February 2021 - Haskell-Cafe

Munich Virtual Haskell Meeting, 2021-02-23 @ 19:30
by Leah Neukirchen 21 Feb '21

21 Feb '21

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, Feburary 23 on Google Meet at 19:30 CET. **Due to meetup limitations in Bavaria, this meeting will take place online!** For details see here: https://muenchen.haskell.bayern/dates.html A Google Meet link to join the room is provided on the page. Everybody is welcome, especially the Haskellers from Bavaria that do not usually come to our Munich meetings due to travel distance! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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dyre-0.9.0 release candidate and call for testing
by Fraser Tweedale 21 Feb '21

21 Feb '21

Dyre is a tool for Haskell program configuration by (re)compilation. After years of project dormancy, I am pleased to announce a Dyre 0.9 release candidate[1]. If you are interested and able to test this candidate release, you can report results at [2]. For more details including changelog, see the accompanying blog post[3]. [1] https://hackage.haskell.org/package/dyre-0.9.0/candidate [2] https://github.com/willdonnelly/dyre/issues/39 [3] https://frasertweedale.github.io/blog-fp/posts/2021-02-21-dyre-0.9-rc.html Thanks, Fraser

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More Efficient Recursion
by A. Mc. 21 Feb '21

21 Feb '21

Hello, I'm just starting off, but I'm wondering if anyone has any suggestions. I've made a recursive function that's goal is to handle a base raised to a very large power. modLoop :: Integer -> Integer -> Integer -> Integer -> Integer -> Integer modLoop iterator exponent result base modulus | (iterator == exponent) = result | (iterator < exponent) = modLoop (iterator + 1) (exponent) (mod (result * base) modulus) base modulus | (iterator > exponent) = (-1) However, it flat breaks or takes a long time when using large numbers. I need to get better at using strict functions, however, I compiled with ghc -o (as it tends to implement the strict version - ?) and I still have the same problem. What's a better way? Thanks in advance and thank you for your time.

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help with existential type signature?
by Ken T Takusagawa 20 Feb '21

20 Feb '21

{- I am experimenting with type-level naturals and with moving values known at runtime to type space. Is there a type signature that I can add to f1 below so that this will compile (maybe also requiring some additional LANGUAGE pragmas)? If I were to change the return value of f1 from "x" to "show x", then the program does compile, suggesting that there's nothing inherently wrong with "x". But I'd prefer the option to compose the output of f1 with other computations separately, e.g., "show . f1" . Thanks, Ken -} {-# LANGUAGE KindSignatures, ScopedTypeVariables, DataKinds, Rank2Types #-} import qualified GHC.TypeLits as TypeLits; import Data.Proxy(Proxy); main = undefined; data Foo (fn :: TypeLits.Nat) = Mkfoo deriving (Show); --f1 :: TypeLits.SomeNat -> ? ; f1 mynat = case mynat of { TypeLits.SomeNat (_ :: Proxy n) -> let { x :: Foo n; x = Mkfoo; } in x }; {- typelits.hs:27:8: error: * Couldn't match expected type `p' with actual type `Foo n' because type variable `n' would escape its scope This (rigid, skolem) type variable is bound by a pattern with constructor: TypeLits.SomeNat :: forall (n :: TypeLits.Nat). TypeLits.KnownNat n => Proxy n -> TypeLits.SomeNat, in a case alternative at smalltypelits2.hs:24:3-33 * In the expression: x In the expression: let x :: Foo n x = Mkfoo in x In a case alternative: TypeLits.SomeNat (_ :: Proxy n) -> let x :: Foo n x = Mkfoo in x * Relevant bindings include x :: Foo n (bound at smalltypelits2.hs:26:5) f1 :: TypeLits.SomeNat -> p (bound at smalltypelits2.hs:23:1) | 27 | } in x | ^ -}

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Grapheme length?
by amindfv＠mailbox.org 20 Feb '21

20 Feb '21

Does there exist a Haskell library or function for getting grapheme lengths of String/Text values? e.g. right now, Prelude.length ("ä" :: String) == 2 Data.Text.length ("ä" :: Text) == 2 But I'd like, either for String or Text, to get 1 instead (there are 2 code points but only 1 grapheme). Thanks, Tom

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Sizes in memory?
by Dušan Kolář 19 Feb '21

19 Feb '21

Dear café, I don't know, whether this is the right place to ask, but I'll try :-) If I would like to know size of the list in the memory (after complete evaluation, e.g. deepseq). The type is [( Int, [([Int],[Int])] )]. So we have K tuples of the type (Int,[([Int],[Int])]) Each tuple has N tuples of the type ([Int],[Int]) and each of these lists M elements long (yes, both a are of the same length). Top level list needs (on 64bit CPU) 8 bytes for pointer to elements, 8 bytes to point to another element, some more "expenses", probably yes, tag, anything else? So we have K*(16+XXX), where XXX is for size of (Int,[([Int],[Int])]). Tuple needs 8 bytes for each pointer, so XXX = 16 + YYY, where YYY is the size of the [([Int],[Int])] list. It needs 8 + 8 bytes on the list, we have N such elements, thus YYY = N * (16 + ZZZ), where ZZZ is size of the ([Int],[Int]). Again tuple, so 16 bytes + 2*(M * (16 + 8)) -- 16 bytes for 2 tuple elements -- 2 for two lists -- every list 16 bytes for list itself -- 8 bytes for Int So ZZZ = 16 + 2*(M * (16 + 8)), thus YYY = N * (16 + 16 + 2*(M * (16 + 8))), thus XXX = 16 + N * (16 + 16 + 2*(M * (16 + 8))), thus size = K * (16 + 16 + N * (16 + 16 + 2*(M * (16 + 8)))) = K* (32 + N* (32 + M*48) ) Am I right? Roughly right? Or totally wrong? E.g. for K = 25, N = 10000, M = 7 we get cca 88MiB ? Why am I asking? In reality the memory consumption is much higher, so I must be missing some extras, e.g. each memory chunk extra 8 bytes for GC, extra for tag, extra to point to VMT? Regards, Dušan

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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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