February 2026 - Haskell-Cafe

HasCallStack constraint for maximum in Prelude?
by Todd Wilson 24 Mar '26

24 Mar '26

Should maximum and minimum have HasCallStack constraints like head and tail do? ghci> :type head head :: GHC.Stack.Types.HasCallStack => [a] -> a ghci> head [] *** Exception: Prelude.head: empty list CallStack (from HasCallStack): error, called at libraries/base/GHC/List.hs:1646:3 in base:GHC.List errorEmptyList, called at libraries/base/GHC/List.hs:85:11 in base:GHC.List badHead, called at libraries/base/GHC/List.hs:81:28 in base:GHC.List head, called at <interactive>:8:1 in interactive:Ghci1 ghci> :type maximum maximum :: (Foldable t, Ord a) => t a -> a ghci> maximum [] *** Exception: Prelude.maximum: empty list CallStack (from HasCallStack): error, called at libraries/base/GHC/List.hs:1646:3 in base:GHC.List errorEmptyList, called at libraries/base/GHC/List.hs:717:28 in base:GHC.List maximum, called at libraries/base/Data/Foldable.hs:745:15 in base:Data.Foldable Todd Wilson

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Appreciation
by Volker Wysk 01 Mar '26

01 Mar '26

Hi! I've set up my Haskell environment with GHCup and Cabal and compiled a program of mine. It has worked really really well. The developers have my thanks and appreciation. Cheers, Volker

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finding out ghc version from closed issue
by Zoran Bošnjak 28 Feb '26

28 Feb '26

Dear members, I am facing this problem with ghc-9.10.3: https://gitlab.haskell.org/ghc/ghc/-/issues/25477 It looks like the problem is already fixed at some later version. My question is... how do I find out the ghc version from the issue or from the git commit? Zoran

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tls: Hybrid key exchange of ECC and Post-Quantum
by Kazu Yamamoto 27 Feb '26

27 Feb '26

Hello, TLS client of OpenSSL version 3.5 or later specifies hybrid key exchange of ECC and Post-Quantum (such as X25519MLKEM768) and an ECC key exchange (such as X25519) in TLS ClientHello. I have implemented both the hybrid algorithms and the OpenSSL specifying-method in the Haskell "tls" library. Interop with OpenSSL is quite good. I would like to discuss how to specify algorithms in ClientHello before releasing. If you are interested, I would like to invite you to this issue: https://github.com/haskell-tls/hs-tls/issues/516 I thank Olivier Chéron for implementing excellent "mlkem" library. --Kazu

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Midlands Graduate School, 13-17 April 2026, Nottingham, UK
by Graham Hutton (staff) 26 Feb '26

26 Feb '26

Dear all, Registration is now open for the Midlands Graduate School (MGS) in Nottingham! Eight fantastic courses on type theory, category theory, lambda calculus, and more. 13-17 April 2026, Nottingham, UK. Registration closes Sunday 22nd March. Best wishes, Graham ========================================================== MIDLANDS GRADUATE SCHOOL 2026 13-17 April 2026, Nottingham, UK https://tinyurl.com/MGS-2026 ========================================================== BACKGROUND The Midlands Graduate School (MGS) in the Foundations of Computing Science provides a yearly programme of courses on the mathematical foundations of computing, and recently celebrated its 25th year. The MGS is primarily aimed at PhD students in their first or second year, but 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 around four courses, depending on their interests. Invited course: - Implement your POPL paper (using Haskell) Stephanie Weirich (Pennsylvania) Introductory courses: - Type Theory using Agda Thorsten Altenkirch (Nottingham) - Category Theory Sergey Goncharov (Birmingham) - Lambda Calculus and Combinatory Algebra Sam Speight (Birmingham) Advanced courses: - Effects and Call-By-Push-Value Paul Blain Levy (Birmingham) - Synthetic Homotopy Theory Axel Ljungström (Nottingham) - Categorical Algebra and Logic Sean Moss (Birmingham) - Inductive and Coinductive Reasoning with Isabelle/HOL Andrei Popescu (Sheffield) REGISTRATION Student participants: £350 (or £600 with accommodation) Industry participants: £800 (or £1050 with accommodation) The registration fee includes all lecture courses and example classes, lunch and coffee breaks, and the conference dinner. The 'with accommodation' options include five nights at Rutland Hall with shared bathroom facilities. Registration closes Sunday 22nd March. Spaces are limited, so please register early to secure your place. 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. LOCAL ORGANISERS For any queries, please contact the local organisers: Ulrik Buchholtz <ulrik.buchholtz(a)nottingham.ac.uk> Dan Marsden <dan.marsden(a)nottingham.ac.uk> ========================================================== -- Professor Graham Hutton School of Computer Science University of Nottingham, UK http://people.cs.nott.ac.uk/pszgmh 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.

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Call for Talks: Haskell Implementors' Workshop 2026
by Christiaan Baaij 20 Feb '26

20 Feb '26

Haskell Implementors' Workshop 2026 https://haskell.foundation/events/2026-haskell-implementors-workshop.html June 5, 2026 Organized by the Haskell Community Co-located with ZuriHac 2026 and Haskell Ecosystem Workshop 2026 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… Important dates --------------- Deadline: April 10, 2026 (AoE) Notification: May 8, 2026 Workshop: June 5, 2026 The 18th Haskell Implementors' Workshop is to be held alongside ZuriHac 2026 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 short lightning talks. Scope and target audience ------------------------- The 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, optimisation 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. Please submit a talk title and abstract of no more than 300 words. Submissions can be made via this form https://forms.gle/S8fpAgFaFEvqoMqH6 until April 10 (anywhere on earth). We will also have a lightning talks session. These have been very well received in recent years, and we aim to increase the time available to them. 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 ----------------- * Andrew Lelechenko * Avi Press * Christiaan Baaij (chair) * Gergo Erdi * Jeffrey Young Contact ------- * Christiaan Baaij <chrisitaan.baaij at gmail.com>

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[ANN] Copilot 4.6.1
by Ivan Perez 12 Feb '26

12 Feb '26

Hi café! We are really excited to announce Copilot 4.6.1 [1, 2]. Copilot is a stream-based EDSL in Haskell for writing and monitoring embedded systems, with an emphasis on correctness and hard realtime requirements. Copilot is typically used as a high-level runtime verification framework, and supports temporal logic (LTL, PTLTL and MTL), clocks and voting algorithms. Compilation to Bluespec, to target FPGAs, is also supported. Copilot is NASA Class D open-source software, and is being used at NASA in drone test flights. Through the NASA tool Ogma [3] (also written in Haskell), Copilot also serves as a programming language and runtime framework for NASA's Core Flight System, Robot Operating System (ROS 2) and FPrime (the software framework used in the Mars Helicopter). Ogma now supports producing flight and robotics applications directly in Copilot, not just for monitoring, but for implementing the logic of the applications themselves. This release improves the Bluespec backend, fixing corner cases related to the generation of Bluespec (and thus Verilog). The release also introduces some general maintenance improvements. Copilot is compatible with versions of GHC from 8.6 to 9.12. This release has been made possible thanks to Chris Hathhorn (Galois) and Trevor Kann (Galois). The team also benefited from discussions with Ryan Scott (Galois). We are grateful to all of them for their contributions, and for making Copilot better every day. For details on this release, see [1]. As always, we're releasing exactly 2 months since the last release. Our next release is scheduled for Mar 7th, 2026. We want to remind the community that Copilot is now accepting code contributions from external participants again. Please see the discussions and the issues in our Github repo [4] to learn how to participate. Current emphasis is on using Copilot for full data processing applications (e.g, system control, arduinos, rovers, drones), improving usability, performance, and stability, increasing test coverage, removing unnecessary dependencies, hiding internal definitions, and formatting the code to meet our coding standards. Users are encouraged to participate by opening issues, asking questions, extending the implementation, and sending bug fixes. Happy Haskelling! Ivan -- [1] https://github.com/Copilot-Language/copilot/releases/tag/v4.6.1 [2] https://hackage.haskell.org/package/copilot [3] https://github.com/nasa/ogma [4] https://github.com/Copilot-Language/copilot

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CLC Elections January 2026
by Teo Camarasu 07 Feb '26

07 Feb '26

Happy new year! The Core Libraries Committee seeks nominations for three seats. ## What is CLC? CLC is a volunteer body, primarily tasked with management of the 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-fi…. ## 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. ## How can I apply? To apply for one of these positions, send an email to [clc.nominations.2026@fastmail.com](mailto:clc.nominations.2026@fastmail.com) with: * The subject “CLC Election January 2026 - {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. Cheers, Teo

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