineStrings] language extension to allow you to more easily write strings spanning multiple lines in your source code. * Improvements to the OverloadedRecordDot extension, allowing the built-in `HasField` class to be used for records with fields of non lifted representations. * The [NamedDefaults] language extension has been introduced allowing you to define defaults for typeclasses other than `Num`. * More deterministic object code output, controlled by the `-fobject-determinism` flag, which improves determinism of builds a lot (though does not fully do so) at the cost of some compiler performance (1-2%). See #12935 for the details * GHC now accepts type syntax in expressions as part of [GHC Proposal #281]. * The WASM backend now has support for TemplateHaskell. * ... and many more A full accounting of changes can be found in the [release notes][]. As always, GHC's release status, including planned future releases, 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. Cheers, Zubin [release notes]: https://downloads.haskell.org/ghc/9.12.1-rc1/docs/users_guide/9.12.1-notes.… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.1-rc1 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [OrPatterns]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0522-o… [MultilineStrings]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0569-m… [GHC Proposal #281]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-v… [NamedDefaults]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0409-e…

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Suggestion for addition to base
by Ivan Perez 23 Nov '24

23 Nov '24

Hi, I have a suggestion of something to add to Control.Arrow. The email address listed here: https://hackage.haskell.org/package/base-4.20.0.1/docs/Control-Arrow.html is libraries(a)haskell.org, which bounces because I'm not subscribed to that mailing list. What's the right mechanism to submit my proposal? Thanks, Ivan

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Re: [Haskell-cafe] attpoarsec and recursive parsing
by Olaf Klinke 23 Nov '24

23 Nov '24

Hi Frederic, Essentially, the '|' character acts like a binary operator in parsing arithmetical expression trees. To that end, the most ergonomic module I know of is provided by parsec [1], which has a port [2] to attoparsec. You have to decide whether the '|' operator should be left- or right- associative. That question vanishes if you decide to re-design your data type as data MaskLocation = MaskLocation Text | MaskLocation'Tmpl Text | MaskLocation'Or (NonEmpty MaskLocation) and use the sepBy1 combinator. Also you should think about whether and how to escape the separator Char inside ordinary MaskLocation strings. Olaf [1] https://hackage.haskell.org/package/parsec-3.1.17.0/docs/Text-ParserCombina… [2] https://hackage.haskell.org/package/attoparsec-expr

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POPL 2025 Call for Participation - Early registration: 20 December
by Ningning Xie 20 Nov '24

20 Nov '24

POPL 2025 -- Call for Participation ** Early registration deadline: 20 December 2024 ** Come and join us for POPL 2025, in January, in Denver, Colorado, the United States. Register here: https://popl25.sigplan.org/attending/registration Location: Curtis Hotel Denver, 1405 Curtis Street, 80202, Denver, Colorado, United States Dates: - Main conference: Wed 22 - Fri 24 January - Workshops, tutorials, co-located events: Sun 19, Mon 20, Tue 21 and Sat 25 January Accepted papers: https://popl25.sigplan.org/track/POPL-2025-popl-research-papers Full details of the conference and co-located events: https://popl25.sigplan.org/ Sponsors: POPL is supported by generous sponsorship from: - Amazon - Jane Street - Google DeepMind - JetBrains - Microsoft - Epic - Google - Ahrefs The 52nd ACM SIGPLAN Symposium on Principles of Programming Languages (POPL 2025) will take place in January in Denver, Colorado, the United States. It's going to be an amazing conference - don't miss it - sign up now! https://popl25.sigplan.org/attending/registration

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Dependent Haskell vs. Explicit Dictionary Application
by Henning Thielemann 20 Nov '24

20 Nov '24

There was an effort to add explicit type class dictionary applications to GHC: https://arxiv.org/abs/1807.11267 Will the new features of Dependent Haskell eventually supersede this approach? The current features in GHC-9.12 do not seem to allow for explicit specification of type class dictionaries.

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[TFP 2025 3rd Call for Papers] 25th International Symposium on Trends in Functional Programming (Oxford, UK)
by Peter Achten 15 Nov '24

15 Nov '24

# TFP 2025 - 3rd Call for Papers (trendsfp.github.io) 26th International Symposium on Trends in Functional Programming 14-16 January 2025, Oxford, UK ## Important Dates Submission deadline (pre-symposium, full papers): Wed 13th Nov 2024 (AOE) Notification (pre-symposium, full papers): Wed 11th Dec 2024 Submission deadline (pre-symposium draft papers): Wed 11th Dec 2024 (AOE) Notification (pre-symposium draft papers): Mon 16th Dec 2024 Submission deadline (post-symposium review): Wed 19th Feb 2025 (AOE) Notification (post-symposium submissions): Wed 26th Mar 2025 The Symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions. This year, TFP will take place in-person at the University of Oxford, UK. It is co-located with the Trends in Functional Programming in Education (TFPiE) workshop, which will take on the day before the main symposium. Please be aware that TFP has several submission deadlines. The first, 13th November, is for authors who wish to have their full paper reviewed prior to the symposium. Papers that are accepted in this way must also be presented at the symposium. The second, 11th December, is for authors who wish to present their work or work-in-progress at the symposium first without submitting to the full review process for publication. These authors can then take into account feedback received at the symposium and submit a full paper for review by the third deadline, 19th February. ## Scope The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five paper categories. High-quality submissions are solicited in any of these categories: * Research Papers: Leading-edge, previously unpublished research work * Position Papers: On what new trends should or should not be * Project Papers: Descriptions of recently started new projects * Evaluation Papers: What lessons can be drawn from a finished project * Overview Papers: Summarizing work with respect to a trendy subject Papers must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: * Functional programming and multicore/manycore computing * Functional programming in the cloud * High performance functional computing * Extra-functional (behavioural) properties of functional programs * Dependently typed functional programming * Validation and verification of functional programs * Debugging and profiling for functional languages * Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. * Interoperability with imperative programming languages * Novel memory management techniques * Program analysis and transformation techniques * Empirical performance studies * Abstract/virtual machines and compilers for functional languages * (Embedded) domain specific languages * New implementation strategies * Any new emerging trend in the functional programming area If you are in doubt on whether your paper is within the scope of TFP, please contact the programme chair, Jeremy Gibbons. ## Best Paper Awards TFP awards two prizes for the best papers each year. First, to reward excellent contributions, TFP awards a prize for the best overall paper accepted for the post-conference formal proceedings. Second, each year TFP also awards a prize for the best student paper. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are the paper's first authors, and a student would present the paper. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, then that paper will receive both prizes. ## Instructions to Authors Submission is via EquinOCS (https://equinocs.springernature.com/service/tfp2025) Please observe that it is temporarily closed while we deal with pre-symposium full paper submissions, it will reopening on 2nd December. Authors of papers have the choice of having their contributions formally reviewed either before or after the Symposium. Further, pre-symposium submissions may either be full (earlier deadline) or draft papers (later deadline). ## Pre-symposium formal review Papers to be formally reviewed before the symposium should be submitted before the early deadline and will receive their reviews and notification of acceptance for both presentation and publication before the symposium. A paper that has been rejected for publication but accepted for presentation may be revised and resubmitted for the post-symposium formal review. ## Post-symposium formal review Draft papers will receive minimal reviews and notification of acceptance for presentation at the symposium. Authors of draft papers will be invited to submit revised papers based on the feedback received at the symposium. A post-symposium refereeing process will then select a subset of these papers for formal publication. ## Paper categories Draft papers and papers submitted for formal review are submitted as extended abstracts (4 to 10 pages in length) or full papers (up to 20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which all authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. ## Format Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS Guidelines web site (https://www.springer.com/gp/computer-science/lncs/conference-proceedings-gu…) ## Organizing Committee Jeremy Gibbons University of Oxford, UK Programme Chair Jason Hemann Seton Hall University, US Conference Chair Peter Achten Radboud University Nijmegen, NL Publicity Chair Marco T. Morazán Seton Hall University, US Steering Committee Chair ## Programme Committee Peter Achten Radboud University Nijmegen, NL Edwin Brady University of St Andrews, UK Laura Castro University of A Coruña, ES Youyou Cong Tokyo Institute of Technology, JP Paul Downen University of Massachusetts Lowell, US João Paulo Fernandes University of Coimbra, PT Ben Greenman University of Utah, US Jurriaan Hage Heriot-Watt University, UK Jason Hemann Seton Hall University, US Zhenjiang Hu Peking University, CN Hans-Wolfgang Loidl Heriot-Watt University, UK Kazutaka Matsuda Tohoku University, JP Zoe Paraskevopoulou Ethereum Foundation, US Alejandro Serrano 47 Degrees, ES Nick Smallbone Chalmers University, SE Alley Stoughton Boston University, US Wouter Swierstra Utrecht University, NL Niki Vazou IMDEA Software Institute, ES Marcos Viera Universidad de la República, UY Viktória Zsók Eötvös Loránd University of Sciences, HU

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[ANNOUNCE] GHC 9.12.1-alpha3 is now available
by Zubin Duggal 15 Nov '24

15 Nov '24

The GHC developers are very pleased to announce the availability of the third alpha release of GHC 9.12.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. We hope to have this release available via ghcup shortly. GHC 9.12 will bring a number of new features and improvements, including: * The new language extension [OrPatterns] allowing you to combine multiple pattern clauses into one. * The [MultilineStrings] language extension to allow you to more easily write strings spanning multiple lines in your source code. * Improvements to the OverloadedRecordDot extension, allowing the built-in `HasField` class to be used for records with fields of non lifted representations. * The [NamedDefaults] language extension has been introduced allowing you to define defaults for typeclasses other than `Num`. * More deterministic object code output, controlled by the `-fobject-determinism` flag, which improves determinism of builds a lot (though does not fully do so) at the cost of some compiler performance (1-2%). See #12935 for the details * GHC now accepts type syntax in expressions as part of [GHC Proposal #281]. * The WASM backend now has support for TemplateHaskell. * ... and many more A full accounting of changes can be found in the [release notes][]. As always, GHC's release status, including planned future releases, 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. [release notes]: https://downloads.haskell.org/ghc/9.12.1-alpha3/docs/users_guide/9.12.1-not… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.1-alpha3 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [OrPatterns]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0522-o… [MultilineStrings]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0569-m… [GHC Proposal #281]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-v… [NamedDefaults]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0409-e… Cheers, Zubin

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