January 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

4 4

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

1 1

NEU: Festschrift and Workshop in Honor of David Basin (Deadline: 21st May 2026)
by Achim D. Brucker 26 Jan '26

26 Jan '26

Dear Fellow Researchers! Our dear colleague and friend David Basin is turning 65 in December 2026 and this has to be celebrated! We therefore organize a Festschrift and a Fest to celebrate his birthday and his extensive research contributions! The Fest/celebration itself will be held as a one-day event on the 15th of January 2027 at ETH Zürich. We have already checked that David is available. You do not have to keep it a secret - on the contrary, it is great if you share this message with anybody who might like to contribute! We hereby cordially invite you to contribute an article to the Festschrift and present it at the Fest. We welcome contributions in all areas close to David's research and interests, including but not limited to security, privacy, formal methods, logic, automated reasoning, model checking, theorem proving, software engineering, bridge, juggling, biking and more. The articles will be lightly reviewed by the Festschrift committee, and the proceedings will be published by Springer Heidelberg in the LNCS series. David will of course love to receive a research article from you, but also short personal articles that celebrate the history and friendship with David will be very much appreciated. We would like to set a deadline for submissions at the 21st of May 2026 via our submission site <https://easychair.org/conferences/?conf=feschi2027>. Articles should be formatted in Springer’s LNCS style and not exceed 5 pages for personal articles and 16 pages for scientific articles. If you have a work that you would like to contribute that does not fit into this page limit, it might be possible, but please reach out to us first. There will be no participation fee for the Fest and coffee breaks, lunch, and dinner will be included. Travel and accommodation costs are at the expense of the participants. We are happy to provide hotel suggestions for Zürich. Submission Information: * Format: 16 pages in LNCS format, templates available at: <https://www.springer.com/gp/computer-science/lncs/conference-proceedings-gu…> * Submission: <https://easychair.org/conferences/?conf=feschi2027> * Website (under construction) with updates and further information: <https://feschi2027.github.io/> * Timeline: * Submission: 21 May 2026 * Notification: end of June 2026 (to be confirmed) * Camera Ready Copy: early September 2026 (to be confirmed) Please let us know if you have any questions! Best wishes Achim D. Brucker (University of Exeter) Sebastian Mödersheim (Danmarks Tekniske Universitet) Christoph Sprenger (ETH Zürich) Luca Viganò (King’s College London)

1 0

Can I coerce type constructors to Any?
by Tom Ellis 23 Jan '26

23 Jan '26

The documentation for Any says: > The type constructor Any is type to which you can unsafely coerce > any lifted type, and back. More concretely, for a lifted type t and > value x :: t, unsafeCoerce (unsafeCoerce x :: Any) :: t is > equivalent to x. https://www.stackage.org/haddock/lts-24.28/base-4.20.2.0/GHC-Base.html#t:Any Is this also true of type constructors? For example, could I coerce `f x` to `Any x`? (assuming f maps lifted types to lifted types) In particular I'm interested in things like data C f where MkC :: (forall a. Monad (f a)) => C newtype CD f = MkCD (C Any) putCD :: C f -> CD f putCD c = MkCD (unsafeCoerce c) getCD :: CD f -> C f getCD (MkCD cd) = unsafeCoerce cd Is that sound? Thanks, Tom

2 3

GSoC 2026 Call for Ideas
by Aaron Allen 13 Jan '26

13 Jan '26

Dear Haskellers, Google Summer of Code is a long-running program by Google that supports Open Source projects. Haskell has been involved in the Google Summer of Code program almost since its inception and we are hoping to participate again in 2026! In order to do this we need a collection of ideas for participants to select from. In the past this has included significant improvements to Haskell projects such as GHC, Cabal, and HLS - and it can include your project as well! This is a great opportunity to find new contributors for your project and many past participants have become involved long-term. You can find more info and instructions on how to participate here: https://summer.haskell.org/ideas.html.

1 0

2nd Call for Participation: BOB 2026 (Berlin, Mar 13)
by Michael Sperber 13 Jan '26

13 Jan '26

Note the early-bird discount runs out on Friday! ========================================================================= BOB 2026 Conference “What happens if we simply use what’s best?” March 13, 2026, Berlin https://bobkonf.de/2026/ Program: https://bobkonf.de/2026/program.html Registration: https://bobkonf.de/2026/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/2026/program.html Talk subjects includes functional programming, software architecture, formal methods, accessibility, UI programming, reactive systems, and domain-driven design. BOB tutorial include sessions on TypeScript, OCaml, Haskell, Agda, accessibility, and reactive systems. Stefan Kaufmann will give the keynote talk on digital sovereignty. Registration is open - many discount options - including limited early-bird discounts - are available, as are grants for members of groups underrepresented in tech. https://bobkonf.de/2026/registration.html

1 0

Critical code generation bug with GHC 9.12.3
by Zubin Duggal 08 Jan '26

08 Jan '26

Hello all, Unfortunately we discovered a major code generation regression in GHC 9.12.3 that results in incorrect runtime results for certain operations involving sub-word divisions. We recommend avoiding upgrading to this minor release at this time. See https://gitlab.haskell.org/ghc/ghc/-/issues/26711 for more details. This is the same bug that affected GHC 9.12.1. A patch re-introducing this bug was accidently backported onto the 9.12.3 branch and went unnoticed throughout the testing period. We are taking steps to ensure similar situations are less likely to occur again, including looking at ways to improve our testing procedure for minor GHC release, potentially using the Stackage infrastructure to run the testsuites of a broad range of packages to detect such issues. We are also starting to prepare GHC 9.12.4 with a fix, which we hope to release soon. However, a significant amount of time has passed since the initial set of backports to GHC 9.12.3 were completed, which means that a number of merge requests addressing issues of various severities has been marked for backporting to the GHC 9.12 branch. We are in the process of triaging this list and preparing a robust and reliable GHC 9.12.4 release. Thanks, Zubin.

1 0