- Haskell-Cafe - Haskell.org

Re: lost polymorphism in Typed Tagless Final Interpreters
by Olaf Klinke 13 Nov '25

13 Nov '25

> So change lit <$> safeRead n to (\x -> > lit x) <$> safeRead n, for example. Does anyone else find this highly non-obvious? SPJ gives an example https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-543683… of a context that can indeed distinguish, semantically, a function and its eta expansion. But the fact that, for the benefit of soundness, there must exist contexts where one can be a well-typed program while the other is not, is still mysterious to me. Can a type system with polymorphism really not do better than this? The discussion thread has some more interesting posts, e.g. https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-544377… https://github.com/ghc-proposals/ghc-proposals/pull/287#issuecomment-112046… At least, thanks to Jaro, the error index now explains when you should eta-expand. https://errors.haskell.org/messages/GHC-83865/ Olaf

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lost polymorphism in Typed Tagless Final Interpreters
by Anthony Carrico 13 Nov '25

13 Nov '25

In his tutorial lecture "Typed Tagless Final Interpreters" https://www.okmij.org/ftp/tagless-final/course/lecture.pdf Oleg Kiselyov deals with the issue of losing polymorphism, tf1'_both :: IO() tf1'_both = case tf1' of Left e -> putStrLn ("Error: " ++ e) Right (x) -> do print (eval x) print (view x) "We have lost polymorphism. The problem is subtle: the function fromTree is indeed polymorphic over repr, as its inferred type shows. However, to extract the de-serialized term, we have to do pattern-matching; the variable x is bound in the case pattern and hence, like a lambda-pattern–bound variable, gets a monomorphic, non-generalizable type. Therefore, we cannot interpret x with several arbitrary interpreters; the extensibility is lost." He shows how to use a newtype wrapper to work around the issue, but also says in a foot note: "With the impredicative polymorphism GHC extension, we do not have to fake first-class polymorphism and do not need Wrapped." Two questions: 1. Is this true? I still get the error. 2. Can anyone summarize the current state of this issue today? It can be hard follow the issue through the history of Haskell/GHC. Thanks to Oleg for publishing this lecture, it is possible to learn quite a bit from it, and thanks to whoever would like to give an update! I've smashed together my source code from the F module and the Serialization module together below. You can see the "defered type error" at the very bottom. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 9.8.4 {-# LANGUAGE NoMonomorphismRestriction #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ImpredicativeTypes #-} module F where class ExpSYM repr where lit :: Int -> repr neg :: repr -> repr add :: repr -> repr -> repr instance ExpSYM Int where lit n = n neg e = -e add e0 e1 = e0 + e1 eval :: Int -> Int eval = id instance ExpSYM String where lit n = show n neg e = "(-" ++ e ++ ")" add e0 e1 = "(" ++ e0 ++ " + " ++ e1 ++ ")" view :: String -> String view = id data Tree = Leaf String | Node String [Tree] deriving (Eq, Read, Show) instance ExpSYM Tree where lit n = Node "Lit" [Leaf (show n)] neg e = Node "Neg" [e] add e0 e1 = Node "Add" [e0, e1] toTree :: Tree -> Tree toTree = id type ErrMsg = String safeRead :: Read a => String -> Either ErrMsg a safeRead s = case reads s of [(x, "")] -> Right x _ -> Left $ "Read error: " ++ s fromTree :: (ExpSYM repr) => Tree -> Either ErrMsg repr fromTree (Node "Lit" [Leaf n]) = lit <$> (safeRead n) fromTree (Node "Neg" [e]) = neg <$> (fromTree e) fromTree (Node "Add" [e0, e1]) = add <$> (fromTree e0) <*> (fromTree e1) fromTree e = Left ("Invalid tree: " ++ show e) tf1 :: ExpSYM repr => repr tf1 = add (lit 8) (neg (add (lit 1) (lit 2))) tf1_tree :: Tree tf1_tree = toTree F.tf1 tf1' :: ExpSYM repr => Either ErrMsg repr tf1' = fromTree tf1_tree tf1'_eval :: IO() tf1'_eval = case tf1' of Left e -> putStrLn ("Error: " ++ e) Right x -> print (eval x) -- >>> tf1'_eval -- 5 tf1'_view :: IO() tf1'_view = case tf1' of Left e -> putStrLn ("Error: " ++ e) Right x -> print (view x) -- >>> tf1'_view -- "(8 + (-(1 + 2)))" tf1'_both :: IO() tf1'_both = case tf1' of Left e -> putStrLn ("Error: " ++ e) Right (x) -> do print (eval x) print (view x) -- >>> tf1'_both -- *** Exception: /tmp/danteIpH3fg.hs:90:19: error: [GHC-83865] -- • Couldn't match type ‘Int’ with ‘[Char]’ -- Expected: String -- Actual: Int -- • In the first argument of ‘view’, namely ‘x’ -- In the first argument of ‘print’, namely ‘(view x)’ -- In a stmt of a 'do' block: print (view x) -- (deferred type error) -- Anthony Carrico

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GHC 9.12.3-rc2 is now available
by Zubin Duggal 12 Nov '25

12 Nov '25

The GHC developers are very pleased to announce the availability of the second release candidate for GHC 9.12.3. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][] and via [GHCup](https://www.haskell.org/ghcup/). GHC 9.12.3 is a bug-fix release fixing several issues of a variety of severities and scopes. A full accounting of these fixes can be found in the [release notes][]. As always, GHC's release status, including planned future releases, can be found on the GHC Wiki [status][]. This release candidate will have a two-week testing period. If all goes well the final release will be available the week of 24 November 2025. The changes from the first release candidate are * Fix a regression resulting in a panic with empty cases (#26442, #25960). * Fix for a potential segfault in the bytecode interpreter (#23210). * Fix for a regression with tabs not being allowed in string gaps (#26415). * Fix for a crash with the non-moving GC (#26204). * A rework of how internal symbols used by the code-generator are handled to adapt to changes in the Apple toolchain, enabling this release to bootstrap on macOS 26 Tahoe (#26166). GHC development is sponsored by: * [Juspay](https://juspay.com/) * [QBayLogic](https://qbaylogic.com/) * [Channable](https://www.channable.com/) * [Haskell Foundation](https://haskell.foundation/) * [Serokell](https://serokell.io/) * [Well-Typed](https://well-typed.com/) * [Tweag](https://www.tweag.io/) * [Dotcom-Monitor](https://www.dotcom-monitor.com/) * [LoadView](https://www.loadview-testing.com/) * [Web Hosting Buddy](https://webhostingbuddy.com/) * [Find My Electric](https://www.findmyelectric.com/) * [Standard Chartered](https://www.sc.com) * [UpCloud](https://upcloud.com) We would like to that these sponsors and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do give this release a try and open a [ticket][] if you see anything amiss. [release notes]: https://gitlab.haskell.org/ghc/ghc/-/blob/ghc-9.12/docs/users_guide/9.12.3-… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.3-rc2 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new

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[ANN] Copilot 4.6
by Ivan Perez 12 Nov '25

12 Nov '25

Hi café! We are really excited to announce Copilot 4.6 [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), 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 major release improves the Bluespec backend, producing more suitable Verilog code. Prior implementations required modifying the resulting Verilog to help connect it to external systems (e.g., sensors, actuators), while the new Bluespec backend produces code that can be used without modification. The new implementation is compatible with versions of GHC from 8.6 to 9.12. This release has been made possible thanks to Sukhman Kahlon (NASA), who carried out a thorough evaluation of our FPGA backend, and to Ryan Scott (Galois), who implemented the updated solution. This work has also greatly benefited from discussions with the Bluespec team on the code generated by Copilot and possible improvements. 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 Jan 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 [2] https://hackage.haskell.org/package/copilot [3] https://github.com/nasa/ogma [4] https://github.com/Copilot-Language/copilot

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ICFP 2026: Call for Papers
by ICFP Publicity 12 Nov '25

12 Nov '25

ICFP 2026 Call for Papers Accepted papers to be invited for presentation at: *The 31st ACM SIGPLAN International Conference on Functional Programming*August 23-29, 2026 Indianapolis, Indiana, USA https://icfp26.sigplan.org ------------------------------ Submission Information - *Submission web site*: https://icfp26.hotcrp.com - *Submission Deadline*: 19 Feb 2026 (AoE) Double-blind review - Reviews: 20 Apr 2026 (AoE) - Author Response: 20 Apr 2026 - 23 Apr 2026 (AoE) - Notification of Conditional Acceptance: 14 May 2026 (AoE) - Revision: 3 Jun 2026 (AoE) - Final Notification: 10 Jun 2026 (AoE) - Camera Ready: 1 Jul 2026 (AoE) ------------------------------ PACMPL issue ICFP 2026 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): - Language Design: concurrency, parallelism, and distribution; modularity; components and composition; meta-programming; macros; pattern matching; type systems; type inference; dependent types; effect types; gradual types; refinement types; session types; interoperability; domain-specific languages; imperative programming; object-oriented programming; logic programming; probabilistic programming; reactive programming; generic programming; bidirectional programming; secure programming. - Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimisation; garbage collection and memory management; runtime systems; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. - Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling; build systems; program synthesis. - Analysis and Transformation: control flow; data flow; abstract interpretation; partial evaluation; program calculation. - Foundations: formal semantics; lambda calculus; program equivalence; rewriting; type theory; logic; category theory; computational effects; continuations; control; state; names and binding; program verification. - Applications: symbolic computing; formal-methods tools; systems programming; distributed systems and web programming; hardware design; databases; scientific and numerical computing; graphical user interfaces; graphics and multimedia; GPU programming; scripting; system administration; security. - Education: teaching introductory programming; mathematical proof; algebra. Submissions will be evaluated according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. PACMPL issue ICFP 2026 also welcomes submissions in two separate categories — Functional Pearls and Experience Reports — that must be marked as such when submitted and that *need not* report original research results. Detailed guidelines on both categories are given at the end of this call. In an effort to achieve a balanced, diverse program, each author may be listed as a (co)author on a maximum of four submissions. Authors who require financial support to attend the conference can apply for PAC funding (http://www.sigplan.org/PAC/) The General Chair and PC Chair may not submit papers. PC members (other than the PC Chair) may submit papers. Please contact the Program Chair if you have questions or are concerned about the appropriateness of a topic. Double-blind Submissions <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#double-bind-submissi…> ICFP 2026 will use a full double-blind reviewing process. This means that identities of authors will not be made visible to reviewers until after conditional-acceptance decisions have been made, and then only for the conditionally-accepted papers. The use of full double-blind reviewing has several consequences for authors. - Submissions: Authors must omit their names and institutions from their paper submissions. In addition, references to authors’ own prior work should be in the third person (e.g., not “We build on our previous work …” but rather “We build on the work of …”). - Supplementary material: Authors must fully anonymize any supplementary material (see below). Links to supplementary material on external websites are not permitted. - Author response: In responding to reviews, authors should not say anything that reveals their identity, since author identities will not be revealed to reviewers at that stage of the reviewing process. - Dissemination of work under submission: Authors are welcome to disseminate their ideas and post draft versions of their paper(s) on their personal website, institutional repository, or arXiv (reviewers will be asked to turn off arXiv notifications during the review period). But authors should not take steps that would almost certainly reveal their identities to members of the Program Committee, e.g., directly contacting PC members or publicizing the work on widely-visible social media or major mailing lists used by the community. The purpose of the above restrictions is to help the Program Committee come to a judgment about the paper without bias, not to make it impossible for them to discover the authors’ identities if they were to try. In particular, nothing should be done in the name of anonymity that weakens the quality of the submission. However, there are occasionally cases where adhering to the above restrictions is truly difficult or impossible for one reason or another. In such cases, the authors should contact the Program Chair to discuss the situation and how to handle it. The FAQ on Double-Blind Reviewing ( https://popl24.sigplan.org/track/POPL-2024-popl-research-papers#FAQ-on-Doub…) addresses many common scenarios and answers many common questions about this topic. But there remain many grey areas and trade-offs. If you have any doubts about how to interpret the double-blind rules or you encounter a complex case that is not clearly covered by the FAQ, please contact the Program Chair for guidance. Preparation of submissions <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#preparation-of-submi…> The deadline for submissions is: *** Thursay, 19 Feb 2026 AoE *** ( https://www.timeanddate.com/time/zones/aoe) This deadline will be strictly enforced. - 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. Please download the latest version of the ACM style from https://www.acm.org/publications/authors/submissions, since the citation format has recently been changed. See also PACMPL’s Information and Guidelines for Authors at https://pacmpl.acm.org/authors.cfm. There is a limit of *25 pages* for a full paper or Functional Pearl and *12 pages* for an Experience Report; in either case, the bibliography and an optional clearly marked appendix will not be counted against these limits. Submissions that exceed the page limits or, for other reasons, do not meet the requirements for formatting, will be desk rejected. - Submission: Submissions will be accepted at https://icfp26.hotcrp.com Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. - Author Response Period: Authors will have a 96-hour period, starting at 00:00 (midnight) AoE on Monday, 20 April, 2026, to read reviews and respond to them. - Appendix and Supplementary Material: Authors have the option to include a clearly marked appendix and/or to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at such an appendix or supplementary material. Supplementary material may be uploaded as a separate PDF document or tarball. Any supplementary material must be uploaded at submission time, not by providing a URL in the paper that points to an external repository. All supplementary material must be anonymised. - Authorship Policies: All submissions are expected to comply with the ACM Policies for Authorship that are detailed at https://www.acm.org/publications/authors/information-for-authors. - Republication Policies: Each submission must adhere to SIGPLAN’s republication policy, as explained on the web at http://www.sigplan.org/Resources/Policies/Republication. Review Process <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#review-process> This section outlines the two-stage process with double-blind reviewing that will be used to select papers for PACMPL issue ICFP 2026. Like last year, ICFP 2026 will adapt a full double-blind reviewing process. More information see below. ICFP 2026 will have two Associate Chairs who will help the PC Chair monitor reviews, solicit external expert reviews for submissions when there is not enough expertise on the committee, and facilitate reviewer discussions. ICFP 2026 will employ a two-stage review process. The first stage in the review process will assess submitted papers using the criteria stated above and will allow for feedback and input on initial reviews through the author response period mentioned previously. As a result of the review process, a set of papers will be conditionally accepted and all other papers will be rejected. Authors will be notified of these decisions on 14 May, 2026. Authors of conditionally accepted papers will be provided with committee reviews along with a set of optional or mandatory revisions. By 3 June, 2026, the authors should provide a revised submission. The second and final reviewing phase assesses whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. The intent and expectation is that the mandatory revisions can feasibly be addressed within a couple of weeks. The second submission should clearly identify how the mandatory revisions were addressed. To that end, the second submission *must be accompanied by a cover letter* mapping each mandatory revision request to specific parts of the paper. The cover letter will facilitate a quick second review, allowing for confirmation of final acceptance within two weeks. Conversely, the absence of a cover letter will be grounds for the paper’s rejection. Information for Authors of Accepted Papers <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#information-for-auth…> As a condition of acceptance, final versions of all papers must adhere to the ACM Small format. The page limit for the final versions of papers will be *increased by two pages* to help authors respond to reviewer comments and mandatory revisions: 27 pages plus bibliography for a regular paper or Functional Pearl, 14 pages plus bibliography for an Experience Report. Authors of accepted submissions will be required to agree to one of the three ACM licensing options, one of which is Creative Commons CC-BY publication; this is the option recommended by the PACMPL editorial board. A reasoned argument in favour of this option can be found in the article Why CC-BY? published by OASPA, the Open Access Scholarly Publishers Association. The other options are copyright transfer to ACM or retaining copyright but granting ACM exclusive publication rights. PACMPL is a Gold Open Access journal, and authors are encouraged to publish their work under a CC-BY license. Gold Open Access guarantees permanent free online access to the definitive version in the ACM Digital Library, and the recommended CC-BY option also allows anyone to copy and distribute the work with attribution. Gold Open Access has been made possible by generous funding through ACM SIGPLAN, which will cover all open access costs in the event authors cannot. Authors who can cover the costs may do so by paying an Article Processing Charge (APC). PACMPL, SIGPLAN, and ACM Headquarters are committed to exploring routes to making Gold Open Access publication both affordable and sustainable. ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking to the definitive version of an ACM article should reduce user confusion over article versioning. After an article has been published and assigned to the appropriate ACM Author Profile pages, authors should visit http://www.acm.org/publications/acm-author-izer-service to learn how to create links for free downloads from the ACM DL. The official publication date is the date the journal is made available in the ACM Digital Library. The journal issue and associated papers will be published no earlier than 1 August, 2026. The official publication date affects the deadline for any patent filings related to published work. Authors of each accepted submission are invited to attend and be available for the presentation of that paper at the conference. The schedule for presentations will be determined and shared with authors after the full program has been selected. ORCID: ORCID provides a persistent digital identifier (an ORCID iD) that you own and control, and that distinguishes you from every other researcher: https://orcid.org/. ACM now require an ORCID iD for every author of a paper, not just the corresponding author. So, the author who is filling out the permission form should make sure they have the ORCID iDs for all of their coauthors before filling out the form. Any authors who do not yet have an ORCID iD can go to https://orcid.org/register to have one assigned. By submitting your article to an ACM Publication, you are hereby acknowledging that you and your co-authors are subject to all ACM Publications Policies, including ACM’s new Publications Policy on Research Involving Human Participants and Subjects. Alleged violations of this policy or any ACM Publications Policy will be investigated by ACM and may result in a full retraction of your paper, in addition to other potential penalties, as per ACM Publications Policy. Artifact Evaluation <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#artifact-evaluation> Authors of papers that are conditionally accepted in the first phase of the review process will be encouraged (but not required) to submit supporting materials for Artifact Evaluation. These items will then be reviewed by an Artifact Evaluation Committee, separate from the paper Review Committee, whose task is to assess how the artifacts support the work described in the associated paper. Papers that go through the Artifact Evaluation process successfully will receive a seal of approval printed on the papers themselves. Authors of accepted papers will be encouraged to make the supporting materials publicly available upon publication of the papers, for example, by including them as “source materials” in the ACM Digital Library. An additional seal will mark papers whose artifacts are made available, as outlined in the ACM guidelines for artifact badging. Participation in Artifact Evaluation is voluntary and *will not influence* the final decision regarding paper acceptance. Special categories of papers <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#special-categories-o…> In addition to research papers, PACMPL issue ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to half the length of a full paper. Authors submitting such papers should consider the following guidelines. Functional Pearls <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#functional-pearls> A Functional Pearl is an elegant essay about something related to functional programming. Examples include, but are not limited to: - a new and thought-provoking way of looking at an old idea; - an instructive example of program calculation or proof; - a nifty presentation of an old or new data structure; - an interesting application of functional programming techniques; - a novel use or exposition of functional programming in the classroom. While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. Functional Pearls are valued as highly and judged as rigorously as ordinary papers, but using somewhat different criteria. In particular, a pearl is not required to report original research, but, it should be concise, instructive, and entertaining. A pearl is likely to be rejected if its readers get bored, if the material gets too complicated, if too much-specialised knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. A submission that is intended to be treated as a pearl must be marked as such on the submission web page and should contain the words “Functional Pearl” somewhere in its title or subtitle. These steps will alert reviewers to use the appropriate evaluation criteria. Pearls will be combined with ordinary papers for the purpose of computing the conference’s acceptance rate. Experience Reports <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#experience-reports> The purpose of an Experience Report is to describe the experience of using functional programming in practice, whether in industrial application, tool development, programming education, or any other area. Possible topics for an Experience Report include, but are not limited to: - insights gained from real-world projects using functional programming; - comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum project-management, business, or legal issues encountered when using functional programming in a real-world project; - curricular issues encountered when using functional programming in education; - real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general. An Experience Report is distinguished from a normal PACMPL issue ICFP paper by its title, by its length, and by the criteria used to evaluate it. Both in the papers and in any citations, the title of each accepted Experience Report must end with the words “(Experience Report)” in parentheses. The acceptance rate for Experience Reports will be computed and reported separately from the rate for ordinary papers. Experience Report submissions can be at most 12 pages long, excluding bibliography. Each accepted Experience Report will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience Reports may be asked to give shorter talks. Because the purpose of Experience Reports is to enable our community to understand the application of functional programming, an acceptable Experience Report need not add to the body of knowledge of the functional-programming community by presenting novel results or conclusions. It is sufficient if the report describes an illuminating experience with functional programming, or provides evidence for a clear thesis about the use of functional programming. The experience or thesis must be relevant to ICFP, but it need not be novel. The review committee will accept or reject Experience Reports based on whether they judge the paper to illuminate some aspect of the use of functional programming. Anecdotal evidence will be acceptable provided it is well-argued and the author explains what efforts were made to gather as much evidence as possible. Typically, papers that show how functional programming was used are more convincing than papers that say only that functional programming was used. It can be especially effective to present comparisons of the situations before and after the experience described in the paper, but other kinds of evidence would also make sense, depending on context. Experience drawn from a single person’s experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. An Experience Report should be short and to the point. For an industrial project, it should make a claim about how well functional programming worked and why; for a pedagogy paper, it might make a claim about the suitability of a particular teaching style or educational exercise. Either way, it should produce evidence to substantiate the claim. If functional programming worked in this case in the same ways it has worked for others, the paper need only summarise the results — the main part of the paper should discuss how well it worked and in what context. Most readers will not want to know all the details of the experience and its implementation, but the paper should characterise it and its context well enough so that readers can judge to what degree this experience is relevant to their own circumstances. The paper should take care to highlight any unusual aspects; specifics about the experience are more valuable than generalities about functional programming. If the paper not only describes experience but also presents new technical results, or if the experience refutes cherished beliefs of the functional-programming community, it may be better to submit it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. The Program Chair will be happy to advise on any concerns about which category to submit to. About PACMPL <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#about-pacmpl> Proceedings of the ACM on Programming Languages (PACMPL https://pacmpl.acm.org/) is a Gold Open Access journal publishing research on all aspects of programming languages, from design to implementation and from mathematical formalisms to empirical studies. Each issue of the journal is devoted to a particular subject area within programming languages and will be announced through publicised Calls for Papers, like this one. Important update on ACM’s new open access publishing model for 2026 ACM Conferences: <https://icfp26.sigplan.org/track/icfp-2026-icfp-papers#important-update-on-…> Starting January 1, 2026, ACM will fully transition to Open Access. All ACM publications, including those from ACM-sponsored conferences, will be 100% Open Access. Authors will have two primary options for publishing Open Access articles with ACM: the ACM Open institutional model or by paying Article Processing Charges (APCs). With over 1,800 institutions already part of ACM Open, the majority of ACM-sponsored conference papers will not require APCs from authors or conferences (currently, around 70–75%). Authors from institutions not participating in ACM Open will need to pay an APC to publish their papers, unless they qualify for a geographic or discretionary waiver. To find out whether an APC applies to your article, please consult the list of participating institutions <https://libraries.acm.org/acmopen/open-participants> in ACM Open and review the APC Waivers and Discounts Policy <https://www.acm.org/publications/policies/policy-on-open-access-apc-waivers…> . To support a smooth transition and encourage broader ACM Open participation, ACM has introduced a temporary subsidy on APC pricing for 2026, funded directly by ACM. This pricing applies to all articles published in ACM and SIG sponsored conferences taking place in 2026. The subsidized conference pricing for 2026 is as follows: Authors No ACM or SIG members At least 1 ACM or SIG member ACM and SIG Sponsored Conference Article $350 $250 From a lower-middle-income country <https://www.acm.org/publications/policies/lower-middle-income-countries> $175 $125 This represents a 65% discount <https://www.acm.org/publications/openaccess>, funded directly by ACM. Authors are encouraged to help advocate for their institutions to join ACM Open during this transition period.

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JFP Special Issue on Program Calculation
by Graham Hutton (staff) 11 Nov '25

11 Nov '25

Dear all, We're delighted to announce that the JFP Special Issue on Program Calculation is now complete, and contains eleven papers that are freely available to read online from the link below. Best wishes, Graham Hutton and Nicolas Wu Special-Issue Editors ==================================================================== JFP Special Issue on Program Calculation https://tinyurl.com/JFP-prog-calc The idea of program calculation, in which programs are derived from specifications using equational reasoning, has been a central theme in functional programming since its inception. In particular, the approach allows us to systematically discover how programs can be defined, while at the same time obtaining proofs that they are correct. This special issue showcases recent advances in the field of program calculation. We are pleased to present eleven papers covering a diverse range of topics, including algorithm design, computational effects, graphical methods, relational reasoning, compiler calculation, and co-induction. These contributions reflect the depth and breadth of contemporary research in the field. We dedicate the special issue to the memory of Richard Bird, the ultimate program calculator. His commitment to clarity, simplicity and elegance set a standard that continues to inspire all who had the pleasure of reading his papers and hearing his talks. ==================================================================== 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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On Streamlining Compliance of Haskell projects with NASA requirements
by Ivan Perez 11 Nov '25

11 Nov '25

Hi, In case it's of interest to anyone here, I'm writing to share "Towards Streamlining Auditing for Compliance with Requirements in Open-source Software at NASA", a paper we presented at the AIAA/IEEE Conference on Digital Avionics Systems (DASC) last September. The paper can be accessed here: https://ntrs.nasa.gov/citations/20250006564 The paper talks about how we are leveraging automated tools to comply with NASA Software Engineering requirements in the Copilot project. The process we follow with Copilot is the same for the Ogma project, so the tools are usable there too. Some of the details discussed (e.g., traceability from issues to code, steps towards issue assignment, review and closure) are public on our Github pages (https://github.com/Copilot-Language/copilot, https://github.com/nasa/ogma) Both Copilot and Ogma are written in Haskell, and I've been posting progress updates on both regularly The gist of the paper is that we can put together git + github data to check if we are doing things minimally right. We can't check for all software engineering requirements, but we can perform many useful checks automatically. While this is a bit tangential, I hope this gives you an idea of how rigorous we have to be when developing Copilot and Ogma and the process we need to make sure these Haskell projects can be used in flight. If you have any comments, feel free to write to me directly at contact(a)ivanperez.io or via the discussions in the Copilot repo: https://github.com/Copilot-Language/copilot/discussions . Happy Haskelling! Ivan

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Cognitive dissonance in the Haskell Report
by Douglas McIlroy 09 Nov '25

09 Nov '25

Section 2.4 of the Haskell report defines "identifier" as a lexeme that begins with a letter. A few paragraphs on it says An operator symbol starting with any other character [than :] is an ordinary identifier. In everyday usage, ordinary people are necessarily people, yet this kind of ordinary identifier falls outside the definition of identifier. Section 3.2 says An operator is either an operator symbol, such as + or $$, or is an ordinary identifier enclosed in grave accents. If "either" is taken in the exclusive sense, this implies that operator symbols are *not* ordinary identifiers, in contradiction to Section 2.4. But if either is taken inclusively, then the lexeme +, an ordinary identifier per Section 2.4, can optionally be enclosed in grave accents. The terminology needs some cleaning. Doug McIlroy

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GHC Steering Committee Call for Nominations
by Simon Marlow 07 Nov '25

07 Nov '25

Dear Haskellers, The GHC Steering Committee is seeking nominations for new members. *To nominate yourself *please send an email to Simon Marlow <marlowsd(a)gmail.com> <marlowsd(a)gmail.com>, briefly summarising your background and relevant experience. See below for details on what committee membership involves, and what kind of experience would be useful. DEADLINE: 6 November 2025 *What's the GHC steering committee?* The committee scrutinizes, debates and eventually decides to accept or reject proposals to change the language or major features supported by GHC. Our processes are described in the GitHub repository <https://github.com/ghc-proposals/ghc-proposals> where proposals are submitted. In particular, please have a look at the Committee bylaws <https://github.com/ghc-proposals/ghc-proposals/blob/master/committee.rst>. *Who are we looking for?* We are looking for members who have the ability to: - understand GHC proposals (e.g. for new language extensions), - find holes and missing corner cases in the specifications, - foresee the interaction with other language or compiler features, - comment constructively and improve proposals through engagement with others, - judge the cost/benefit ratio of changes, and - come to a justifiable conclusion. Ideally, committee members should: - have substantial experience of writing or teaching Haskell; - have a track record of active contributions to the Haskell community; or - have expertise in language design and implementation, in either Haskell or related languages. It is our aim that this committee be diverse; by representing different viewpoints, we will make decisions that benefit larger segments of our community. Even if you are uncertain whether your background qualifies you for the role, you are warmly encouraged to apply. The committee's work requires a small, but non-trivial amount of time, especially when you are assigned a proposal for shepherding. We estimate the workload to be around 2 hours per week, and our process works best if members usually respond to technical emails within 1-2 weeks (within days is even better). Please keep that in mind if your email inbox is already overflowing. Committee members serve for 3 years, but are free to re-nominate themselves when their 3-year term expires. Self-nominations are the norm. You can nominate someone else, but please obtain their explicit consent to do so. (We don't want to choose someone who turns out to be unable to serve.) *What happens after nominations?* In accordance with our Committee bylaws <https://github.com/ghc-proposals/ghc-proposals/blob/master/committee.rst>, the committee deliberates nominations *in private* before communicating the decision. The committee doesn't have a fixed size, although we like to have a minimum of 9 members. There are currently two members rotating off, but as the committee currently has 10 members, we will be looking to appoint at least one but possibly (hopefully!) more. On behalf of the committee, Simon Marlow

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GHC 9.14.1-rc2 is now available
by Zubin Duggal 07 Nov '25

07 Nov '25

The GHC developers are very pleased to announce the availability of the second release candidate of GHC 9.14.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. The changes from the first release candidate are: * Bump the exceptions submodule to 0.10.11 * Bump the os-string submodule to 2.0.8 * Fix a driver regression causing an infinite loop with cyclic imports GHC 9.14 will bring a number of new features and improvements, including: * Significant improvements in specialisation: * The `SPECIALISE` pragma now allows use of type application syntax * The `SPECIALISE` pragma can be used to specialise for expression arguments as well as type arguments. * Specialisation is now considerably more reliable in the presence of `newtype`s * Significant GHCi improvements including: * Correctness and performance improvements in the bytecode interpreter * Features in the GHCi debugger * Support for multiple home units in GHCi * Implementation of the [Explicit Level Imports proposal][levels] * `RequiredTypeArguments` can now be used in more contexts * SSE/AVX2 support in the x86 native code generator backend * A major update of the Windows toolchain and improved compatibility with macOS Tahoe * ... and many more A full accounting of changes can be found in the [release notes][]. Given the many specialisation improvements and their potential for regression, we would very much appreciate testing and performance characterisation on downstream workloads. Note that while this release makes many improvements in the specialisation optimisation, polymorphic specialisation will remain disabled by default in the final release due to concern over regressions of the sort identified in [#26329][T26329]. Users needing more aggressive specialisation can explicitly enable this feature with the `-fpolymorphic-specialisation` flag. Depending upon our experience with 9.14.1, we may enable this feature by default in a later minor release. We would like to thank the Zw3rk stake pool, Well-Typed, Mercury, Channable, Tweag I/O, Serokell, SimSpace, the Haskell Foundation, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work have made the Haskell ecosystem what it is today. As always, do give this release a try and open a [ticket][] if you see anything amiss. [downloads.haskell.org] https://downloads.haskell.org/ghc/9.14.1-rc2 [release notes]: https://downloads.haskell.org/ghc/9.14.1-rc2/docs/users_guide/9.14.1-notes.… [ticket]: https://gitlab.haskell.org/ghc/homepage/-/issues/new [macos]: https://gitlab.haskell.org/ghc/ghc/-/issues/26166 [T26329]: https://gitlab.haskell.org/ghc/ghc/-/issues/26329 [levels]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0682-e…

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