- Haskell-Cafe - Haskell.org

Attoparsec: match many, skipping rest
by amindfv＠gmail.com 17 Apr '20

17 Apr '20

Say I've got an Attoparsec parser like: p = string "prio:" *> digit And some text, e.g. s = "this is just a prio:5 example, I'd prio:6 very much like to prio:4 parse" What's the most idiomatic way to apply p to s so that I get out a list ['5', '6', '4']? Thanks, Tom

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streaming package: How to demux a stream properly?
by ☂Josh Chia (謝任中) 15 Apr '20

15 Apr '20

I have a streaming package question. Suppose I have a: Stream (Of (Int, a)) m () I would like to demultiplex it into Vector (Stream (Of a) m ()) using the Int as the index of an item into the Vector of output streams. How can I do this efficiently (constant memory and linear time)? Does the following work? import qualified Streaming.Prelude as SP import qualified Data.Vector as V type StreamOf a m r = Stream (Of a) m r demuxStream :: forall a m. MonadIO m => Int -> StreamOf (Int, a) m () -> m (Vector (StreamOf a m ())) demuxStream numSyms stream = let emptyStreams = V.replicate numSyms (pure ()) processItem v (iD, x) = V.modify (\vm -> VM.modify vm (>> SP.yield x) iD) v in SP.fold_ processItem emptyStreams id stream My guess is that it takes more than constant memory as it goes through the entire input stream before returning. Josh

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Haskell Symposium 2020 -- Call for Papers: Regular track & demos
by Tom Schrijvers 14 Apr '20

14 Apr '20

================================================================================ ACM SIGPLAN CALL FOR SUBMISSIONS Haskell Symposium 2020 New Jersey, United States 27--28 August, 2020 http://www.haskell.org/haskell-symposium/2020/ ================================================================================ The ACM SIGPLAN Haskell Symposium 2020 will be co-located with the 2020 International Conference on Functional Programming (ICFP). Like last year, we will be using a lightweight double-blind reviewing process. See further information 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; * 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. 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. 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, and demo proposals should be labelled clearly as such. Lightweight Double-blind Reviewing ---------------------------------- Haskell Symposium 2020 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 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: 20 March 2020 (Fri) Notification: 24 April 2020 (Fri) Regular track and demos: Submission deadline: 15 May 2020 (Fri) Notification: 26 June 2020 (Fri) 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://haskell20.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 are free to upload both 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 principal editor 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. Travel Support ============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC program, see its web page (http://pac.sigplan.org) 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 permssion-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 ================= Arthur Azevedo de Amorim Carnegie Mellon University Manuel Chakravarty Tweag I/O / IOHK Jan Christiansen Flensburg University of Applied Sciences Youyou Cong Tokyo Institute of Technology Pierre-Evariste Dagand CNRS Anton Ekblad Chalmers University of Technology Jurriaan Hage Universiteit Utrecht Graham Hutton University of Nottingham José Pedro Magalhães Standard Chartered Clare Martin Oxford Brookes University Andrey Mokhov Jane Street Shin-Cheng Mu Academia Sinica Nikolaos Papaspyrou National Technical University of Athens Simon Peyton Jones Microsoft Research Cambridge Norman Ramsey Tufts University Exequiel Rivas INRIA Tom Schrijvers (chair) KU Leuven Martin Sulzmann Karlsruhe University of Applied Sciences If you have questions, please contact the chair at: tom.schrijvers(a)kuleuven.be ================================================================================

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Re: [Haskell-cafe] Eff verbose/ambiguous/type safety
by Anthony Clayden 13 Apr '20

13 Apr '20

Hi Daneel, welcome > Recently I discovered for my self an alternative for monad > transformers - extensible effects. > > http://okmij.org/ftp/Haskell/extensible/ > So straight away a word of warning: the 'ok' there is Oleg Kiselyov, who has produced many wonderful demonstrations of the power of Haskell's type system. (HList is anoher inspiration.) Many would say he has warped the type system beyond what it could reasonably be expected to achieve. So we measure the brain power needed to understand type signatures and messages in milli-Olegs. > Overall it is really cool and Eff1.hs started working right away, > but I noticed a problem with mixing several effects in 1 function. > Type checker requires pretty verbose type signature, which looks > unreasonable plus the error reported to a beginner developer > is not intuitive. Extensible effects/all of Oleg's work is not aimed at beginners. I advise beginners to stay well clear. You can expect enormous, inscrutable type errors. I agree with Georgi you should give type signatures everywhere you can. And your code should be used only by type experts. AntC

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Eff verbose/ambiguous/type safety
by Daneel Yaitskov 13 Apr '20

13 Apr '20

Hello Group, Recently I discovered for my self an alternative for monad transformers - extensible effects. http://okmij.org/ftp/Haskell/extensible/ Overall it is really cool and Eff1.hs started working right away, but I noticed a problem with mixing several effects in 1 function. Type checker requires pretty verbose type signature, which looks unreasonable plus the error reported to a beginner developer is not intuitive. So my questions are: - why it happens? - what could I do to get more friendly error message? - is there a workaround enforcing GHC make an extra mile and work without these "extra hints"? - any more maintainable Eff implementations? Here it an example I wish should working. $ stack gchi Eff1.hs *Eff1 > doLift = lift (putStrLn "EFF!") *Eff1 > doWrite = tell "HELLO" *Eff1 > doLiftAndWrite = doLift >> doWrite *Eff1 > runLift (runWriter doLiftAndWrite) <interactive>:82:20: error: * Couldn't match type `Writer [Char]' with `Lift IO' arising from a use of `doLiftAndWrite' * In the first argument of `runWriter', namely `doLiftAndWrite' In the first argument of `runLift', namely `(runWriter doLiftAndWrite)' In the expression: runLift (runWriter doLiftAndWrite) The error message looks unclear, because documentation makes me think that Writer and Lift are members of same Eff monad and they are composable and doLiftAndWrite has been evaluated successfully. And my workaround: *Eff1 > runLift $ do { x :: ((), [String]) <- runWriter doLiftAndWrite; return x; } EFF! ((),["HELLO"]) Plus GHCi swallows a type error: *Eff1 > badCast = do { x :: ((), [Int]) <- runWriter doLiftAndWrite; return x; } *Eff1 > :t badCast badCast :: (MemberU2 Lift (Lift IO) r, OpenUnion52.FindElem (Writer [Char]) r) => Eff r ((), [Int]) Definitely Extensible Effects are more flexible that MTL, but compile type safety looks jeopardized. Type verbosity overhead is kind of acceptable, but having possibility of run-time errors is too big price for flexibility. -- Daneel S. Yaitskov

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Targeting old Windows versions
by Andreas Klebinger 08 Apr '20

08 Apr '20

Hello devs, GHC is planning to use the Large Address Space mode of allocation for future releases on windows. See https://gitlab.haskell.org/ghc/ghc/issues/12576 This is a significant optimization for the GC and well tested as we use it on Linux already. However it will regress memory useage on versions of windows *older* than Windows 8.1/Sever 2012. Please let us know if you are targeting older versions than that either by responding to the mailing list, commenting on the ticket or contacting me directly if you have privacy concerns. Depending on how many people are affected by this change we might consider measures to reduce the impact. Cheers, Andreas

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Uppercase OverloadedLabels
by Alejandro Serrano Mena 07 Apr '20

07 Apr '20

Dear Haskell-Café / GHC devs, Is there any reason why the `OverloadedHaskell` extension does not work with labels starting with an uppercase letter? Kind regards, Alejandro

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New stack.yaml.lock file to master source repo?
by David Banas 06 Apr '20

06 Apr '20

What’s the general consensus on whether or not the new stack.yaml.lock file should be stored in the master source repo.? Thanks, -db

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Looking for a library for DerivingVia idioms
by Jack Kelly 06 Apr '20

06 Apr '20

Hello Cafe, Here is a problem a friend (CC'd - please include him in replies) and I wanted to solve using DerivingVia. We have a solution (below). I'm wondering whether a library for these newtypes exists, and if not, whether it should. This email is Literate Haskell. The problem: given two types `Foo` and `Bar`, and an injection `foo2bar :: Foo -> Bar`, use DerivingVia to derive instances for `Foo` in terms of `Bar`. > {-# LANGUAGE DerivingVia, MultiParamTypeClasses #-} > {-# LANGUAGE ScopedTypeVariables, TypeOperators #-} > > import Data.Function (on) > import Data.Coerce (coerce) > > data Foo = Foo deriving Eq via (Foo `InjectedInto` Bar) > data Bar = Bar deriving Eq > > foo2bar :: Foo -> Bar > foo2bar Foo = Bar Section 4.3 of the DerivingVia paper[1] shows how to use a newtype wrapper to derive instances for one type in terms of another, so long as their generic representation is the same. We can use the same technique. We need a newtype that's representationally equal to `a`, to pass to DerivingVia: > newtype InjectedInto a b = InjectedInto a We also need a class, so that we can find our injection given the type: > class Injective a b where > -- Law: to x = to y => x = y > to :: a -> b > > instance Injective Foo Bar where > to = foo2bar We need an instance that uses the InjectedInto newtype: > instance (Eq b, Injective a b) => Eq (a `InjectedInto` b) where > (==) = (==) `on` (to :: a -> b) . coerce The `deriving Eq via (Foo `InjectedInto` Bar)` above compiled successfully, so everything seems to work well. I'm wondering: 1. Is a library collecting useful DerivingVia wrappers? 2. If not, suppose I wanted to stand up such a library. Is there a canonical library providing classes like `Injection`? I found the `type-iso` package, which provides an `Injective` typeclass[2], but it has few revdeps and a dubious instance Default a => Injective (Maybe b) (Either a b). 3. What about DerivingVia for types not of kind `Type`? It might be possible to repeat this structure one level up for `Eq1` &c. Is it worth supporting `Eq1` etc in a world with QuantifiedConstraints? 4. Is there a better name than `InjectedInto`? It doesn't scan well. A type operator, perhaps? Thanks for reading. I look foward to your responses. -- Jack [1]: https://www.kosmikus.org/DerivingVia/deriving-via-paper.pdf [2]: https://hackage.haskell.org/package/type-iso-1.0.1.0/docs/Data-Types-Inject…

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Two PhD positions in Utrecht
by Wouter Swierstra 06 Apr '20

06 Apr '20

============================================================== Two PhD positions in functional programming ============================================================== The Department of Information and Computing Sciences at Utrecht University is currently advertising two PhD positions in Functional Programming. The candidates will join the Intelligent Systems group, working with Johan Jeuring, Gabriele Keller, and Wouter Swierstra. Besides research, the successful candidate will be expected to help supervise MSc students and assist in the teaching of courses. The positions should be filled by September 2020, although the exact starting date is negotiable. --------------------------------- Research topics --------------------------------- These two positions are tied to two specific topics. * Programming tutors for Functional Languages (Johan Jeuring) The focus of the position is on designing new technologies to support students working in an intelligent tutoring system for functional programming. We expect to use techniques from dependently typed programming, refinement types, program synthesis, automated theorem proving, and more to analyse student programs, and to help students in taking the next step when developing a program. The candidate will investigate the design and use of multiple technologies for this purpose, add them to Ask-Elle, our intelligent tutoring system, perform experiments with the system, and improve the technologies based on the outcome of the experiments. * Compiler verification for a smart contract language (Wouter Swierstra & Gabriele Keller) This project aims to develop a certifying compiler for Plutus Tx, a subset of the purely functional language Haskell that is used to implement smart contracts for the Cardano blockchain. The Plutus smart contract framework is being developed by IOHK for Cardano and the present project is a joint effort of IOHK and Utrecht University. The Plutus Tx compiler is based on the GHC Haskell compiler and adds a translation step from GHC Core to a minimal lambda calculus. Programs in this lambda calculus are executed during transaction validation in a sandboxed execution environment in a manner that is crucial to the security of the blockchain. This project aims to formalise the semantics of the languages involved, to reason about the transformation and optimisation steps that the compiler performs, and finally, to generate a proof object certifying the correctness of the generated code together with that code. --------------------------------- What we are looking for --------------------------------- The ideal candidates should have a degree in Computer Science, be highly motivated, speak and write English well, and be proficient in producing scientific reports. Furthermore, candidates should be able to demonstrate experience with functional programming languages, such as Haskell, OCaml, ML, Agda, Idris, or Coq. --------------------------------- What we offer --------------------------------- The candidates are offered a full-time position for four or five years, depending on the teaching load. The gross salary ranges between €2,325 in the first year and €2,972 in the fourth year per month for full-time employment. A part-time position of at least 0.8 fte may also be possible. The salary is supplemented with a holiday bonus of 8% and an end-of-year bonus of 8.3% per year. The position also includes a generous allocation of fully-paid vacation days. In addition we offer: a pension scheme, partially paid parental leave, and flexible employment conditions. Conditions are based on the Collective Labour Agreement Dutch Universities. The research group will provide the candidate with necessary support on all aspects of the project. More information is available on the website: Terms and employment: http://bit.ly/1elqpM7 Utrecht is consistently ranked as one of the best places in the world to live. --------------------------------- How to apply --------------------------------- To apply please attach a letter of motivation, a curriculum vitae, and (email) addresses of two referees. Make sure to also include a transcript of the courses you have followed (at bachelor and master level), with the grades you obtained, and to include a sample of your scientific writing, such as your MSc or BSc thesis. It is possible to apply for this position if you are close to obtaining your undergraduate degree. In that case include a letter of your supervisor with an estimate of your progress, and do not forget to include at least a sample of your technical writing skills. The application deadline for the first position closes on April 29th. You can apply through the University's website: https://www.uu.nl/en/organisation/working-at-utrecht-university/jobs/5-year… The application for the second position is not yet open -- but feel free to contact me directly if you're interested. If you're still unsure about the topic, feel free to apply using the link above; we're happy to discuss which topic fits your interests best during the application process.

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