April 2025 - 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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IFL 2025: First call for papers
by Mart Lubbers 29 Apr '25

29 Apr '25

===================================================================== IFL 2025 37th Symposium on Implementation and Application of Functional Languages Montevideo, Uruguay October 1-3, 2025 https://www.fing.edu.uy/inco/congresos/ifl2025 ===================================================================== ### Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2025 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. ### Topics of interest Topics of interest to IFL include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialisation - run-time code generation - partial evaluation - (abstract) interpretation - meta-programming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques ### Peer-review process Differently from previous editions of IFL, IFL 2025 solicits two kinds of submissions: * Regular papers (12 pages including references) * Draft papers for presentations ('weak' limit between 8 and 15 pages) Regular papers will undergo a rigorous review by the program committee, and will be evaluated according to their correctness, novelty, originality, relevance, significance, and clarity. A set of regular papers will be conditionally accepted for publication. Authors of conditionally accepted papers will be provided with committee reviews along with a set of mandatory revisions. Regular papers not accepted for publication will be considered as draft papers, at the request of the author. Draft papers will be screened to make sure that they are within the scope of IFL, and will be accepted for presentation or rejected accordingly. Prior to the symposium: Authors of conditionally accepted papers and accepted presentations will submit a pre-proceedings version of their work that will appear in the draft proceedings distributed at the symposium. The draft proceedings does not constitute a formal publication. We require that at least one of the authors present the work at IFL 2025. After the symposium: Authors of conditionally accepted papers will submit a revised versions of their paper for the formal post-proceedings. The program committee will assess whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. Our interest is to ultimately accept all conditionally accepted papers. If you are an author of a conditionally accepted paper, please make sure that you address all the concerns of the reviewers. Authors of accepted presentations will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal post-proceedings. The program committee will evaluate these submissions according to their correctness, novelty, originality, relevance, significance, and clarity, and will thereby determine whether the paper is accepted or rejected. ### Important dates Submission of regular papers: June 16, 2025 Regular papers notification: August 4, 2025 Submission of draft papers: August 4, 2025 Draft papers notification: August 11, 2025 Deadline for early registration: September 5, 2025 Submission of pre-proceedings version: September 8, 2025 IFL Symposium: October 1-3, 2025 Submission of papers for post-proceedings: December 15, 2025 Notification of acceptance: February 28, 2026 Camera-ready version: March 30, 2026 Deadlines are end of day Anywhere on Earth (UTC-12) (https://www.timeanddate.com/time/zones/aoe) ### Submission details All contributions must be written in English. Papers must use the ACM two columns conference format, which can be found at: http://www.acm.org/publications/proceedings-template Submit your paper here: https://easychair.org/conferences/?conf=ifl20240 Important note to authors about the new ACM open access publishing model ACM has introduced a new open access publishing model for the International Conference Proceedings Series (ICPS). Authors based at institutions that are not yet part of the ACM Open program and do not qualify for a waiver will be required to pay an article processing charge (APC) to publish their ICPS article in the ACM Digital Library. To determine whether or not an APC will be applicable to your article, please follow the detailed guidance here: https://www.acm.org/publications/icps/author-guidance. Further information may be found on the ACM website, as follows: - Full details of the new ICPS publishing model: https://www.acm.org/publications/icps/faq - Full details of the ACM Open program: https://www.acm.org/publications/openaccess - Please direct all questions about the new model to icps-info(a)acm.org. ### Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. ### Organisation PC Chairs: Alberto Pardo, Universidad de la República, Montevideo, Uruguay Marcos Viera, Universidad de la República, Montevideo, Uruguay Local Chairs: Alberto Pardo, Universidad de la República, Montevideo, Uruguay Marcos Viera, Universidad de la República, Montevideo, Uruguay ### Program committee: Matteo Cimini, University of Massachusetts Lowell Facundo Domínguez, Tweag João Paulo Fernandes, Universidade do Porto Jeremy Gibbons, Oxford University Jurriaan Hage, Heriot Watt University Jason Hemann, Seton Hall University Maja Hanne Kirkeby, Roskilde University Mart Lubbers, Radboud University Bruno C. D. S. Oliveira, The University of Hong Kong Rinus Plasmeijer, Radboud University Andre Rauber Du Bois, Universidade Federal de Pelotas Rodrigo Ribeiro, Universidade Federal de Ouro Preto Alejandro Russo, Chalmers University of Technology João Saraiva, University of Minho Wenhao Tang, University of Edinburgh Zhixuan Yang, Imperial College London Brent Yorgey, Hendrix College Beta Ziliani, Manas.Tech Viktória Zsók, Eötvös Loránd University ### Venue IFL 2025 will be held physically in Montevideo, Uruguay. See the website for more information. https://www.fing.edu.uy/inco/congresos/ifl2025 ### Acknowledgments This call-for-papers is an adaptation and evolution of content from previous instances of IFL. We are grateful to prior organisers for their work, which is reused here.

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Is there a way to make this code compose generic ?
by PICCA Frederic-Emmanuel 25 Apr '25

25 Apr '25

Hello, I have this data DataSourceShape = DataSourceShape ([HSize], [Maybe HSize]) | DataSourceShape'Range !DIM1 !DIM1 a function which combine 2 DataSourceShape and produce an new one (monoid operation ?) combine'Shape ∷ DataSourceShape → DataSourceShape → DataSourceShape I use this in this class with familly types. class DataSource a where data DataSourcePath a ∷ Type data DataSourceAcq a ∷ Type ds'Shape ∷ MonadSafe m ⇒ DataSourceAcq a → m DataSourceShape withDataSourceP ∷ (Location l, MonadSafe m) ⇒ ScanFile l → DataSourcePath a → (DataSourceAcq a → m r) → m r and here an instance for one of my type DataFrameQCustom (I have plenty of them). they are all constructed the same way. data DataSourceAcq DataFrameQCustom = DataSourceAcq'DataFrameQCustom (DataSourceAcq Attenuation) (DataSourceAcq Geometry) (DataSourceAcq Image) (DataSourceAcq Mask) (DataSourceAcq Timestamp) (DataSourceAcq Timescan0) (DataSourceAcq Scannumber) ds'Shape(DataSourceAcq'DataFrameQCustom a g i m idx t0 s) = do sa ← ds'Shape a sg ← ds'Shape g si ← ds'Shape i sm ← ds'Shape m sidx ← ds'Shape idx st0 ← ds'Shape t0 ss ← ds'Shape s pure $ foldl1 combine'Shape [sa, sg, si, sm, sidx, st0, ss] withDataSourceP f (DataSourcePath'DataFrameQCustom a g i m idx t0 s) gg = withDataSourceP f a $ λa' → withDataSourceP f g $ λg' → withDataSourceP f i $ λi' → withDataSourceP f m $ λm' → withDataSourceP f idx $ λidx' → withDataSourceP f s $ λs' → withDataSourceP f t0 $ λt0' → gg (DataSourceAcq'DataFrameQCustom a' g' i' m' idx' t0' s') My question is, how should avoid writting by hand all these ds'Shape / withDatasourceP implementations, which seems quite mechanical. thanks for your help Frederic

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ICFP 2025 Student Research Competition Call for Submissions
by Kimball Germane 24 Apr '25

24 Apr '25

ICFP 2025 invites students to participate in the Student Research Competition in order to present their research and get feedback from prominent members of the programming language research community. The SRC consists of three rounds: * Submission of an extended abstract * Poster session at ICFP 2025 * Finalists' presentations at ICFP 2025 During the first round students submit an extended abstract detailing their research to be reviewed by the program committee. Those students whose abstracts get accepted advance to the poster session round which will take place during ICFP 2025. Winners of the poster session advance to the next round, where they will give a 5-minute presentation about their work on the ICFP main stage in front of a live audience. Please note that a work cannot be simultaneously submitted to multiple SRCs, including both the ICFP SRC and the SPLASH SRC which are co-located this year. ### IMPORTANT DATES AND TIMES * Submission Deadline: 19 June 2025 (Thursday) * Author Notification: 14 July 2025 (Monday) * ICFP 2025 Conference in Singapore: 12 October 2025 (Sunday) - 18 October 2025 (Saturday) ### SUBMISSION OF EXTENDED ABSTRACTS * Submission Website: https://icfp25src.hotcrp.com Each submission (referred to as "abstract" below) should include the student author's name and e-mail address; institutional affiliation; research advisor's name; ACM student member number; category (undergraduate or graduate); research title; and an extended abstract addressing the following: * Problem and Motivation: Clearly state the problem being addressed and explain the reasons for seeking a solution to this problem. * Background and Related Work: Describe the specialized (but pertinent) background necessary to appreciate the work in the context of ICFP areas of interest. Include references to the literature where appropriate, and briefly explain where your work departs from that done by others. * Approach and Uniqueness: Describe your approach in addressing the problem and clearly state how your approach is novel. * Results and Contributions: Clearly show how the results of your work contribute to programming language design and implementation in particular and to computer science in general; explain the significance of those results. * Submissions must be original research that is not already published at ICFP or another conference or journal. One of the goals of the SRC is to give students feedback on ongoing, unpublished work. Furthermore, the abstract must be authored solely by the student. If the work is collaborative with others and/or part of a larger group project, the abstract should make clear what the student's role was and should focus on that portion of the work. * Formatting: Submissions must be in PDF format, printable in black and white on US Letter sized paper, and interpretable by common PDF tools. All submissions must adhere to the "ACM Small" template that is available (in both LaTeX and Word formats) from https://www.acm.org/publications/authors/submissions. For authors using LaTeX, a lighter-weight package, including only the essential files, is available from http://sigplan.org/Resources/Author/#acmart-format. The submission must not exceed 3 pages in PDF format. Reference lists do not count towards the 3-page limit. Further information is available at the ICFP SRC website: https://icfp25.sigplan.org/track/icfp-2025-student-research-competition

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"Magical Haskell" book is finally finished and available :)
by Anton Antich 22 Apr '25

22 Apr '25

Dear all, I have started working on this book almost 10 years ago, wrote about a third and then took a big pause until last year, when I got approached by Springer and finished the book in another 6 months, dedicating the 2nd part to Agentic AI and now it's finally out and available and I would really love to get the community's feedback :) I tried to introduce Haskell from the first mathematical principles, but very gently, so that it is accessible to a much wider audience as we all know the learning curve can be a bit steep. Here is a sample free chapter on medium: https://medium.com/superstringtheory/functor-bifunctor-trifunctor-c1424f962… The book itself is available on Springer: https://link.springer.com/book/10.1007/979-8-8688-1282-8 And there is a 20% discount with the code APAUT - or just scan the QR code below. I put a lot of effort into this book, being passionate about the language we all love and do hope it will help more people to become proficient and fluent in Haskell! Best wishes, Anton Antich, CPO and founder, Integrail.ai [image: image.png]

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Release: webdriver-precore
by John Walker 21 Apr '25

21 Apr '25

Hi All, We are happy to announce the release of [webdriver-precore]( https://hackage.haskell.org/package/webdriver-precore) ~ *A typed wrapper for [W3C WebDriver](https://www.w3.org/TR/2025/WD-webdriver2-20250306/) protocol* This library is intended to be used as a base for other libraries that provide a WebDriver client implementation and higher level functions for browser automation. More details can be found in the [project README]( https://github.com/pyrethrum/webdriver) John & Adrian

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type-class and subclasses
by lloyd allison 16 Apr '25

16 Apr '25

I have a general question: I would like to have a type-class C in which types that are instances of C have some type parameters and then a subclass S of C where some of the type parameters of an instance of S are more constrained in some way -- perhaps by being equal and/or being instances of some other class. I have an entirely artificial example (att.) that illustrates this situation and struggled (it's a while since I wrote much Haskell) to get it past the ghc type checker until stumbling across GADTs which do seem to do the trick, so I could leave it at that but... ...I am very far from sure that this is the correct way to look at the problem and am hoping for some illumination. regards Lloyd. -- No AI was used in composing this message.

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TyDe 2025 - Call for Papers & Extended Abstracts
by Andras Kovacs 11 Apr '25

11 Apr '25

========================================================================= The Tenth International Workshop on TYPE-DRIVEN DEVELOPMENT Call for papers and extended abstracts Singapore, 12 October 2025 https://icfp25.sigplan.org/home/tyde-2025 ========================================================================= The Workshop on Type-Driven Development (TyDe) aims to show how static type information may be used effectively in the development of computer programs. Co-located with ICFP and SPLASH, this workshop brings together leading researchers and practitioners who are using or exploring types as a means of program development. We welcome all contributions, both theoretical and practical, on a range of topics including: * dependently typed programming; * generic programming; * design and implementation of programming languages, exploiting types in novel ways; * exploiting typed data, data dependent data, or type providers; * static and dynamic analyses of typed programs; * tools, IDEs, or testing tools exploiting type information; * pearls, being elegant, instructive examples of types used in the derivation, calculation, or construction of programs. ### Important dates ### * Mon 9 Jun 2025 (AoE): Submission deadline for papers and extended abstracts * Wed 16 Jul 2025: Notification of acceptance * Wed 30 Jul 2025: Submission of camera-ready papers to ACM * Sun 12 Oct 2025: Workshop ### Proceedings and Copyright ### We will have formal proceedings for full-length papers, published by the ACM. Accepted papers will be included in the ACM Digital Library. Authors must grant ACM publication rights upon acceptance, but may retain copyright if they wish. Authors are encouraged to publish auxiliary material with their paper (source code, test data, and so forth). The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. The official publication date is the date the papers are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. ### Submission Details ### Submissions should fall into one of two categories: * regular research papers (12 pages); * extended abstracts (3 pages). The bibliography will not be counted against the page limits for either category. Regular research papers are expected to present novel and interesting research results, and will be included in the formal proceedings. Extended abstracts should report work in progress that the authors would like to present at the workshop. Extended abstracts will be distributed to workshop attendees but will not be published in the formal proceedings. We welcome submissions from PC members (with the exception of the two co-chairs), but these submissions will be held to a higher standard. Submission is handled through HotCRP: > https://tyde25.hotcrp.com All submissions should be in portable document format (PDF) and formatted using the ACM SIGPLAN style guidelines: > https://www.sigplan.org/Resources/Author/ Note that submissions should use the new ‘acmart’ format and the two-column ‘sigplan’ subformat (not to be confused with the one-column ‘acmsmall’ subformat). Extended abstracts must be submitted with the label ‘Extended Abstract’ clearly in the title. ### Presentations ### We expect that each accepted submission is presented at the workshop. Presentations are around 20 minutes plus questions. Remote presentation is possible. ### Participant Support ### Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover participation-related expenses. PAC also offers other support, such as for child-care expenses during the meeting or for accommodations for members with physical disabilities. For details on the PAC program, see its web page: > https://www.sigplan.org/PAC/

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Extended deadline - CFP - Haskell Implementors' Workshop 2025
by Andreas Herrmann 07 Apr '25

07 Apr '25

Extended deadline to April 18, 2025 Call for proposals for the Haskell Implementors' Workshop https://haskell.foundation/events/2025-haskell-implementors-workshop.html June 6, 2025 Organized by the Haskell Community Co-located with ZuriHac 2025 and Haskell Ecosystem Workshop 2025 Hosted by the Haskell Foundation at Eastern Switzerland University of Applied Sciences (OST) https://www.ost.ch/en/university-of-applied-sciences/campus/rapperswil-jona… ## Overview * Extended Deadline: April 18, 2025 * Notification: May 5, 2025 * Workshop: June 6, 2025 The 17th Haskell Implementors' Workshop is to be held alongside ZuriHac 2025 this year near Zurich. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure to share their work and to discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings. The workshop will be informal and interactive, with open spaces in the timetable and room for ad-hoc discussion, demos, and lightning talks. In the past the Haskell Implementors’ Workshop was co-located with ICFP (International Conference on Functional Programming). However, in recent years it has become more and more challenging to attract a large enough audience and sufficiently many speakers for an appealing program. ZuriHac and the Haskell Ecosystem Workshop have become an important annual gathering of a large part of the Haskell community. This year the Haskell Implementors’ Workshop will be co-located with these events to be accessible to a broader audience. ## Scope and Target Audience The Haskell Implementors' Workshop is an ideal place to describe a Haskell extension, describe works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Members of the wider Haskell community are encouraged to attend the workshop - we need your feedback to keep the Haskell ecosystem thriving. Students working with Haskell are especially encouraged to share their work. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimization and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment ## Talks We invite proposals from potential speakers for talks and demonstrations. We are aiming for 20-minute talks with 5 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Submissions can be made via the form linked below until April 18, 2025 (anywhere on earth). https://docs.google.com/forms/d/e/1FAIpQLSdczGbxJYGc4eusvPrxwBbZl561PnKeYno… We will also have a lightning talks session. Lightning talks should be ~7mins and are scheduled on the day of the workshop. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. ## Program Committee * Luite Stegeman * Jaro Reinders * Emily Pillmore * Rodrigo Mesquita * Ian-Woo Kim * Andreas Herrmann (chair) ## Contact * Andreas Herrmann

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Re: [Haskell-cafe] type level function with different return kinds
by Barak A. Pearlmutter 06 Apr '25

06 Apr '25

Can a type level function have different return kinds depending on its arguments? I think this question is best answered by quoting https://aphyr.com/posts/342-typing-the-technical-interview “Haskell is a dynamically-typed, interpreted language.”

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