December 2023 - 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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[TFP (and TFPiE) 2024] Call For Participation (January 9-12, Seton Hall University, NJ, USA)
by Peter Achten 22 Dec '23

22 Dec '23

# TFP 2024 -- Call For Participation (trendsfp.github.io) ## Venue TFPiE and TFP will take place in-person at Seton Hall University, New Jersey in the United States. ## Dates TFPiE Workshop: Tuesday 9th January, 2024 TFP Symposium: Wednesday 10th - Friday 12th January, 2024 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. ## Keynote speakers We are happy to have the following keynotes in the programme: * Jeremy Gibbons, Oxford University * Benjamin Pierce, University of Pennsylvania * John Reppy, University of Chicago ## Programme The programme schedule can be found here: trendsfp.github.io/schedule.html ## Excursion and banquet After lunch on Thursday there is a private guided tour of the Thomas Edison National Historical Park and Museum. Thursday evening we have the TFP banquet at Forno's of Spain. During dinner the winners of the best paper awards of last year's TFP will be announced.

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RFC: Changes to the Haskell Symposium
by J. Garrett Morris 20 Dec '23

20 Dec '23

Dear Haskellers, In recent years, submissions to the Haskell Symposium have dropped precipitously: from 37 in 2017 to just 15 in 2023. This has made it increasingly challenging for the program committee to put together a program that is vibrant and intellectually rigorous. The Haskell Symposium steering committee has developed two proposals to reinvigorate the Symposium. The first is to invite a new category of presentations of work-in-progress results; the second is to submit accepted papers to the Journal of Functional Programming for publication rather than continuing the Symposium's proceedings. A full description of the proposed changes can be found on the Haskell Foundation's GitHub: https://github.com/haskellfoundation/tech-proposals/pull/62. We invite feedback from the community, particularly on how these changes would effect your participation (either as authors or attendees) in the future. Yours, /g J. Garrett Morris PC Co-chair, 2024 Haskell Symposium For the Haskell Symposium Steering Committee

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Call for Participation: BOB 2024 (Berlin, March 15)
by Stefan Wehr 15 Dec '23

15 Dec '23

========================================================================= BOB 2024 Conference “What happens if we simply use what’s best?” March 15, 2024, Berlin https://bobkonf.de/2024/ Program: https://bobkonf.de/2024/program.html Registration: https://bobkonf.de/2024/registration.html ========================================================================= BOB conference is a place for developers, architects, and decision-makers to explore technologies beyond the mainstream in software development and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experience. The program features 14 talks and 8 tutorials on current topics: https://bobkonf.de/2024/program.html Talk subjects includes functional programming, property-based testing, service API design, programming for spacecraft, accessibility, hypermedia, business processes, software analytics, event-based communication and zero-knowledge proofs. BOB will feature tutorials on F#, Haskell, Lean, SwiftUI, Copilot, the K Framework, functional domain modelling, and Liberating Structures. Andreas Rossberg will give the keynote talk. Registration is open - online tickets are all under 200€, and many discount options are available, as are grants for members of groups underrepresented in tech: https://bobkonf.de/2024/registration.html

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Package takeover request for socks
by Markus Läll 13 Dec '23

13 Dec '23

Hi I would like to take over maintainership of the socks package (as in the SOCKS network proxy protocol). Vincent announced archiving the package in September[1] (I'm also cc'ing him here). The github repository is here[2]. Unless there are objections, my plan is to simply maintain it under my own github account, because when I took over rapid, I created a new github organization for it. But until there are other people interested in contributing, then having an org for each library is not worth it IMO. [1] https://twitter.com/vincenthz/status/1704395906374889934 [2] https://github.com/vincenthz/hs-socks -- Markus Läll

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ICFP 2024: Call for Workshops and Co-Located Events
by ICFP Publicity 12 Dec '23

12 Dec '23

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2024 29th ACM SIGPLAN International Conference on Functional Programming September 2 - 7, 2024 Milan, Italy https://icfp24.sigplan.org/ The 29th ACM SIGPLAN International Conference on Functional Programming will be held in Milan, Italy on September 2 - 7, 2024, with the option of virtual participation. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as symposiums) to be affiliated with ICFP 2024 and sponsored by SIGPLAN. These events should be less formal and more focused than ICFP itself, include sessions that enable interaction among the attendees, and foster the exchange of new ideas. The preference is for one-day events, but other schedules can also be considered. The workshops are scheduled to occur on September 2nd (the day before ICFP) and September 6-7th (the two days after ICFP). A separate call for Call for Tutorial, Panel, and Discussion Proposals is going to be circulated later. ---------------------------------------------------------------------- Submission details Deadline for submission: January 11, 2024 Notification of acceptance: January 19, 2024 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2024 workshop co-chairs (Chandrakana Nandi and Yannick Forster) via email to chandra(a)certora.com yannick.forster(a)inria.fr by January 11, 2024. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified whether their event proposal is accepted by January 19, 2024, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2024-files/icfp24-workshops-form.txt.docx PC Template: http://www.icfpconference.org/icfp2024-files/pc-template.xlsx Further information about SIGPLAN sponsorship is available at: http://www.sigplan.org/Resources/Proposals/Sponsored/ ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2024 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Chandrakana Nandi (Certora) Workshop Co-Chair: Yannick Forster (Inria) General Chair: Marco Gaboardi (Boston University) Program Chair: Brigitte Pientka (McGill University) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Chandrakana Nandi and Yannick Forster), via email to chandra(a)certora.com end yannick.forster(a)inria.fr

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Issues with static linking of a C ".a" library with cabal
by Hécate 11 Dec '23

11 Dec '23

Hi everyone, I'm hitting a number of roadblocks while trying to embed a ".a" file (and its headers) in Haskell code. The setup is the following: I have a bindings library that handles all of the cAPI FFI declarations, and a consumer that does not directly depend on the underlying C library. As such, I use a combination of extra-bundled-libraries, include-dirs and install-includes stanzas in my cabal file: elifflag(bundled-libsodium) extra-bundled-libraries: Csodium include-dirs: include install-includes: include/sodium.h include/sodium/core.h include/sodium/crypto_aead_aes256gcm.h […] (full file here: https://github.com/haskell-cryptography/libsodium-bindings/blob/37-libsodiu…) It all seemed to work on my day-to-day system, but further tests on a system that did not have libsodium installed system-wide showed that there's still a missing part: |/usr/bin/ld.gold:error:cannotfind-lsodium collect2:error:ldreturned1exitstatus `gcc'failedinphase`Linker'.(Exitcode:1)| At this point I am simply baffled, because I thought that I had the right invocation for all of this to work. What else can I do? Am I missing something obvious? I have opened a ticket here with a bit more details: https://github.com/haskell/cabal/issues/9509 Cheers, Hécate -- Hécate ✨ 🐦: @TechnoEmpress IRC: Hecate WWW:https://glitchbra.in RUN: BSD

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OverloadedStrings, but with compile-time invariants
by Max Ulidtko 10 Dec '23

10 Dec '23

Greetings @cafe, Transparent wrapping of string literals into custom types, driven automatically by type inference, is of course very useful. I'm getting huge mileage off GHC's OverloadedStrings when working with e.g. the Text type: class IsString a where fromString :: String -> a instance IsString Text {- ... -} greeting :: Text greeting = "Hello" But I hope there's possibly room for improvement. I've got a bag of several use-cases; perhaps the simplest is the following. Consider a type called NonEmptyText which is a (wrapper around) Text, but with an added invariant that the contained text is never the empty string "". Depending on codebase, such a type can be very useful too, allowing to offload the burden of keeping track of the invariant onto machine; the typechecker will show me all the spots to handle, current or future ones. IOW, this type enables writing a non-boolean-blind null check, toNonEmptyText :: ToText str => str -> Maybe NonEmptyText — which makes not only the programmer, but also the compiler, aware of the null-check result. I'm focusing the example on Text to somewhat maintain generality of the idea. NonEmpty Char can be an alternative formulation of "non-empty String"; length-indexed vectors could possibly handle this too. But there're all sorts of invariants besides non-emptiness one may wish to express. I'll omit other examples; but to summarize, I've got plenty of practical motivation to get "smart literals". And to the point: IsString as it stands, forces us to write much unsatisfactory instances: instance IsString NonEmptyText where fromString (c:cs) = NonEmptyText.new c cs fromString [] = error "unavoidable runtime panic" -- argh ... despite fromString being invoked, presumably with OverloadedStrings, on a string literal — which is statically known at compile-time. I'd imagine upgrading the interface with GHC's own TypeLits: class KnownSymbol lit => IsString' str lit where fromString' :: proxy lit -> str This 2-parameter formulation of IsString enables pretty straightforward user code, along the lines of: type family MyInvariant (s :: Symbol) :: Constraint where MyInvariant "" = TypeError ('Text "Empty string literal") MyInvariant _s = () instance (MyInvariant lit, KnownSymbol lit) => IsString' NonEmptyText lit where fromString' = nothingImpossible . NeT.fromText . toText . symbolVal where nothingImpossible (Just x) = x nothingImpossible Nothing = error "typelevel invariant violated" text1, text2 :: NonEmptyText text1 = fromString' $ Proxy @"hello, types" text2 = fromString' $ Proxy @"" -- compile error here, as expected! With the primes, this is possible to write today. With AllowAmbiguousTypes, the unwieldy syntax can be abbreviated somewhat, e.g. smartLiteral @"overloaded string literal with compiler-checked invariant" — but is still rather awkward... in the same way that ubiquitous T.pack is annoying enough to want to enable OverloadedStrings and stop seeing it all over the place. The question is, could this IsString' somehow become the IsString that OverloadedStrings is friends with? I guess, not soon, with compatibility in mind... But in principle? Is it even worth trying to pursue the GHC Proposal path? Perhaps a new extension OverloadedTypelevelStrings? Relatedly... Can a GHC Plugin do this? And overall, to zoom out of "XY Problem" pitfalls: am I missing a neater way to have "partial" IsString instances, those that can reject some literals at compile-time? Having this code work is the goal: text1, text2 :: NonEmptyText text1 = "hello, types" text2 = "" -- compile error Best regards, Max

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[ANN] mig-2.1 minimal server library
by Anton Kholomiov 10 Dec '23

10 Dec '23

I'm happy to announce a new release of the mig library. It's a minimal and simple library to build servers. This release adds some goodies for servers that produce HTML: * cookies (new [input Cookie]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Types-Route.h…) type and [function to set]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Class-Respons…) the cookies) * [type-safe stable URLs]( https://hackage.haskell.org/package/mig-0.2.1.0/docs/Mig-Core-Class-Url.html) from server definition * an example [HtmlTemplate]( https://github.com/anton-k/mig/tree/main/examples/mig-example-apps/HtmlTemp…) on how to use template engine and type-safe URLs with the library Also it adds a stack template to create new hello-world server with stack (on how to use it see the section on how to start [a new mig project]( https://anton-k.github.io/mig/#how-to-start-a-new-project) in the tutorial). Links: * [github](https://github.com/anton-k/mig) * [tutorial](https://anton-k.github.io/mig/) * [hackage for the server library]( https://hackage.haskell.org/package/mig-server) * [hackage for the core library](https://hackage.haskell.org/package/mig) * [examples]( https://github.com/anton-k/mig/tree/main/examples/mig-example-apps#mig-exam… )

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PLDI'24 Artifact Evaluation Committee: Call for nominations
by Ningning Xie 08 Dec '23

08 Dec '23

Dear all, We are looking for motivated students and researchers to be members of the PLDI 2024 Artifact Evaluation Committee (AEC). The artifact evaluation process aims to promote, share and catalog the research artifacts of papers accepted to the PLDI research track. This year, we are accepting self-nominations for the AEC. The self-nomination form is available at: https://forms.gle/wWGfW33qMY2gUqqz7 The first round of nomination ends on Dec 22nd, 2023. As a committee member, the primary responsibilities would be to review the artifacts submitted corresponding to the already accepted papers in the main research track. In particular, you may have to run the associated tool, check whether the results in the main paper can be reproduced, and inspect the data. We expect the bulk of the review work to take place between early February and mid April. Each artifact will take about 8h to review, and reviewers will be assigned about 3 artifacts to review. Come join us in improving the quality of research in our field! Best regards, Manuel Rigger and Ningning Xie

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CGO'24 - Student Research Competition (SRC) and more
by L. Cicolini 08 Dec '23

08 Dec '23

Dear all, The International Symposium on Code Generation and Optimization (CGO) is hosting a Student Research Competition (SRC). Our SRC is a unique opportunity for undergraduate and graduate students to present their original research before a panel of judges and attendees at CGO'24. The winner will be invited to the Grand 2024 ACM SRC competition. If you are an undergraduate or postgraduate student doing research in compiler-related topics, consider applying. We look forward to meeting you! CGO'24 is a compiler conference happening in Edinburgh, March 2nd to 6th 2024. This conference is a great opportunity for anyone interested in compilers to connect with great researchers and professionals and will host four existing workshops: LLVM [1], the BuildIt Framework [2], LATHC [3], and C4ML [4]. Find more information on https://conf.researchr.org/home/cgo-2024 and share this message with anyone who might be interested! Best, Luisa Cicolini [1] LLVM Performance Workshop: https://llvm.org/devmtg/2024-03/ [2] BuildIt: Building DSLs made easy with the BuildIt Framework: https://buildit.so/tutorial/ [3] LATHC: Languages, Architectures, and Tools for Heterogeneous Computing Workshop: https://jnamaral.github.io/LATHC24/ [4] C4ML - Compilers For Machine Learning: https://www.c4ml.org/c4ml-2024

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Haskell.org Call for Nominations (for the terms 2024-2027)
by Ida Bzo 05 Dec '23

05 Dec '23

Dear Haskellers, We are pleased to announce that nominations are now open for the Haskell.org committee. You can nominate yourself or a friend for a three-year term (2024-2027) by sending an email to [committee at haskell.org] by January 10, 2024. Self-nominations and re-nominations are also welcome. Please include any relevant information about yourself or your nominee that you think will help us make our decision. Committee members do not have to be technical experts in Haskell. We are looking for people who are enthusiastic about improving the Haskell community and come from a variety of backgrounds, such as academia, industry, open-source development, and community building. Our goal is to represent the various facets of the Haskell world, including gender, race, location, and industry or research. The committee's responsibilities include setting policies, providing guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with Haskell.org funds and donations. Being a committee member does not require a significant amount of time, but members should be responsive during discussions and should be able to attend monthly calls and participate in the Haskell.org Slack and mailing lists. Candidates for the committee should possess strong leadership, communication, and judgment skills. They must be able to set aside personal or business-related biases and make decisions with the good of the open-source Haskell community in mind. For more information about the committee's roles and responsibilities, please visit https://www.haskell.org/haskell-org-committee/ If you have any questions about the nomination process, please feel free to email us at [committee at haskell.org] or contact one of us individually. λCheers, Ida Bzo

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Download stats not updated on Hackage
by Harendra Kumar 05 Dec '23

05 Dec '23

Hi, It seems the download count for packages on Hackage is stuck for quite some time now. Is this issue known to the Hackage maintainers? -harendra

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by Ihab Mohsen 05 Dec '23

05 Dec '23

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by Ihab Mohsen 05 Dec '23

05 Dec '23

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garn: a Nix/Cabal/Stack alternative
by Julian Arni 03 Dec '23

03 Dec '23

Dear list, garn is a new take on Nix - configured in Typescript rather than the Nix language, and with a nicer and simpler CLI (thanks optparse-applicative!). The repository is https://github.com/garnix-io/garn, and the website is https://garn.io/. I'm mentioning it here because we just added better Haskell support, which makes it now a plausible alternative to managing your Haskell projects with Nix, Stack or Cabal. An example Haskell project: import * as garn from "https://garn.io/ts/v0.0.18/mod.ts"; import * as pkgs from "https://garn.io/ts/v0.0.18/nixpkgs.ts"; export const project = garn.haskell.mkHaskellProject({ description: "My project", src: ".", executables: ["server", "openapi-generation"] overrideDependencies: { "servant": "0.19.1" }, ghcVersion: "ghc94" }).addExecutable("format")`${pkgs.ormolu}/bin/ormolu --mode inplace $(find . -name '*.hs')` Which allows for `garn build project` to build the project, and `garn enter project` to be in a devshell with cabal, ghc94 and all the dependencies installed, and `garn run project.format` for formatting all files with ormolu. (`garn init` is capable of generating most of that file for you.) # Comparisons First, compared to all of the options below, garn is much less mature - there'll be rougher edges or features not yet supported. It's also still changing very quickly, and backwards compatibility is not, for now, a priority. - Cabal: Like Cabal, garn still uses a cabal file, and in fact expects you to develop with cabal. garn brings in the dependencies itself and makes them known to ghc/cabal, and they're snapshot-based rather than resolver-based. In addition, garn makes it easy to include system (non-Haskell) dependencies, it sandboxes builds and tests for better reproduciblity (making CI in particular quite easy); it allows for shared caches; it supports scripts for project janitoring and management; and it supports devshells. - Stack: Like Stack, garn uses a snapshot-based approach to dependencies. In fact, it's based on the stackage releases. It also supports overriding versions from Hackage, but not yet from git. Unlike Stack, it allows specifying system dependencies (non-Haskell dependencies); it sandboxes builds and tests for better reproduciblity (making CI in particular quite easy); it allows for shared caches; it supports scripts for project janitoring and management; and it supports devshells. - Nix: the focus of garn has been a simplified and more productive user experience when compared to Nix. A typed language means you can get autocompletion, documentation, and type errors in your editor, for example. And, even if Typescript is by no means perfect, it's much easier to program in than Nix, so it becomes more fun to abstract away functionality (e.g.: an addGhcidToProject function). garn "transpiles" to Nix, and Nix can be embedded in garn, so the choice is not binary. # Feedback If you try it out, please let us know what you think, or if you run into any roadblocks. You can email me at this address, or open issues on GitHub, or join the Discord channel at https://discord.com/invite/XtDrPsqpVx. It also happens to be partly written in Haskell; hopefully that makes it more easy and fun to get involved! Cheers, Julian

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ANN: hledger 1.32
by Simon Michael 02 Dec '23

02 Dec '23

G'day all, I'm very pleased to announce hledger 1.32, including work from contributors Jonathan Dowland, S. Zeid, Charlie Ambrose, Jacob Weisz, Peter Sagerson, Philipp Klocke, Stephen Morgan, and bobobo1618. Highlights: More precision control, beancount output, TSV output, --summary-only, strict/idempotent import, CSV rule enhancements, timedot letters, fixes - there's a lot in this one, set aside some time to read the notes. - https://github.com/simonmichael/hledger/releases/1.32 - https://hledger.org/release-notes.html#2023-12-01-hledger-132 - https://hledger.org/install hledger is free, fast, reliable, multicurrency, double-entry, plain text accounting software for unix, mac, windows, and the web; built around human-readable, version-controllable plain text files; inspired by and largely compatible with Ledger CLI; convertible to and from Beancount; written in Haskell for reliability and longevity. For help getting started or more info, see https://hledger.org and join our Matrix/IRC chat or mail list: https://hledger.org/support . Newcomers, experts, contributors, sponsors, feedback are welcome! For more about plain text accounting, see https://plaintextaccounting.org . May you be free from danger; may you have happy, peaceful, prosperous holidays. -Simon

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Request to take over floating-bits package
by Julia Longtin 01 Dec '23

01 Dec '23

This is an attempt to follow the process at wiki.haskell.org/Taking_over_a_package I have set up a git repository at github.com/julialongtin/floating-bits/ , and fixed compatibility issues with ghc-9.4. I would like to be made maintainer of this package in hackage. Julia Longtin

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Does the constraints library give (a :- b) -> Dict (a |- b)?
by Tom Ellis 01 Dec '23

01 Dec '23

The constraints library [1] has implied :: forall a b. (a => b) => a :- b from which I can obtain implied' :: Dict (a |- b) -> a :- b implied' Dict = implied but can I get the converse of this, that is coimplied :: (a :- b) -> Dict (a |- b) coimplied = error "Can it be done?" I don't see how. The library itself only mentions "|-" in its definition. Would this combinator violate some guarantee that's essential for the safe use of type classes? I'm a bit confused why the same library allows abstraction over type variables with forall_[2] forall_ :: (forall a. Dict (p a)) -> Dict (Forall p) but not abstraction over "term" variables (i.e. of kind Constraint), as in coimplied. Alternatively, can someone help me achieve my goal another way? That is, I need to be able to dischange the quantified constraint forall i. T0 i => T2 i given that I can already discharge the constraint forall i. T1 i => T2 i and I have available the value forall i. T0 i => Dict (T1 i) >From the latter I can easily get forall i. T0 i :- T1 i But without coimplied I don't know how to proceed. Thanks, Tom [1] https://hackage.haskell.org/package/constraints-0.14/docs/Data-Constraint-F… [2] https://hackage.haskell.org/package/constraints-0.14/docs/Data-Constraint-F…

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[Haskell] [ANNOUNCE] Haskell Language Server 2.5.0.0 released
by Zubin Duggal 01 Dec '23

01 Dec '23

Binaries for this release are available at https://downloads.haskell.org/~hls/haskell-language-server-2.5.0.0/. These binaries can be installed using [GHCup](https://www.haskell.org/ghcup/), using the vanilla metadata channel: ghcup --url-source=https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-vanilla-0.0.8.yaml install hls 2.5.0.0 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: - 9.2.8 - 9.4.8 - 9.6.3 - 9.8.1 # Changelog - Bindists for GHC 9.4.8 - Drop support for GHC 9.0 - Re-add stan plugin - Load default operator fixities in Fourmolu plugin Happy editing! - Zubin

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