January 2025 - 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…

5 7

Re: [Haskell-cafe] Course-of-value recursion by defining a sequence as a self-referential infinite list
by Douglas McIlroy 10 Feb '25

10 Feb '25

> catalanNumbers :: Num a => [a] > catalanNumbers = > let xs = 1 : PowerSeries.mul xs xs > in xs This example of a generating function come to life as a program deserves to be better known. Bill Burge presented it 50 years ago in "Recursive Programming Techniques", Addison-Wesley, 1975. I revisited it in "Power series, power serious", JFP 9 (1999) 323-335, where, with overloadied arithmetic, it became ts = 1 : ts^2 The technique is laid bare in ten one-liners at https://www.cs.dartmouth.edu/~doug/powser.html. Doug

4 5

[haskell-cafe] HList maintainer
by Daniil Iaitskov 29 Jan '25

29 Jan '25

Hi List, I am trying to reach HList package maintainer. Maintainer email form cabal file does not exist. There is no feedback on PR <https://bitbucket.org/HList/hlist/pull-requests/3>. Best regards, Daniil Iaitskov

1 0

CGO 25 - Call for Participation - Early Registration
by Luisa Cicolini 27 Jan '25

27 Jan '25

ACM/IEEE International Symposium on Code Generation and Optimization (CGO 2025) Dear all, This is the last week to take advantage of the early registration<https://2025.cgo.org/attending/registration> (deadline: 31st Jan.) for CGO’25, taking place in Las Vegas (1-5 March 2025). The International Symposium on Code Generation and Optimization (CGO) is a premier venue to bring together researchers and practitioners working at the interface of hardware and software on a wide range of optimization and code generation techniques and related issues. The conference spans the spectrum from purely static to fully dynamic approaches, and from pure software-based methods to specific architectural features and support for code generation and optimization. We have an exciting list of accepted papers<https://2025.cgo.org/track/cgo-2025-papers>, workshops and tutorials<https://2025.cgo.org/track/cgo-2025-workshops-and-tutorials> ahead, don’t forget to register! Best, Luisa

1 0

Midlands Graduate School 2025
by Graham Hutton 27 Jan '25

27 Jan '25

Dear all, The Midlands Graduate School (MGS) in the Foundations of Computing Science will be held 7-11 April 2025 in Sheffield, UK. Eight fantastic courses on category theory, type theory, coalgebra, semantics and more. Please share! Best wishes, Graham Hutton ========================================================== Midlands Graduate School 2025 7-11 April 2025, Sheffield, UK https://tinyurl.com/MGS-2025 BACKGROUND: The Midlands Graduate School (MGS) in the Foundations of Computing Science provides an intensive course of lectures on the mathematical foundations of computing. The MGS has been running since 1999, and is aimed at PhD students in their first or second year of study, but the school is open to everyone, and has increasingly seen participation from industry. We welcome participants from all over the world! COURSES: Eight courses will be given. Participants usually take all the introductory courses and choose additional options from the advanced courses depending on their interests. Invited course - Proof-Theoretic Semantics, David Pym Introductory courses - Category Theory, Thorsten Altenkirch - The Curry-Howard Correspondence, Anupam Das - Quantum Computing, Venanzio Capretta Advanced courses - Coalgebra, Paul Blain Levy - Linear Logic, Abhishek De and Charles Grellois - Modular Proofs in Isabelle/HOL, Chelsea Edmonds - Refinement Types in Haskell, Brandon Hewer REGISTRATION: Will be open shortly; further details to be announced. SPONSORSHIP: We offer a range of sponsorship opportunities for industry (bronze, silver, gold and platinum), each with specific benefits. Please see the website for further details. ========================================================== This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

1 0

CLC Elections January 2025
by Andrew Lelechenko 27 Jan '25

27 Jan '25

Core Libraries Committee seeks nominations for three expiring seats (including my own). <https://discourse.haskell.org/t/clc-elections-january-2025/11126#what-is-cl…>What is CLC? CLC is a volunteer body, primarily tasked with management of API of base <https://hackage.haskell.org/package/base> package, but also owning so-called Core Libraries and responsible for PVP <https://pvp.haskell.org/>. You can find more at our home page <https://github.com/haskell/core-libraries-committee/?tab=readme-ov-file>. <https://discourse.haskell.org/t/clc-elections-january-2025/11126#who-should…>Who should apply? Anyone who meets the following criteria should apply: Candidates should have enough bandwidth to review merge requests to base on a semi-frequent basis (~3-4 per month), and sustain this for their 3 years term in a healthy manner. Candidates should be able to contribute opinions and analysis to issues raised by the community as a part of the CLC proposal process on a semi-frequent basis (~7 per month). Candidates should be good communicators, and at least be able to articulate to the CLC team when they will be available vs. unavailable. Candidates should be productive, and be able to follow through on merge requests and conversations to their completion in a diligent and timely manner. We encourage any and all who satisfy these requirements to apply. Please note that we are not looking for the biggest galaxy brain in the room – quite the opposite. We are looking for productive, motivated individuals who want to help support the ecosystem that we love. As such, we hope to build a broad sample of the community. <https://discourse.haskell.org/t/clc-elections-january-2025/11126#how-can-i-…>How can I apply? To apply for one of these positions, send an email to clc.nominations.2025(a)gmail.com <mailto:clc.nominations.2025@gmail.com> that consists of the following data: The subject “CLC Election September 2025 - {your name}”. Why you think you’re a good fit given the above criteria. If applicable, please point us to some code you’ve written. Please apply before Feb 2. Best regards, Andrew

1 1

Course-of-value recursion by defining a sequence as a self-referential infinite list
by Vanessa McHale 20 Jan '25

20 Jan '25

Laziness turns out to allow course-of-value recursion where one might use memoization in other languages. But I hadn’t seen this articulated! Famously, one can use this to define the Fibonacci numbers, viz. fibs :: [Integer] fibs = 1 : 1: zipWith (+) fibs (tail fibs) Or the Catalan numbers: catalan :: [Integer] catalan = 1 : 1 : [ sum [ (-1)^(k+1) * (pc (n - ((k*(3*k-1)) /. 2)) + pc (n - ((k*(3*k+1))/.2))) | k <- [1..n] ] | n <- [2..] ] where pc m | m >= 0 = part !! m | otherwise = 0 infixl 6 /. (/.) = quot I wrote up the example: http://vmchale.com/static/serve/Comb.pdf Reinhard Zumkeller has a lot of examples on OEIS: https://oeis.org/A000081 Cheers, Vanessa McHale

3 3

Call for Nominations: Haskell.org Committee (2025-2028)
by Rebecca Skinner 18 Jan '25

18 Jan '25

Dear Haskellers, We are pleased to announce that nominations are now open for the Haskell.org <http://haskell.org/> committee. You can nominate yourself or a friend for a three-year term (2025-2028) by sending an email to [committee at haskell.org <http://haskell.org/>] by January 31, 2025. 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 <http://haskell.org/> infrastructure, planning for the long term, and being fiscally responsible with Haskell.org <http://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 <http://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 <http://haskell.org/>], or contact one of us individually.

1 0

2nd Call for Participation: BOB 2025 (Berlin, Mar 14)
by Stefan Wehr 17 Jan '25

17 Jan '25

The early-bird deadline is Jan 17 (TODAY)! ========================================================================= BOB 2025 Conference “What happens if we simply use what’s best?” March 14, 2025, Berlin https://bobkonf.de/2025/ Program: https://bobkonf.de/2025/program.html Registration: https://bobkonf.de/2025/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 16 talks and 8 tutorials on current topics: https://bobkonf.de/2025/program.html Talk subjects includes functional programming, property-based testing, language server implementation, domain-specific languages, domain-driven design, local-first software, formal methods, and microservices. BOB tutorial include sessions on frontend development, local-first programming, data science, Elixir, and software documentation, combining mob programming, TDD, and AI. Annette Bieniusa will give the keynote talk. Registration is open - many discount options are available, as are grants for members of groups underrepresented in tech. Early-bird discounts apply until Jan 17. https://bobkonf.de/2025/registration.html

1 0

PhD Scholarships in Quantum Computing
by Venanzio Capretta 13 Jan '25

13 Jan '25

PhD Scholarships School of Computer Science University of Nottingham The School of Computer Science at the University of Nottingham is advertising 10 fully-founded PhD studentships starting on 1 October 2025: https://jobs.nottingham.ac.uk/vacancy.aspx?ref=SCI298 We are calling for good candidates to apply for a PhD project in Quantum Computing. The range of potential topics for the PhD project will be: * Quantum Data Types and Algorithms; * Applications of Quantum Computing to Artificial Immune System; * Quantum Functional Programming Languages; * Compilation of Higher-Order Quantum Code; * Quantum computing in AI. Expertise in any of this topics is welcome but not necessary. Applicants should have a good background in programming, data structures, discrete mathematics. The application deadline is 6 April 2025. Before applying you must agree with a potential supervisor and write a research project. So, if you intend to apply, please contact one of us as soon as possible: Ulrik Buchholtz (ulrik.buchholtz(a)nottingham.ac.uk) Venanzio Capretta (venanzio.capretta(a)nottingham.ac.uk) Julie Greensmith (pszjg1(a)exmail.nottingham.ac.uk) ------------------------------------- Venanzio Capretta School of Computer Science University of Nottingham, UK -------------------------------------

1 0