- Haskell-Cafe - Haskell.org

How to deal with type family applications in type equalities?
by Jakob Brünker 18 Feb '21

18 Feb '21

Imagine you want to have a type level function that calculates the element-wise OR on two bitstrings, encoded as length-indexed vectors filled with Bools (this is a simplification of something I need for a project). The vectors should be "aligned to the right" as it were, such that the new right-most value is True if the right-most value of the first vector OR the right-most value of the second vector was True. Example (replacing True and False with 1 and 0 for readability): ElementwiseOr [1,0,1,1] [0,1,1,0,0,0,0,1] = [0,1,1,0,1,0,1,1] Its type would be something like ElementwiseOr :: Vec n Bool -> Vec m Bool -> Vec (Max n m) Bool I have written the following code: --------------------------------------------------------------------------- {-# LANGUAGE TypeFamilies, GADTs, DataKinds, StandaloneKindSignatures, PolyKinds, TypeOperators, RankNTypes, TypeApplications, UndecidableInstances #-} import Data.Kind import Data.Type.Equality data Nat = Z | S Nat infixr 5 :< type Vec :: Nat -> Type -> Type data Vec n a where Nil :: Vec Z a (:<) :: a -> Vec n a -> Vec (S n) a type Cong :: forall f -> a :~: b -> f a :~: f b type family Cong f p where Cong f Refl = Refl type Max :: Nat -> Nat -> Nat type family Max n m where Max Z m = m Max n Z = n Max (S n) (S m) = S (Max n m) -- The reason for this slightly convoluted Ordering type is that it seems -- to make the "right-alignment" easier. type NatOrdering :: Ordering -> Nat -> Nat -> Type data NatOrdering o n m where NOLTE :: NatOrdering EQ n m -> NatOrdering LT n (S m) NOLTS :: NatOrdering LT n m -> NatOrdering LT n (S m) NOEQZ :: NatOrdering EQ Z Z NOEQS :: NatOrdering EQ n m -> NatOrdering EQ (S n) (S m) NOGTE :: NatOrdering EQ n m -> NatOrdering GT (S n) m NOGTS :: NatOrdering GT n m -> NatOrdering GT (S n) m type MaxSIsSMaxEQ :: forall n m . NatOrdering EQ n m -> Max n (S m) :~: S (Max n m) type family MaxSIsSMaxEQ o where MaxSIsSMaxEQ NOEQZ = Refl MaxSIsSMaxEQ (NOEQS o) = Cong S (MaxSIsSMaxEQ o) type TransportVec :: n :~: m -> Vec n a -> Vec m a type family TransportVec p v where TransportVec Refl v = v type ElementwiseOr :: NatOrdering o n m -> Vec n Bool -> Vec m Bool -> Vec (Max n m) Bool type family ElementwiseOr o u v where ElementwiseOr (NOLTE o) u (s :< v) = TransportVec (MaxSIsSMaxEQ o) (s :< ElementwiseOr o u v) -- XXX -- other equations ommitted --------------------------------------------------------------------------- To me, the equation at the end marked with XXX seems like it should work. However, it produces the following error: • Couldn't match kind: Max n ('S m) with: 'S (Max n m) Expected kind ‘'S (Max n m) :~: 'S (Max n m)’, but ‘MaxSIsSMaxEQ o’ has kind ‘Max n ('S m) :~: 'S (Max n m)’ • In the first argument of ‘TransportVec’, namely ‘(MaxSIsSMaxEQ o)’ So it expects something of kind `S (Max n m) :~: S (Max n m)` - it seems like Refl fits that bill, but that doesn't work either, because replacing (MaxSIsSMaxEQ o) with Refl means that TransportVec returns a type of the wrong kind, producing a very similar error. I suspect the reason this doesn't work is because type equality isn't equipped to properly handle type family applications. I could potentially work around this if I defined Max as data Max n m r where MaxZZ :: Max Z Z Z MaxSZ :: Max (S n) Z (S n) MaxZS :: Max Z (S m) (S m) MaxSS :: Max n m r -> Max (S n) (S m) (S r) However, dealing with a proof object like this is a lot less convenient than just being able to use a type family for most purposes, so I'd like to avoid this if possible. Is there a way to make this work while sticking with type families? Thanks Jakob

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Blog post about masking asynchronous exceptions
by coot＠coot.me 18 Feb '21

18 Feb '21

Hello Haskell-Caffee, I wrote a blog post about masking asynchronous exceptions and interruptible operations: https://coot.me/posts/mask.html Best regards, Marcin Szamotulski

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First Call for Papers for the Haskell Symposium 2021
by Hage, J. (Jurriaan) 17 Feb '21

17 Feb '21

Dear all, This is the first call for papers for the upcoming Haskell Symposium. Please forward to anyone that you believe might be interested. Apologies for receiving multiple copies of this announcement. Best regards, Jurriaan Hage Chair PS. Note that the website is not up yet (but will be soon). ================================================================================ ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2021 ** virtual ** Thu 26 -- Fri 27 August, 2021 http://www.haskell.org/haskell-symposium/2021/ ================================================================================ The ACM SIGPLAN Haskell Symposium 2021 will be co-located with the 2021 International Conference on Functional Programming (ICFP). Due to COVID-19 it will take place **virtually** this year. Like last year, we will be using a lightweight double-blind reviewing process. See further information below. Different from last year is that we offer a new submission category: the tutorial. Details can be found below. The Haskell Symposium presents original research on Haskell, discusses practical experience and future development of the language, and promotes other forms of declarative programming. Topics of interest include: * Language design, with a focus on possible extensions and modifications of Haskell as well as critical discussions of the status quo; * Theory, such as formal semantics of the present language or future extensions, type systems, effects, metatheory, and foundations for program analysis and transformation; * Implementations, including program analysis and transformation, static and dynamic compilation for sequential, parallel, and distributed architectures, memory management, as well as foreign function and component interfaces; * Libraries, that demonstrate new ideas or techniques for functional programming in Haskell; * Tools, such as profilers, tracers, debuggers, preprocessors, and testing tools; * Applications, to scientific and symbolic computing, databases, multimedia, telecommunication, the web, and so forth; * Functional Pearls, being elegant and instructive programming examples; * Experience Reports, to document general practice and experience in education, industry, or other contexts; * Tutorials, to document how to use a particular language feature, programming technique, tool or library within the Haskell ecosystem; * System Demonstrations, based on running software rather than novel research results. Regular papers should explain their research contributions in both general and technical terms, identifying what has been accomplished, explaining why it is significant, and relating it to previous work, and to other languages where appropriate. Experience reports and functional pearls need not necessarily report original academic research results. For example, they may instead report reusable programming idioms, elegant ways to approach a problem, or practical experience that will be useful to other users, implementers, or researchers. The key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. It is not enough simply to describe a standard solution to a standard programming problem, or report on experience where you used Haskell in the standard way and achieved the result you were expecting. A new submission category for this year's Haskell Symposium is the tutorial. Like with the experience report and the functional pearl, the key criterion for such a paper is that it makes a contribution from which other Haskellers can benefit. What distinguishes a tutorial is that its focus is on explaining an aspect of the Haskell language and/or ecosystem in a way that is generally useful to a Haskell audience. Tutorials for many such topics can be found online; the distinction here is that by writing it up for formal review it will be vetted by experts and formally published. System demonstrations should summarize the system capabilities that would be demonstrated. The proposals will be judged on whether the ensuing session is likely to be important and interesting to the Haskell community at large, whether on grounds academic or industrial, theoretical or practical, technical, social or artistic. Please contact the program chair with any questions about the relevance of a proposal. If your contribution is not a research paper, please mark the title of your experience report, functional pearl, tutorial or system demonstration as such, by supplying a subtitle (Experience Report, Functional Pearl, Tutorial Paper, System Demonstration). Submission Details ================== Early and Regular Track ----------------------- The Haskell Symposium uses a two-track submission process so that some papers can gain early feedback. Strong papers submitted to the early track are accepted outright, and the others will be given their reviews and invited to resubmit to the regular track. Papers accepted via the early and regular tracks are considered of equal value and will not be distinguished in the proceedings. Although all papers may be submitted to the early track, authors of functional pearls and experience reports are particularly encouraged to use this mechanism. The success of these papers depends heavily on the way they are presented, and submitting early will give the program committee a chance to provide feedback and help draw out the key ideas. Formatting ---------- Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines. Authors should use the `acmart` format, with the `sigplan` sub-format for ACM proceedings. For details, see: http://www.sigplan.org/Resources/Author/#acmart-format It is recommended to use the `review` option when submitting a paper; this option enables line numbers for easy reference in reviews. Functional pearls, experience reports, tutorials and demo proposals should be labelled clearly as such. Lightweight Double-blind Reviewing ---------------------------------- Haskell Symposium 2021 will use a lightweight double-blind reviewing process. To facilitate this, submitted papers must adhere to two rules: 1. Author names and institutions must be omitted, and 2. References to authors' own related work should be in the third person (e.g., not "We build on our previous work" but rather "We build on the work of "). The purpose of this process is to help the reviewers come to an initial judgment about the paper without bias, not to make it impossible for them to discover the authors if they were to try. Nothing should be done in the name of anonymity that weakens the submission or makes the job of reviewing the paper more difficult (e.g., important background references should not be omitted or anonymized). In addition, authors should feel free to disseminate their ideas or draft versions of their paper as they normally would. For instance, authors may post drafts of their papers on the web or give talks on their research ideas. A reviewer will learn the identity of the author(s) of a paper after a review is submitted. Page Limits ----------- The length of submissions should not exceed the following limits: Regular paper: 12 pages Functional pearl: 12 pages Tutorial: 12 pages Experience report: 6 pages Demo proposal: 2 pages There is no requirement that all pages are used. For example, a functional pearl may be much shorter than 12 pages. In all cases, the list of references is not counted against these page limits. Deadlines --------- Early track: Submission deadline: 19 March 2021 (Fri) Notification: 23 April 2021 (Fri) Regular track and demos: Submission deadline: 21 May 2021 (Fri) Notification: 23 June 2021 (Wed) Deadlines are valid anywhere on Earth. Submission ---------- Submissions must adhere to SIGPLAN's republication policy (http://sigplan.org/Resources/Policies/Republication/) and authors should be aware of ACM's policies on plagiarism (https://www.acm.org/publications/policies/plagiarism) Program Committee members are allowed to submit papers, but their papers will be held to a higher standard. The paper submission deadline and length limitations are firm. There will be no extensions, and papers violating the length limitations will be summarily rejected. Papers should be submitted through HotCRP at: https://haskell21.hotcrp.com/ Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Supplementary material: Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. This supplementary material should not be submitted as part of the main document; instead, it should be uploaded as a separate PDF document or tarball. Supplementary material should be uploaded at submission time, not by providing a URL in the paper that points to an external repository. Authors can distinguish between anonymized and non-anonymized supplementary material. Anonymized supplementary material will be visible to reviewers immediately; non-anonymized supplementary material will be revealed to reviewers only after they have submitted their review of the paper and learned the identity of the author(s). Resubmitted Papers: authors who submit a revised version of a paper that has previously been rejected by another conference have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the conference chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Proceedings =========== Accepted papers will be included in the ACM Digital Library. Their authors will be required to choose one of the following options: - Author retains copyright of the work and grants ACM a non-exclusive permission-to-publish license (and, optionally, licenses the work with a Creative Commons license); - Author retains copyright of the work and grants ACM an exclusive permission-to-publish license; - Author transfers copyright of the work to ACM. For more information, please see ACM Copyright Policy (http://www.acm.org/publications/policies/copyright-policy) and ACM Author Rights (http://authors.acm.org/main.html) Accepted proposals for system demonstrations will be posted on the symposium website but not formally published in the proceedings. Publication date: The official publication date of accepted papers is the date the proceedings 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. Artifacts ========= Authors of accepted papers are encouraged to make auxiliary material (artifacts like source code, test data, etc.) available with their paper. They can opt to have these artifacts published alongside their paper in the ACM Digital Library (copyright of artifacts remains with the authors). If an accepted paper's artifacts are made permanently available for retrieval in a publicly accessible archival repository like the ACM Digital Library, that paper qualifies for an Artifacts Available badge (https://www.acm.org/publications/policies/artifact-review-badging#available) Applications for such a badge can be made after paper acceptance and will be reviewed by the PC chair. Program Committee ================= Edwin Brady University of St Andrews Koen Claessen Chalmers University of Technology Dominique Devriese Vrije Universiteit Brussel Andy Gill University of Kansas Jurriaan Hage (chair) Universiteit Utrecht Zhenjiang Hu Peking University Ranjit Jhala University of California Patricia Johann Appalachian State University Yukiyoshi Kameyama University of Tsukuba George Karachalias Tweag Ralf Laemmel University of Koblenz-Landau Daan Leijen Microsoft Research Ben Lippmeier Ghost Locomotion Neil Mitchell Facebook Alberto Pardo Universidad de la Republica, Uruguay Matt Roberts Macquarie University Janis Voigtlaender University of Duisburg-Essen Nicolas Wu Imperial College London If you have questions, please contact the chair at: j.hage(a)uu.nl ================================================================================

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Final Call for Participation: BOB 2021 (February 26, online)
by Stefan Wehr 17 Feb '21

17 Feb '21

Subject: Final Call for Participation: BOB 2021 (February 26, online) ============================================================================ BOB 2021 Conference “What happens if we simply use what’s best?” February 26, 2021, online (UTC+0100) http://bobkonf.de/2021/ Program: http://bobkonf.de/2021/program.html Registration: http://bobkonf.de/2021/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 14 talks and 8 tutorials on current topics: http://bobkonf.de/2021/program.html The subject range includes functional programming, logic programming, revision control, formal methods, mindfulness, event sourcing, front-end development, and more. Jeremy Gibbons will give the keynote talk. BOB 2021 will take place online. We are working towards fostering lively exchange of exciting ideas and enable meaningful social interactions. Registration is open online: http://bobkonf.de/2021/registration.html Registration is €30 for a regular ticket, €15 for a student ticket. (If you need financial aid, let us know.) We intend to make this the most diverse, colorful, fun BOB ever!

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International Conference on Informatics Revolution for Smarter Healthcare (IRSH 2021) Prague
by matteo＠confscience.com 16 Feb '21

16 Feb '21

Call for papers ************************************************* International Conference on Informatics Revolution for Smarter Healthcare (IRSH 2021) Prague- Czech Republic, October 14-15, 2021 https://confscience.com/irsh/ Submission deadline: April 1, 2021 All papers accepted in IRSH 2021 will be published in Springer CCIS (Communications in Computer and Information Science). CCIS is abstracted/indexed in Scopus, SCImago, EI-Compendex, Mathematical Reviews, DBLP, Google Scholar, and Thomson Reuters Conference Proceedings Citation (Former ISI Proceedings) *************************************************************************** IMPORTANT DATES: - Paper Submission: April 1, 2021 - Acceptance Notification: July 1, 2021 - Final Manuscript Due: September 1, 2021 *************************************************************************** The IRSH 2021 conference will be held in Conjunction with: International Conference on Applied Data Science and Intelligence (ADSI 2021) International Conference on Recent Theories and Applications in Transportation and Mobility - (RTATM 2021) *************************************************************************** TOPICS: Authors are invited to submit their original papers to address the topics of the conference, including but not limited to: FUNDAMENTALS AND THEORIES - Interoperability and Data Integration - Confidentiality and Data Security - Data protection - Data Sharing - Security, Privacy, and Trust - Emergent healthcare standards - Emergent healthcare architectures - ICT, Ageing and Disability - Physiological and behavioural modelling - Pandemic and disease modeling - Usability and user experience of medical devices - Human behaviour - Clinical investigation regulatory frameworks - Integrated healthcare approaches - eHealth data standards and interoperability (e.g. HL7/FHIR) - Databases and data warehousing - Big Data and Open Data for healthcare - Design and Development of Methodologies for Healthcare - Emergent Communication Technologies - Real-time interaction theories - Emergent Technologies for Ambient Assisted Living - User Interface Design for healthcare - Sustainability - New approaches for accuracy and effectiveness - Data mining and bioinformatics - Enhanced living environments - Analysis and evaluation of healthcare systems INTELLIGENT HEALTHCARE - Pattern recognition and Machine - Learning for healthcare - Cognitive Informatics - Big Data in Healthcare - Wellbeing Informatics - Data Mining and Data Analytics - Data Visualization - Smart environments - Smart Ambient Assisted Living - Intelligent healthcare solutions - Agent-based solutions for healthcare - Collaboration systems - Intelligent Electronic Health Records - Internet of Things for healthcare - Cyber-Physical Systems for healthcare - Ambient Computing and Reasoning - Context Awareness - Smart devices for eldercare - Autonomy and active ageing - Emergent technologies for intelligent Computer Vision - Service production and delivery - Gamification - Multi-modal interaction - Computer-aided detection and diagnosis - Crowdsourcing for smarted healthcare SERVICES, SYSTEMS, AND INFRASTRUCTURES - Emergent healthcare services - Pervasive health systems and services - Remote healthcare management - Emergent healthcare infrastructure - Industry Revolution 4.0 for healthcare - eHealth - Electronic health records - Assistive technologies - Disease surveillance and patient monitoring systems - Prevention and detection systems - Home monitoring - Healthcare management systems - ICT-based therapeutic systems - ICT-based rehabilitation technologies - Wearable health informatics - Emergent technologies for data analytics - Ambient Assisted Leaving (AAL) - Decision Support Systems - Emergent Technologies for Remote AAL Monitoring - Emergent Technologies and Accessibility - 5G for healthcare - Healthcare supply chain and logistics - Wireless Body Networks - Telemedicine and mobile telemedicine - Mobile Systems - Software Defined infrastructures - Patient empowerment systems - Smart technology for remote patient visits - Biosensors - Medical devices APPLICATIONS - eHealth applications - Application of health informatics in clinical cases - Mobile technologies for healthcare applications - Software Systems in healthcare - Social networking and healthcare - Case Studies - Personalization and patient experience - AR and VR applications - Patient billing - Accounting systems - Personnel and payroll - Materials management - Voice recognition systems - Asset management solutions - Disease management - Feedback integration - Clinical software - Crowd-computing applications - Future directions - Drone-based solutions - Software Defined Networks for healthcare *************************************************************************** OUTSTANDING PAPERS: Based on the peer review scores as well as the presentations at the conference, the authors of outstanding papers will be invited to extend their works for a potential publication in journals special issues with high impact factors. *************************************************************************** PAPER SUBMISSION: Papers must be submitted electronically as PDF files via easychair (https://easychair.org/conferences/?conf=irsh2021) All papers will be peer reviewed. Length of Full papers: 12-15 pages long (written in the LNCS/CCIS one-column page format, 400 words per page) Length of Short papers: less than 12 pages For more information, please refer to the conference website: https://confscience.com/irsh/ *************************************************************************** CONTACT For more information, please send an email to info-irsh(a)confscience.com

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Kind pairs and kind currying/uncurrying
by CASANOVA Juan 14 Feb '21

14 Feb '21

Hello again, At this point, I think what I am trying to do is simply not possible in Haskell, not even with as many extensions as I would want. Perhaps with template Haskell, but that's ground that I am still afraid to tread. But I thought I would ask anyway. (All the code in the rest of the email assumes as many extensions as GHC asks me to compile). The background is more complicated and controversial than the question itself, but I'll give a little bit of it so that you know why I'm trying to do this. I am trying to define a type class that takes other type classes as arguments. That is fairly straightforward by itself. A single parameter type-class has kind (k -> Constraint). So I can write my class SecondOrderClass (cl :: k -> Constraint) The problem, however, is that I want to use this class for multi-parameter type classes as well. A two parameter type class has kind (k1 -> k2 -> Constraint) The most general unifier of these two kinds is, unfortunately, a free kind (k), which thus includes kinds that are not type classes (like types). Therefore, it is not possible to directly specify a class that takes as parameter a type class with any number of arguments. So, thus I thought: I've seen this before. This is currying and uncurrying. Except I'm doing it with kinds except of with types. A brief search didn't yield me any library that would do this. So I thought: This should be fairly easy to do by myself if I throw enough extensions at it. Well, I failed. I tried two variations and neither worked. To be clear, what I tried to do is to * Define a type synonym that would produce a kind representing a pair of two given kinds. * Define type synonyms for currying and uncurrying kinds using the pair kind. The first variation I tried I feel is the most semantically accurate, as it is clear from the beginning of what I am trying to do. The main "insight" I used was using/assuming extensionality of kinds (and partial application of type synonyms) to define a kind pair: type KindPair (a :: ka) (b :: kb) (f :: ka -> kb -> *) = f a b A pair of kinds is simply anything that "owes the two kinds to the type checker". Thus, if given anything expecting those two kinds, it would feed them to it. This by itself compiles fine. But then when trying to define currying and uncurrying, it won't type check: type KCurry (tf :: (KindPair ka kb) -> kc) (ta :: ka) (tb :: kb) = kc type KUncurry (tf :: ka -> kb -> kc) (tp :: (KindPair ka kb)) = kc The error I get is: The type synonym ‘KindPair’ should have 3 arguments, but has been given 2 Partial application of type synonyms is fine, I use it all the time. But it seems it is not fine when it is "kind synonyms"??? It is not even asking me for any extension to allow it. So I tried a more syntactic approach, abusing the notion that GHC treats kinds and types very similarly: type KindPair (a :: ka) (b :: kb) = ((ka -> kb -> *) -> *) type KCurry (tf :: (KindPair ka kb) -> kc) = ka -> kb -> kc type KUncurry (tf :: ka -> kb -> kc) = (KindPair ka kb) -> kc This type checks, but it is just a mirage. If you check the kinds of KCurry and KUncurry it does not look good: (KindPair ka kb -> kc) -> * Uhh... that's not what I said. It's taking the right hand side entirely as a type, not as a kind. This becomes obvious when we try to use them: ftest :: KCurry (KUncurry (,)) Int Int -> (Int, Int) This does not type check, regardless of the definition (ideally ftest = id would work, but we do not even get that far). The error is: Expected kind ‘* -> * -> *’, but ‘KCurry (KUncurry (,))’ has kind ‘*’ So, my question is: Is what I am trying to do possible in Haskell? Is what I am trying to do completely absurd? I do not think it is, but maybe I am being naive? Thanks in advance, and sorry for the long email (again). I tried to be as straight to the point as possible. Juan. The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Request for Nominations to the GHC Steering Committee
by Alejandro Serrano Mena 14 Feb '21

14 Feb '21

Dear Haskell community, The three-year term limit of some of the GHC Steering committee members has expired. We are really grateful for the awesome work that Iavor, Richard, and Joachim have done through these years (especially the latter, who has acted as secretary.) As a result, the GHC Steering committee is seeking nominations for new members. The committee scrutinizes, nitpicks, improves, weights and eventually accepts or rejects proposals that extend or change the language supported by GHC and other (public-facing) aspects of GHC. Our processes are described at https://github.com/ghc-proposals/ghc-proposals which is also the GitHub repository where proposals are proposed. We are looking for members who have the ability * to understand such language extension proposals, * to find holes and missing corner cases in the specifications, * foresee the interaction with other language features and specifications, * comment constructively and improve the proposals, * judge the cost/benefit ratio and * finally come to a justifiable conclusion. We look for committee members who have some of these properties: * have substantial experience in writing Haskell applications or libraries, which they can use to inform judgements about the utility or otherwise of proposed features, * have made active contributions to the Haskell community, for some time, * have expertise in language design and implementation, in either Haskell or related languages, which they can share with us. 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. The GHC developers themselves are already well represented already. We seek Haskell _users_ more than GHC hackers. There is no shortage of people who are eager to get fancy new features into the language, both in the committee and the wider community. But each new feature imposes a cost, to implement, to learn, (particularly) through its unexpected interaction with other features. We need to strike a balance, one that encourages innovation (as GHC always has) while still making Haskell attractive for real-world production use and for teaching. We therefore explicitly invite “conservative” members of the community to join the committee. To make a nomination, please send an email to me (as the interim committee secretary) at trupill at gmail.com until February 28th. I will distribute the nominations among the committee, and we will keep the nominations and our deliberations private. We explicitly encourage self-nominations. You can nominate others, but please obtain their explicit consent to do so. (We don’t want to choose someone who turns out to be unable to serve.) On behalf of the committee, Alejandro Serrano

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Surprising lack of generalisation
by Tom Ellis 11 Feb '21

11 Feb '21

Dear all, I don't understand why the type of pBD defined in the where clause of pFG cannot be inferred to a general type in the presence of TypeFamilies. In particular I don't understand why nonetheless the type of pBD definied in the where clause of pF (only slightly simpler) *can* be inferred. Can anyone explain? Thanks Tom {-# LANGUAGE TypeFamilies #-} -- This code came up in the context of writing a parser, but that's -- not terribly important import Prelude hiding ((<$>)) data B = B data D f = F f | GF AP f | DF AM f data AM = AM data AP = AP pB :: Parser B pB = Parser -- Works fine pF :: Parser (D B) pF = pBD GF AP <|> pBD DF AM where pBD f p = f p <$> pB -- Shows the (presumably) inferred type for pBD pFWithType :: Parser (D B) pFWithType = pBD GF AP <|> pBD DF AM where pBD :: (t -> B -> b) -> t -> Parser b pBD f p = f p <$> pB -- One would hope this type would be inferred too pFGWithType :: Parser B -> Parser (D B) pFGWithType pBArg = pBD GF AP <|> pBD DF AM where pBD :: (t -> B -> b) -> t -> Parser b pBD f p = f p <$> pBArg -- But omitting it leads to a type error if TypeFamilies is enabled. -- There is no error if TypeFamilies is not enabled. pFG :: Parser B -> Parser (D B) pFG pBArg = pBD GF AP <|> pBD DF AM where pBD f p = f p <$> pBArg -- The specifics of the parser don't matter data Parser a = Parser (<|>) :: Parser a -> Parser a -> Parser a (<|>) _ _ = Parser (<$>) :: (a -> b) -> Parser a -> Parser b (<$>) _ _ = Parser

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10 PhD studentships in Nottingham
by Graham Hutton 11 Feb '21

11 Feb '21

Dear all, The School of Computer Science at the University of Nottingham is seeking applications for 10 fully-funded PhD studentships: https://tinyurl.com/ten-phds-2021 Applicants in the area of the Functional Programming Laboratory (https://tinyurl.com/fp-notts) are strongly encouraged! If you are interested in applying, please contact a potential supervisor as soon as possible (the application deadline is 15th March): Thorsten Altenkirch - constructive logic, proof assistants, homotopy type theory, category theory, lambda calculus. Graham Hutton - mathematics of program construction, category theory, program correctness and efficiency, Haskell. Nicolai Kraus - homotopy type theory, higher category theory, constructive mathematics, and related topics. Henrik Nilsson - functional reactive programming, domain- specific languages, generalised notions of computation. The studentships are open to applicants of any nationality. Best wishes, Graham +-----------------------------------------------------------+ 10 Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK tinyurl.com/ten-phds-2021 Applications are invited for a number of fully funded PhD studentships offered by the School of Computer Science at the University of Nottingham, starting on 1st October 2021. The topics for the studentships are open, but should relate to the interests of one of the School’s research groups: Computational Optimisation and Learning Lab; Computer Vision Lab; Cyber Security; Functional Programming; Intelligent Modelling and Analysis; Mixed Reality Lab and Uncertainty in Data and Decision Making. The studentships are for three and a half years and include a stipend of (minimum) £15,609 per year and tuition fees. Applicants are normally expected to have a 2:1 Bachelor or Masters degree or international equivalent, in Computer Science or a related discipline, and must obtain the support of a potential supervisor in the School prior to submitting their application. Please contact potential supervisors at least two weeks prior to the closing date for applications. Informal enquiries may be addressed to Professor Tony Pridmore (tony.pridmore(a)nottingham.ac.uk) To apply, please submit the following items by email to Lindsay.Norman(a)nottingham.ac.uk: (1) A copy of your CV, including your actual or expected degree class(es), and results of all University examinations; (2) An example of your technical writing, such as a project report or dissertation; (3) Contact details for two academic referees - it is the applicant’s responsibility to ensure that references are requested and sent to Lindsay.Norman(a)nottingham.ac.uk; (4) A research proposal – max 2 x sides A4; (5) A covering letter, which must include the name of the member of staff who has agreed to support your application (without this your application cannot be considered), and the University’s job reference number (SCI1979). Closing date for applications: Monday 15th March 2021. +-----------------------------------------------------------+ 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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lexing nested comments in GHC/Parser/Lexer.x
by Benjamin Redelings 11 Feb '21

11 Feb '21

Hi, I notice that in Lexer.x for GHC nested comments are parsed using a special function, with the comment "nested comments require traversing by hand, they can't be parsed using regular expressions." While I see that nested comments can't be parsed using regular expressions, the lexer has a state stack, and so it seems like nested comments could be parsed by pushing a new "nested comment" state onto the lexer state every time a "{-" is encountered. But perhaps this is not done because its not clear how to make the entire comment into a single token that way? This might be a naive question -- I don't have that much experience with lexers. Thanks! -BenRI

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