December 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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[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 14 Dec '23

14 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 11 Dec '23

11 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 09 Dec '23

09 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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