- Haskell-Cafe - Haskell.org

Enable -Wall by default?
by Fumiaki Kinoshita 22 Dec '20

22 Dec '20

I mean literally. An article [0] reminded me of the fact that I enable -Wall in 99% of time -- and most packages I use have it enabled too. It's well known that -Wall doesn't enable* all *warnings, but a subset of warnings that * are well accepted by the community * rarely produce false positives Well, they look like good reasons to enable the warnings by default. Same goes for -Wcompat, except that it is not as popular as -Wall. Seeing potential problems when compiling code is far less of a pain than leaving breakages unnoticed. Am I missing some obvious reason not to do this? [0] https://www.snoyman.com/blog/2020/12/haskell-bad-parts-3

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Re: [Haskell-cafe] type class Boolean
by Johannes Waldmann 22 Dec '20

22 Dec '20

> most useful kind of instance would be symbolic booleans Yes. Ersatz has this: https://hackage.haskell.org/package/ersatz-0.4.8/docs/Ersatz-Bit.html#t:Boo… I also use this class (re-exported) in https://hackage.haskell.org/package/obdd Then modules typically start with import Prelude hiding ( not, and, or, (&&), (||) ) -- possibly more import Ersatz slightly annoying but you get used to it ... - J.

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(Co-)Maintainer-needed: imagemagick, binary-conduit
by Alexander V Vershilov 21 Dec '20

21 Dec '20

Hello, I'm searching for the (co-)maintainers for packages that I don't use at my daily work. If anyone use those, please volunteer: https://hackage.haskell.org/package/imagemagick https://hackage.haskell.org/package/binary-conduit https://hackage.haskell.org/package/network-transport-zeromq -- Alexander Vershilov

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Where do trivial functions on the usual containers live?
by Olaf Klinke 21 Dec '20

21 Dec '20

> I am looking for a place where I can find or put some simple > functions that I > expect but do not find in the `containers` or elsewhere in the > standard > libraries. Many hackage contributors seem to maintain their own <maintainer>-utils package containing bits and bobs they personally deem useful but can not find elsewhere. I might add my own one soon. Not saying this is a good thing, though. > Some examples I have in mind: > > * Group a collection by an equivalence relation: > > classify ∷ Ord π ⇒ (a → π) → [a] → [[a]] GHC.Exts.groupWith or Data.List.groupBy might do want you want, depending on whether you want to aggregate globally or only consecutive runs of equivalent elements. > > * From a finite map, get a map from its range to its fibers: > > fibers ∷ (Ord k, Ord v) ⇒ Map k v → Map v (NonEmpty k) The usual Semigroup instance of Map k v destroys data upon key collision. For problems like this I have a newtype wrapper newtype SemiMap f v = SemiMap {getSemiMap :: f v} instance (Semigroup v, Ord k) => Semigroup (SemiMap (Map k) v) so that one can implement 'fibers' using foldMapWithKey. Is this worth its own package? > > * Put a type into the diagonal of its square: > > diagonal = λx → (x, x) This may be too trivial to be a library function, and its proper type should rather be x -> Pair x since we know both components have the same type. Pair is a Monad and this function is its return. In package ghc Pair has an Applicativeinstance at least. Regards, Olaf

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Intermediate Modern Haskell
by Jacques Carette 20 Dec '20

20 Dec '20

I will be teaching a second Haskell course next semester. Let's assume that Introducing functional programming Getting started with Haskell and GHCi Basic types and definitions Designing and writing programs Data types, tuples and lists Programming with lists Defining functions over lists Playing the game: I/O in Haskell Reasoning about programs Generalization: patterns of computation Higher-order functions Developing higher-order programs Overloading, type classes and type checking Algebraic types (i.e. the first chapters of Thompson's Haskell: the Craft of Functional Programming book is "beginner, classic Haskell". The next few chapters, namely Abstract data types Lazy programming Programming with monads Domain-Specific Languages Time and space behaviour would be (re)done at the start of such a second course. The question for cafe is: what else? I will likely cover: - Typeclassopedia - finally tagless - Template Haskell - Optics - GADTs - recursion schemes I should probably cover parser combinators, pretty-printing, cabal&stack. I know that http://dev.stephendiehl.com/hask/ <http://dev.stephendiehl.com/hask/> gives me one heck of a smorgasbord of options, which is kind of a problem. Things I know I will not cover: - dependent types (if I was going to do that, I'd switch to Idris/Agda) - concurrency (don't ask) Jacques

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Where do trivial functions on the usual containers live?
by Ignat Insarov 20 Dec '20

20 Dec '20

Hello. I am looking for a place where I can find or put some simple functions that I expect but do not find in the `containers` or elsewhere in the standard libraries. Some examples I have in mind: * Group a collection by an equivalence relation: classify ∷ Ord π ⇒ (a → π) → [a] → [[a]] * From a finite map, get a map from its range to its fibers: fibers ∷ (Ord k, Ord v) ⇒ Map k v → Map v (NonEmpty k) * Put a type into the diagonal of its square: diagonal = λx → (x, x) These are all very common and trivial mathematical constructions and I really want such things to be common place in Haskell, so that they may have a canonical, polished definition. It is unfeasible to put them into the standard libraries. _(Attempts were made.)_ So I wonder if there is a package around that would accept them!

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[CFP]Automatic Debugging, Error and Impact Reporting Approaches workshop
by mgajda＠mimuw.edu.pl 20 Dec '20

20 Dec '20

International workshop on Automatic Debugging, Error and Impact Reporting Approaches ==================================================================================== Automatic debugging, error fixing, and reporting is getting more and more popularity across different research communities. We aim to provide a forum for exchange of latest work and document best practices across different disciplines. - Automatic reproducers like those generated by LLDB - Automatic unit test extraction and using decorators and code inspection - Automatic reproducer/error case minimization - Interactive error recovery systems and their applications - Error algebras and error-tracing algebras - Formalizations of debugging - Fault impact reporting and accuracy estimation - Provenance tracking in term rewriting - Error analytics pipelines and databases - Error monitoring in intrusion detection systems - Delta debugging - Automatic generation of error dashboards - Global fault analysis of systems with both computational and human component, and cyber-physical systems. Note that workshop particularly encourage treatment of all kinds of errors, be they compile-time, run-time, discrepancies between intent and specification, hardware failures, or human errors to the degree to which they are amenable to automatic analysis and reporting. Intended audience ----------------- - Experts in automated debugging, error fixing wishing to exchange knowledge with their peers. - Industry practitioners interested to learn more about approaches to automated analysis of errors. - Academics in quest for formalization of automated error analysis and fixing - Financial industry practitioners interested in computer science approaches to satisfy risk reporting guidelines with respect to error impact tracing and reporting of accuracy - Management scientist looking for automatic global fault reporting and impact reporting methodologies. Because of this, we encourage you to reference used theories and methodologies, and provide a introductions with use cases. The workshop is held as a part of [Programming Conference 2021](https://2021.programming-conference.org/). Goals ----- - Our main goal is to encourage social exchange of ideas related to automatic error analysis and impact reporting. - We call for position papers, research expositions, early stage work, and industry experience papers to encourage exchange of on-going research and implementation efforts. Important dates =============== Abstract registration deadline : January 15th, 2021 Submission deadline : January 20th, 2021 Author response period : February 14th-21st, 2021 Acceptance notification : March 1st, 2021 Programme committee ------------------- - Kenichi Asai, [Ochanomizu University](http://pllab.is.ocha.ac.jp/~asai/) - Michał J. Gajda, [Migamake Pte Ltd](https://migamake.com) *(chair)* - Michał "phoe" Herda [\#lisp @ Freenode IRC](https://netsplit.de/channels/details.php?room=%23lisp&net=freenode) - Henrik Nilsson [University of Nottingham](http://www.cs.nott.ac.uk/~psznhn/) - Vladimir Panteleev [DustMite](https://github.com/CyberShadow/DustMite) - Francesco Spegni [Università Politecnica delle Marche](https://www.dii.univpm.it/francesco.spegni) - Dmitry Vostokov [DumpAnalysis.org](https://DumpAnalysis.org) - Vadim Zaytsev [Universiteit Twente](http://grammarware.net) Submissions =========== We welcome original, unpublished papers of the following forms: full articles, extended abstracts, case studies. Please submit your paper or extended abstract [by EasyChair](https://easychair.org/conferences/?conf=adeira2021). You may [contact the committee chair](https://www.linkedin.com/in/mjgajda/) with any questions you have about submission. The paper evaluation will use the following criteria: \* Novelty \* Clarity of exposition \* Transferrability of innovation(s) to other domains \* Scientific rigour and form of the article \* Cross-pollination potential with other papers \* Preliminary papers will be offered feedback on how to improve the quality in all these factors before and during the workshop All submissions should use [ACM DL template](https://www.acm.org/publications/proceedings-template).

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[CFP]Automatic Debugging, Error and Impact Reporting Approaches workshop
by mgajda＠mimuw.edu.pl 20 Dec '20

20 Dec '20

Dear Friends, The emerging interest in the rigorous and agile data science and functional ETL pipelines has driven the developments of many new libraries, frameworks and methodologies. We invite industry practitioners and academics to present their latest work on agile functional data pipelines as a workshop at the EDI'2021 conference. Call for papers =============== The following would be accepted: - Exposures of methodologies for building agile functional data pipelines - Presentations of frameworks used for data pipelines in industry and academia - Experience reports from building of data pipelines in industry and academia - Benchmarks and optimization of agile data pipeline frameworks - Solutions to data integration and exchange problems within ETL pipelines. In particular, we encourage: - Agile functional data methodologists - Authors of ETL frameworks, streaming libraries and databases, reactive streaming, streaming benchmarks, high-performance workflow engines - Position papers on the use of agile and functional data methodologies - Descriptions of environments for interactive and live programming with data - Benchmarks of state-of-the-art solutions for agile functional data pipelines. Details at https://afdp.github.io Important dates =============== Abstract registration deadline : December 27th, 2020 (extended) Submission deadline : January 3rd, 2020 (extended) Acceptance notification : January 12th 2021 Final manuscripts : January 23rd, 2021 Presentation at the conference : March 23rd-26th 2021 Submission instructions ======================= Original, unpublished papers are solicited for presentation at the AFDP workshop. Prospective authors are invited to submit papers (electronically, PDF only) that are no longer than 6 pages for full papers, including all figures and references. For work-in-progress, we also solicit extended abstracts for presentations. The submitted paper must be formatted according to the guidelines of Procedia Computer Science. All accepted papers will be published in Procedia Computer Science. Location ======== This workshop will be held as a part of [The 4th International Conference on Emerging Data and Industry 4.0 (EDI40)](http://cs-conferences.acadiau.ca/EDI40-21/) held on March 23 - 26, 2021 in Warsaw, Poland. -- Best regards Michał

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[CFP]Agile and Functional Data Pipelines workshop
by mgajda＠mimuw.edu.pl 20 Dec '20

20 Dec '20

Dear Friends, The emerging interest in the rigorous and agile data science and functional ETL pipelines has driven the developments of many new libraries, frameworks and methodologies. We invite industry practitioners and academics to present their latest work on agile functional data pipelines as a workshop at the EDI'2021 conference. Call for papers =============== The following would be accepted: - Exposures of methodologies for building agile functional data pipelines - Presentations of frameworks used for data pipelines in industry and academia - Experience reports from building of data pipelines in industry and academia - Benchmarks and optimization of agile data pipeline frameworks - Solutions to data integration and exchange problems within ETL pipelines. In particular, we encourage: - Agile functional data methodologists - Authors of ETL frameworks, streaming libraries and databases, reactive streaming, streaming benchmarks, high-performance workflow engines - Position papers on the use of agile and functional data methodologies - Descriptions of environments for interactive and live programming with data - Benchmarks of state-of-the-art solutions for agile functional data pipelines. Details at https://afdp.github.io Important dates =============== Abstract registration deadline : December 27th, 2020 (extended) Submission deadline : January 3rd, 2020 (extended) Acceptance notification : January 12th 2021 Final manuscripts : January 23rd, 2021 Presentation at the conference : March 23rd-26th 2021 Submission instructions ======================= Original, unpublished papers are solicited for presentation at the AFDP workshop. Prospective authors are invited to submit papers (electronically, PDF only) that are no longer than 6 pages for full papers, including all figures and references. For work-in-progress, we also solicit extended abstracts for presentations. The submitted paper must be formatted according to the guidelines of Procedia Computer Science. All accepted papers will be published in Procedia Computer Science. Location ======== This workshop will be held as a part of [The 4th International Conference on Emerging Data and Industry 4.0 (EDI40)](http://cs-conferences.acadiau.ca/EDI40-21/) held on March 23 - 26, 2021 in Warsaw, Poland. -- Best regards Michał

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[TFPIE'21] Second Call For Papers: Trends in Functional Programming *in Education* 2021, 16 February 2021 (with Lambda Days 2021 & TFP 2021)
by Peter Achten 18 Dec '20

18 Dec '20

----------------------------------- TFPIE 2021 2nd Call for papers ----------------------------------- https://wiki.tfpie.science.ru.nl/TFPIE2021#TFPIE_2021 (February 16 2021, co-organized with TFP 2021 and Lambda Days 2021) Because of the covid pandemic, the events are online this year. The goal of the International Workshops on Trends in Functional Programming in Education is to gather researchers, professors, teachers, and all professionals that use or are interested in the use of functional programming in education. TFPIE aims to be a venue where novel ideas, classroom-tested ideas, and work in progress on the use of functional programming in education are discussed. The one-day workshop will foster a spirit of open discussion by having a review process for publication after the workshop. TFPIE 2021 welcomes submissions in the above mentioned areas. This year many teaching programmes have had to make a rapid transition to online teaching, and we explicitly solicit papers that explore this area of teaching functional programming. Topics of interest include, but are not limited to: - FP and beginning CS students - FP and Computational Thinking - FP and Artificial Intelligence - FP in Robotics - FP and Music - Advanced FP for undergraduates - FP in graduate education - Engaging students in research using FP - FP in Programming Languages - FP in the high school curriculum - FP as a stepping stone to other CS topics - FP and Philosophy - The pedagogy of teaching FP - FP and e-learning: MOOCs, automated assessment etc. - Best Lectures - more details below In addition to papers, we are requesting best lecture presentations. What's your best lecture topic in an FP related course? Do you have a fun way to present FP concepts to novices or perhaps an especially interesting presentation of a difficult topic? In either case, please consider sharing it. Best lecture topics will be selected for presentation based on a short abstract describing the lecture and its interest to TFPIE attendees. The length of the presentation should be comparable to that of a paper. On top of the lecture itself, the presentation can also provide commentary on the lecture. Submissions Potential presenters are invited to submit an extended abstract (4-6 pages) or a draft paper (up to 20 pages) in EPTCS style. The authors of accepted presentations will have their preprints and their slides made available on the workshop's website. Papers and abstracts can be submitted via easychair at the following link: https://easychair.org/conferences/?conf=tfpie2021 After the workshop, presenters are invited to submit (a revised version of) their article for review. The PC will select the best articles for publication in the Electronic Proceedings in Theoretical Computer Science (EPTCS). Articles rejected for presentation and extended abstracts will not be formally reviewed by the PC. Dates - Submission deadline: January 11 2021, Anywhere on Earth. - Notification: January 15 2021 - Workshop: February 16 2021 - Submission for formal review: April 20 2021, Anywhere on Earth. - Notification of full article: June 7 2021 - Camera ready: July 1st 2021 Program Committee - Peter Achten, Radboud University, Netherlands (chair) - Edwin Brady, University of St Andrews, UK - Laura Castro, Universidade da Coruña, Spain - Stephen Chang, University of Massachusetts Boston, USA - Youyou Cong, Tokyo Institute of Technology, Japan - Matthew Flatt, University of Utah, USA - Seth Fogarty, Trinity University, USA - Alex Gerdes, University of Gothenburg, Sweden - Gabriele Keller, Utrecht University, Netherlands - Prabhakar Ragde, University of Waterloo, Canada - Melinda Tóth, Eötvös Loránd University, Budapest, Hungary Registration TFPIE is part of Lambda Days. Please visit the Lambda Days 2021 pages when registration information becomes available. Only papers that have been presented at TFPIE may be submitted to the post-reviewing process. Information on Lambda Days is available at https://www.lambdadays.org/lambdadays2021 Information on TFP is available at http://tfp2021.org

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Re: [Haskell-cafe] Intermediate Modern Haskell
by Johannes Waldmann 18 Dec '20

18 Dec '20

Hi, when I was in a similar situation two years ago (an advanced course, after something basic) I had this list of topics https://imweb.imn.htwk-leipzig.de/~waldmann/edu/modul/seminar/fkps/ This was not a lecture but "Seminar", meaning that students give talks. Not all topics were actually presented - depending on number, and preferences, of participants. Meanwhile, I would probably add something on Foldable (cf. Section 7 of https://arxiv.org/abs/1808.08329) and I do plan to use https://thinkingwithtypes.com/ next time. Best regards, Johannes. PS: overview of my basic course: https://imweb.imn.htwk-leipzig.de/~waldmann/talk/17/wflp/ with that regard - on your list of topics: "Basic: ... algebraic data types" - comes last - good heavens! For me, it's first. See also Joachim's CIS 194. "likely cover: ... recursion schemes" - by all means do! Second lecture of basic course! Full marks for Brent's version of CIS194 https://www.cis.upenn.edu/~cis194/spring13/lectures.html

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PhD studentships in Robotics (Edinburgh Center for Robotics)
by Ekaterina Komendantskaya 17 Dec '20

17 Dec '20

Dear colleagues, Edinburgh Center for Robotics has published a fresh call for PhD scholarship applications. There will be space for verification and programming language research: see item 3: *" explainable AI, certification, verification, safety, security, multi-agent interactions;"* It is an ideal opportunity for PhD students who wish to apply their knowledge in Programming Languages and Verification to the growing field of Robotics and Autonomous systems research. See below the official advert and contact me directly if you need more information. *************************************** The Edinburgh Centre of Robotics is offering exciting PhD Studentships <https://www.edinburgh-robotics.org/projects-list> for September 2021 through the EPSRC Centre for Doctoral Training in Robotics and Autonomous Systems (CDT-RAS). A limited number of industry PhD placements may also be available and information about these opportunities is posted here <https://www.edinburgh-robotics.org/industry-studentships>. *Closing date 29**th** January 2021* Heriot-Watt University and the University of Edinburgh are jointly offering an exciting opportunity to study a PhD through an innovative 4-year PhD training programme, drawing on our extensive experience with postgraduate teaching and research supervision in the area of Robotics and Autonomous Systems. The Centre's goal is to train innovation-ready robotics researchers to be part of a multi-disciplinary team, focusing on *Safe Interaction for Robotics.* PhD candidates specialise in one of the following four areas, whilst gaining a deep understanding of technical aspects and theoretical foundations: 1. *Physical Interactions**: *control, actuation, compliance, sensing, mapping, planning, embodiments, swarms; 2. *People Interactions*: human-robot interaction, affective robotics, smart spaces, human-robot teaming, collaborative decision-making, cobots, multimodal interfaces; 3. *Self-Interactions*: condition monitoring, prognosis, explainable AI, certification, verification, safety, security, multi-agent interactions; 4. *Interaction Enablers*: vision, embedded and parallel computing, novel and soft fabrication methods, optimisation, (transparent) machine learning, deep reinforcement learning and other AI techniques inc. natural language processing (NLP). Achieving impact with robotics also requires non-technical skills, for example an understanding of technology translation, creativity and entrepreneurial processes. These are an essential component of the CDT programme, captured in the #Cauldron <https://www.edinburgh-robotics.org/content/cauldron> training programme. Why join us? · Flexible programme structure allows students to tailor courses to match research area of interest in year one, ensuring students have a strong general grounding in responsible research, current theory, methods and applications as a solid foundation for their PhD research. · Cohort approach with activities including group projects and away days encourages peer support and cross fertilisation of ideas. · Opportunity for international placement in prestigious research laboratory in year three. · Bespoke #Cauldron training programme to provide non-technical skills such as understanding of technology translation, creativity, and entrepreneurial processes. · Gateway seminars with prestigious speakers. · Fund to seed innovation. · Fully funded posts available paying stipend and Home fees, limited posts for overseas students More information on PhD placements and how to apply: www.edinburgh-robotics.org/apply *************** Prof. Ekaterina Komendantskaya Department of Computer Science School of Mathematical and Computer Sciences Earl Mountbatten Building [ building 21, office G26 ] Heriot-Watt University Edinburgh, UK, EH14 4AS +44 (0)131 451 8283 http://www.macs.hw.ac.uk/~ek19/ http://www.laiv.ac.uk

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Compilers written in Haskell that generate assembly
by Vanessa McHale 17 Dec '20

17 Dec '20

Hi all, Have any of you written compilers or backends that generate assembly? Lots of projects seem to "end" at LLVM. I have a project (https://hackage.haskell.org/package/kempe) where I used something like maximal munch and a linear register allocator, but I was wondering if there was anything more sophisticated out there! So far I've found http://hackage.haskell.org/package/linearscan-hoopl also Cheers, Vanessa McHale

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Question on optimization
by Todd Wilson 17 Dec '20

17 Dec '20

Dear Cafe, In code of the form case as ++ bs of [] -> ... x:xs -> ... under what circumstances is the overhead of cons-cell creation for the elements of `as` that would happen in an eager language avoided? Always? Is the answer the same if `f` is defined by simple list recursion and we call `f (as ++ bs)`? What if `f` is defined in a different module (or comes from the Prelude)? I know that questions of this type can be answered in specific cases by looking at the (intermediate) code generated by the compiler, but more generally, are there some succinct principles that everyday programmers can hang on their cubicle walls that summarize a lot of what they can expect from the compiler in terms of optimizations? Todd Wilson

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"Natural" polymorphism for n*(n+1)/2
by M Douglas McIlroy 17 Dec '20

17 Dec '20

Some nominally rational functions, e.g n*(n+1)/2, yield integer values for integer arguments. I seek either a way to wrap such a function so it has type Num a => a->a or a convincing argument that it can't be done. Doug

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Teaching High-School one-semester FP (using Haskell)
by Zachi Baharav 16 Dec '20

16 Dec '20

Dear mighty Haskell list, I am a high-school teacher in the US (California, Palo Alto). We have an advanced CS course that students can take after completing the AP-CS-A (in Java). This is usually Juniors and Seniors, after at least 2 or 3 courses of coding (Python+Java). Traditionally, I've done a potpourri of subjects, mostly search algorithms as applied to 'games'. Of course a little bit of data-structure comes in, and each year we did some 'other' interesting subject (like halftoning, barcodes, and so on) and investigated and implemented algorithms there. THIS year, for the second half, I would like us to do Functional-Programming. From what I have seen, my students over the years tend to write more and more convoluted codes, electing first to search on stackOverFlow for some similar pattern, rather than think and find a concise and clean solution. I think FP would supply them with a new way of thinking, which will help with whatever they will write later on. I've been coding with Haskell for about 7 years, so feel ok leading the class. The question: I have many books on Haskell (i think 'all', but who knows. Haskell, Real world Haskell, Learn you a haskell for great good, programming in haskell, and many more). ---> I am looking for something more hands-on and 'fun' for HS students. may i say the buzzword 'project based', or maybe better 'problem based' for our case. Something that I could teach a little, and then we can solve a bunch of problems, and teach a little more, and so on. It doesn't have to be a Book!! Just an outline of a course someone did with associated bunch of problems would be awesome. We have 18 weeks in a semester. My thought right now (if I don't find anything) is to relyon Euler project early problems. These are often clean and simple in Haskell. Or otherwise some CSES problems (which we've done in Java). Ok, long enough email. If you have material and can share here, great! If you have material and want to reach out privately, please do ( zbaharav(a)kehillah.org) If you have a good pointer, that would be appreciated. Thanks in advance for any help! Zachi (Dr. Zachi Baharav, HS teacher (after 20 years in Industry and academia))

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Psa : perhaps time for maintainers to start migrating off Travis ci
by Carter Schonwald 16 Dec '20

16 Dec '20

Hey everyone: it looks like, from my perspective and experiences, that Travis ci should perhaps now be viewed as not open source friendly. Or even converging on hostile? 1) crazy long queue times/ latency for oss ci actions to run 2) very low concurrency on oss builds. 3) very low build build minute caps for oss that require high touch customer support contact to adjust. I’ve started moving my own projects slowly to gh actions for now, though there’s also gitlab ci , src hut and other options that may suit different folks. There’s definitely some ways to keep on having the clever cabal caching we know and love that folks like the Haskell-ci folks and others have hacked out for Travis be available on other platforms, though I don’t think there’s consolidated docs for those yet ? Def seen it discussed though. https://github.com/haskell-CI/haskell-ci/issues/411 Heres a url to my dupe ticket where I share an example naive use of the setup Haskell gh actions Config, definitely not perfect. But kinda amazing to have Mac and Linux and windows ci all in one ! :)

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error: ghcide must be compiled with same GHC version as the project
by Dennis Raddle 15 Dec '20

15 Dec '20

I'm trying to use the Haskell extension with VS Code which, is my understanding, uses ghcide. When I built ghcide, I had the resolver in the stack.yaml set to "nightly-2020-09-02". Associated with this is GHC 8.10.2. When I first created my project I had the resolver set to something like lts-14.20. Associated with this is GHC something like 8.6.5. Later I changed my project resolver to nightly-2020-09-02, same as ghcide. But I'm still getting this error. Perhaps I haven't properly rebuilt my project using the new resolver? Dennis

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Re: [Haskell-cafe] Teaching High-School one-semester FP (using Haskell)
by Johannes Waldmann 13 Dec '20

13 Dec '20

Hi. Teaching is a topic that I do care about, and spend a lot of time on. I want to underline two points that Chris Smith made in his message about the ideas behind "Code World". I am speaking from current experience in teaching an "Intro to CS" course for non-CS students ("Informatik" on https://imweb.imn.htwk-leipzig.de/~waldmann/lehre.html) > easy graphics and UI library with a purely functional style. > ... lets your students experience declarative programming > and build cooI things easily, without needing to > get bogged down in stuff like I/O [...] etc. So much this! Have some basic pictures (rectangle, circle), combinators "row" (besides) and "column" (above), with some basic transformations. This will get students started with the algebraic method (description in the term algebra, evaluation in the domain algebra). I gave homework "produce pictures that resemble letters, traffic signs, etc." Does not need fancy graphics at all - I looked at the "diagrams" package but then decided to go with rectangular black/white pictures only. These restrictions make the "draw a sign" exercise more interesting. And after all, CS is about abstraction. It also allows me to connect to Boolean operations (combine pictures of identical sizes with point-wise Nand, etc.) that are treated in the lecture as well. > get bogged down in stuff like [...] recursion, etc. Again, yes! For my current course, I indeed think that primitive recursion (that is, iteration) is quite enough. E.g., students can draw a chess board from a single white square by flipping the color, and iterated composition. (Of course a CS major should know about the halting problem - but only to avoid it when programming. A non-CS major shouldn't write programs at all.) Advertisment: So I made an experimental "sub-Haskell" language (with online interpreter, integrated into an auto-grading system) (source currently at https://gitlab.imn.htwk-leipzig.de/autotool/all0/-/tree/661-programmable-gr… - I should break it out) with drastically restricted features * just one type (Pictures) * "let" but it's not "letrec" - definitions cannot be recursive * functions but they must be named (no lambdas) * strict evaluation (except for if-then-else which is hard-coded) * just one hard-wired higher order functions: iterate :: (a -> a) -> Nat -> a -> a * well, here I need Nat, and for if-then-else I need Bool, so that's the other two domain types besides Picture * formal parameters of functions must have a type declaration * extensive tracing of evaluation * and check(For) for property-based testing * code with with holes, where students have to fill in the holes, but cannot change anything outside. I can switch off some features (e.g., no functions, or, just unary) and I can switch the underlying domain: instead of pictures, can use sequences (it's not too much of a difference, a sequence is just a one-dimensional picture). I will use this for showing (one method of) compression: find a short straight-line program that produces a given string. Also I was thinking of introducing the "music" domain (scores/performances as in Haskore/Euterpea) but I am undecided about the best way of rendering MIDI in the browser (without 3rd-party JS, see below). Perhaps it doesn't have to be rendered (just print the events) as CS is about abstraction ... So while I try to stay in a Haskell subset (syntactically and semantically) the goal is not to teach some concrete programming language but algebraic/structural thinking. That is, mathematical thinking. Not "computational thinking" - because there is no such thing. If it works, it *is* mathematical... - J.W. NB: another necessary goal of an "intro to all things computer/computing" course is to make students aware of the surveillance economy. Fits nicely in the "computer networks" chapter. > ... get your students started quickly and easily in class, > without waiting for them to install GHC, Cabal/Stack, etc. Yes. That's hard even for CS majors. Well and for me as well. I normally provide software installed in the computer pool, but .. cf. https://github.com/haskell/cabal/issues/7172 Of course "the browser is the OS" is what Evil Corp wants students and teachers to think ..

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Re: [Haskell-cafe] What features should an (fictitious) IDE for Haskell have?
by Luke Lau 11 Dec '20

11 Dec '20

haskell-language-server recently got some basic integration with the retrie plugin, and can offer to do folding and unfolding: https://github.com/haskell/haskell-language-server/pull/266 Luke Lau

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Re: [Haskell-cafe] Teaching High-School one-semester FP (using Haskell)
by Andrew Butterfield 11 Dec '20

11 Dec '20

Dear all > On 11 Dec 2020, at 00:07, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: .... stuff elided > So recursion looks to me as funtional programming's GOTO, and if it is tail recursive, it actually is one. Yes, but it's "GOTO considered beneficial" ;-) Thanks to Zachi for asking the original question - I've already seen lots more ideas for me to chase up for my course! Regards, and Happy Holidays, Andrew > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Software Foundations & Verification Research Group School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

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Re: [Haskell-cafe] What features should an (fictitious) IDE for Haskell have?
by Olaf Klinke 08 Dec '20

08 Dec '20

On 20-12-07 18:22, André Popovitch wrote: > One feature I've always wanted would be able to "manually inline" a > function. For example, if I have > > foo a = a + a > bar = foo 3 > > I'd like to be able to right-click on the invocation of `foo` and > select > some option to inline it, rewriting the file to read > > foo a = a + a > bar = 3 + 3 > > I think this would make it simple to dive into an abstraction to see > what's > really going on. For example, imagine a new user is confused by this > expression: > > Just 3 >> Just 4 > > They could right-click on `>>` to see that it is identical to: > > case Just 3 of > Just _ -> Just 4 > Nothing -> Nothing > > > Which would allow them to easily see that this expression will always > evaluate to "Just 4" (and hopefully simplify it). Enlightened, they > could > then hit ctrl-z to revert the code to its original state. > > Haskell's purity and laziness makes it one of the only mainstream > languages > where equational reasoning is really possible, so this feature would > play > on Haskell's strengths and provide a feature that couldn't be > replicated in > other languages' IDEs. It would also allow new users to more easily > dive in > and understand certain abstractions (although how it would work in > many > cases would be hard to determine). +1 for this. It is like a step-by-step evaluation/debugger but limited to function calls at the syntactic level. I can image so many past situations where this would have prevented lots of head-scratching. This hypothetical IDE feature might go well with a tree view where inlining a function application is equivalent to expanding a level in a tree (within the same source line number), like so: Folded: [+] button appears on mouse over "foo". bar = foo[+] 3 Expanded: One code line now occupies two lines in the IDE window. [-] button reverts code to previous state. bar = foo 3 [-] 3 + 3 I've seen similar things in action in a demo implementation of [1,2]. Also great for teaching, I guess. Olaf [1] https://www.cs.bham.ac.uk/~pbl/papers/functionalexplainsupp.pdf [2] https://dl.acm.org/doi/10.1145/3110258

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[TFP'21] second call for papers: Trends in Functional Programming 2021, 18-19 February (online event with Lambda Days 2021 & TFPIE 2021)
by Peter Achten 08 Dec '20

08 Dec '20

------------------------------------------------------------------------- Second call for papers 22nd Symposium on Trends in Functional Programming tfp2021.org ------------------------------------------------------------------------- Did you miss the deadline to submit a paper to Trends in Functional Programming http://tfp2021.org/? No worries -- it's not too late! Submission is open until January 15th 2021, for a presentation slot at the event and post-symposium reviewing. The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions. * TFP offers a supportive reviewing process designed to help less experienced authors succeed, with two rounds of review, both before and after the symposium itself. Authors have an opportunity to address reviewers' concerns before final decisions on publication in the proceedings. * TFP offers two "best paper" awards, the John McCarthy award for best paper, and the David Turner award for best student paper. * TFP is co-located with Lambda Days in beautiful Krakow. Lambda Days is a vibrant developer conference with hundreds of attendees and a lively programme of talks on functional programming in practice. Due to the covid pandemic, the event is online with a lot of attention to interaction and getting to socialize with the community. Important Dates --------------- Submission deadline for pre-symposium review: 20th November, 2020 -- passed -- Submission deadline for draft papers: 15th January, 2021 Symposium dates: 18-19th February, 2021 Visit http://tfp2021.org/ for more information.

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What features should an (fictitious) IDE for Haskell have?
by Gueven Bay 08 Dec '20

08 Dec '20

Greetings, Imagine that you can decide what functionality and features an Integrated Development Environment for Haskell should get. You can make this IDE with your wishlist similar in power as other IDEs for Java for example. Now list as many features as you can, what would make the life of a Haskell developer as comfortable as possible!? I am dying to know how this list of features at the end (of this thread) will look like.

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Call for Participation: BOB 2021 (February 26, online, early-bird until Dec 31)
by Stefan Wehr 08 Dec '20

08 Dec '20

“What happens if we simply use what’s best?” BOB Conference 2021 February 26, 2021, online (0100+UTC) http://bobkonf.de/2021/ Program: http://bobkonf.de/2021/program.html Registration: http://bobkonf.de/2021/registration.html Keynote: Jeremy Gibbons 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 The early-bird registration is €10 for a regular ticket, €5 for a student ticket. (If you need financial aid, let us know.) We intend to make this the most diverse, colorful, fun BOB ever! NOTE: The early-bird rates expire on December 31, 2020!

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Help lead Haskell: Executive Director sought for Haskell Foundation
by Richard Eisenberg 08 Dec '20

08 Dec '20

Posting on behalf of the Haskell Foundation Working Group. Please forward widely! -------- The Haskell Foundation is seeking an Executive Director. Please find the job description here: https://haskell.foundation/ <https://haskell.foundation/executive-director>ed-job-description <https://haskell.foundation/ed-job-description/> The Haskell Foundation (HF) is an independent, non-profit organization dedicated to driving Haskell adoption and open-source development. HF seeks a full-time Executive Director (ED) to lead and develop the organization going forward. The ED is responsible for furthering the HF’s mission and vision, ensuring that resources are in place to accomplish its goals. The HF is supported by an active group of volunteers, and the ED will be a key focal point for that community, in addition to hiring support staff. This is a salaried position. Salary will be commensurate with the experience, qualities, and location of the individual, and will also reflect the Foundation’s status as a non-profit organisation funded by donations.

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ANN: hledger-1.20
by Simon Michael 07 Dec '20

07 Dec '20

I'm pleased to announce hledger 1.20 ! Changes include: - Strict mode - check command - rendering, speed, and valuation fixes. https://hledger.org/release-notes has the full details. Thanks to release contributors Stephen Morgan, Dmitry Astapov, TANIGUCHI Kohei, and legrostdg. hledger (http://hledger.org) is a robust, cross-platform, plain text accounting tool, with command-line, terminal and web UIs. It is an actively maintained, largely compatible reimplementation of Ledger CLI with many improvements. You can use it to track money, time, investments, cryptocurrencies, inventory and more. See also the Plain Text Accounting site (http://plaintextaccounting.org) http://hledger.org/download shows all the ways to install hledger on mac, windows or unix (stack, cabal, brew, nix, CI binaries, your package manager..). Or, run this bash script to install or upgrade to the latest release: $ curl -sO https://raw.githubusercontent.com/simonmichael/hledger/master/hledger-insta… $ less hledger-install.sh # security review $ bash hledger-install.sh New users, check out https://hledger.org/quickstart. To get help, see http://hledger.org#help-feedback, and join our chat channel via Freenode (#hledger, http://irc.hledger.org) or Matrix (#freenode_#hledger:matrix.org, http://riot.hledger.org) Beginners and experts, contributors, sponsors, and all feedback are most welcome! Be safe and well, -Simon

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Re: [Haskell-cafe] What features should an (fictitious) IDE for Haskell have?
by Johannes Waldmann 07 Dec '20

07 Dec '20

> very cool feature would be if I could select a program phrase > and let it find /similar/ phrases, where a similarity metric > could be edit-distance with respect to language tokens ... I often wanted a tool that finds (nearly) duplicate AST sub-trees in a large code base, and suggests refactorings. Of course, in an IDE, it could alert me on-the-fly that I'm typing some code that's already present elsewhere. How might one go about implementing this? Actual (approximate) sub-tree matching seems the easy part; but I have no clear idea about whether this should just use syntax, or needs types as well (my guess is: yes) what libraries are there to provide the (annotated) ASTs, etc. - J.W.

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Haskell on windows 7 without chocolatey?
by Jim Blorg 06 Dec '20

06 Dec '20

Is there a way to install haskell on windows 7 manually via zip without all the requirements for scripting?

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Re: [Haskell-cafe] [Ann] group-theory
by Emily Pillmore 06 Dec '20

06 Dec '20

Hi Marcin, > > Nice work; I also implemented free groups in the free-algebra package > > Wonderful - I'll take a look. Those were a bit rough, and it'd be good to see how you encoded them in `free-algebra`. Maybe we can collaborate on expanding the knowledgebase and utility of free group calculations in our respective packages :) > > However 'FG' is the free group in the class of all groups. > > I'm glad you noted this as well! The `DList` encoding is interesting - I'll need to study that a bit :) Someone has also linked me to Sjoerd's `free-functors` library, which seems is also very interesting: https://github.com/sjoerdvisscher/free-functors/blob/master/src/Data/Functo… > > There's an interesting difference between 'FreeGroup' (using notation of > your package) and 'FG'. First one is not a free group in the class of all > groups, but only free in the class of groups for which multiplication is > strict in the left argument > > Correct. Regarding motivation, I provided both because I know some people do like to operate in the fast-and-loose strict subset of Haskell. It's no skin off my back. But I was very unsatisfied with strict (vis. "moral") Free constructions with respect to bottoms, and, inspired by Dan Doel's article ( http://comonad.com/reader/2015/free-monoids-in-haskell/ ), attempted to find a free construction for groups that was more in line with Haskell's domain-like types. I note this article in the documentation for FG. Maybe this deserves a broader blog post? > > I identified that `Foldable` can only be defined for algebraic structures > for which `Endo b` has the same algebra type; this fails for groups: only > automorphisms form a group. Having a seprate `GroupFoldable` class makes > sense. > > That class ended up being good for two things: * Great puns. * Very convenient word evaluation If you'd like to add to it, I'd be stoked. Cheers, Emily On Sat, Dec 05, 2020 at 2:53 PM, < coot(a)coot.me > wrote: > > Hi Emily, > > Nice work; I also implemented free groups in the free-algebra package. > > > There's an interesting difference between 'FreeGroup' (using notation of > your package) and 'FG'. First one is not a free group in the class of all > groups, but only free in the class of groups for which multiplication is > strict in the left argument, i.e. ones that satisfy the equation `⊥ <> a > == ⊥`. That's because `++` is strict in the left argument. For the same > reason `[]` is free monoid in the class of left strict monoids. > > > > However 'FG' is the free group in the class of all groups. Another > isomorphic construction of a free group (in the class of all groups) can > be based on `DList` - the free monoid. You might want to check out https:/ > / hackage. haskell. org/ package/ free-algebras-0. 1. 0. 0/ docs/ Data-Algebra-Free. > html ( > https://hackage.haskell.org/package/free-algebras-0.1.0.0/docs/Data-Algebra… > ) which gives a uniform interface for all free algebras (not just groups). > You'll recover there `Monad` instance of `FG` (and `FreeGroup`), and a bit > more. > > > I identified that `Foldable` can only be defined for algebraic structures > for which `Endo b` has the same algebra type; this fails for groups: only > automorphisms form a group. Having a seprate `GroupFoldable` class makes > sense. > > > > Cheers, > > Marcin Szamotulski > > > > > > > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Saturday, December 5th, 2020 at 19:52, Emily Pillmore < emilypi@ cohomolo. > gy ( emilypi(a)cohomolo.gy ) > wrote: > > > > >> Hello all, >> >> >> >> I am pleased to announce the release of the group-theory ( >> https://hackage.haskell.org/package/group-theory ) package: a package aimed >> at implementing the theory of finite groups in a reasonably ergonomic, >> well-documented, and featureful way. It's hardly finished, but my >> co-maintainer, Reed Mullanix (@totbwf) and I thought this was a good mvp >> with which we could announce. >> >> >> >> Contributions are very welcome, and I hope everyone enjoys. >> >> >> >> Cheers, >> >> Emily Pillmore >> >> > >

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STM rollback?
by amindfv＠mailbox.org 05 Dec '20

05 Dec '20

I'd like to be able to give up on an STM transaction: roll back and don't retry. I've cooked up something with exceptions but it feels a bit icky to use exceptions for something like this - is there a better way?: data Rollback = Rollback deriving (Show) instance Exception Rollback rollback :: STM x rollback = throwSTM Rollback atomicallyWithRollback :: STM x -> IO (Maybe x) atomicallyWithRollback a = (Just <$> atomically a) `catch` (\Rollback -> pure Nothing) The alternative I've found is something like: otherWay :: STM x -> IO (Maybe x) otherWay a = atomically $ (Just <$> a) `orElse` pure Nothing But this turns any "retry" in "a" into a rollback, and I'd like to have the option to do either (retry or rollback). Thanks, Tom

6 6

untyped printf
by Ben Franksen 04 Dec '20

04 Dec '20

I am writing an interpreter for a very simple untyped language and I want to provide a built-in function to format a list of values in a printf like fashion. There is the beautiful Text.Printf module with its multi-variadic printf function, and I would like to use that to do all the heavy lifting. I can easily implement an instance PrintfArg Value for my Value type, but what I need in addition to that is an "untyped" version of printf i.e. something like format :: String -> [Value] -> String with the property that format fmt [] = printf fmt format fmt [x] = printf fmt x format fmt [x,y] = printf fmt x y ... I am pretty sure this is possible using a bit of class hackery and perhaps GADTs or continuation passing thrown in, but I can't seem to get it working. Any hint that helps to untangle the knots in my brain would be appreciated! Cheers Ben

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2020 Haskell.org Call for Nominations
by Jasper Van der Jeugt 03 Dec '20

03 Dec '20

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. Three members of the committee are at the ends of their current terms: Ryan Trinkle, George Wilson and Tikhon Jelvis. The Haskell Foundation has also recently put out a call for nominations. How does Haskell.org differ from the new Haskell Foundation? Why should you nominate yourself for a role? While the Haskell.org committee shares many goals with the Haskell Foundation, it remains an independent organization to operate the haskell.org website, provide resources and infrastructure, and run programs such as GSoC, like it has been doing for years. The Haskell.org committee has voted to affiliate itself with the Haskell Foundation and several of its members were involved in the founding of the Haskell Foundation. We believe that Haskell.org can represent the community as a stakeholder in the Haskell Foundation and help drive it forward. This makes it an exciting time to nominate yourself, as we'll be able to serve the community better, working closely with other groups in the Haskell community. To nominate yourself, please send an email to committee at haskell.org by the 7th of January, 2020. Current members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Committee members do not need to be deep technical Haskell experts. Instead we look for enthusiasm towards improving the Haskell community. We aim to represent the different facets of the community. We aim to be diverse in terms of industry or research, and in terms of gender, race and location. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds and donations. As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. More details about the committee's roles and responsibilities are on https://www.haskell.org/haskell-org-committee/ If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Warm regards Jasper

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Question on Coerce
by Brent Walker 02 Dec '20

02 Dec '20

In the following code, function fmap does not compile because variable 'y' on line marked <***> has type (Expr a) where an (Expr b) is expected. The code can be fixed simply by returning (Val x) on the rhs of the function but then we are allocating a new object for something we could potentially reuse since (Val n) has the same runtime representation in (Expr a) and (Expr b) (it has no dependence on the type variable). I was reading about coerce (Data.Coerce) recently and thought this could be a place it could be used but simply replacing the 'y' on the rhs with 'coerce y' does not compile (error message below). Is it possible to use coerce in this context? Thanks for any help, Brent ======================================= data Expr a = Var a | Val Int | Add (Expr a) (Expr a) deriving Show instance Functor Expr where fmap :: (a -> b) -> Expr a -> Expr b fmap f (Var a) = Var (f a) fmap f (Add e0 e1) = Add (fmap f e0) (fmap f e1) fmap _ y@(Val x) = y -- <***> ======================================= /dev/proj/src/Ex12.hs:69:22: error: • Couldn't match representation of type ‘a’ with that of ‘b’ arising from a use of ‘coerce’ ‘a’ is a rigid type variable bound by the type signature for: fmap :: forall a b. (a -> b) -> Expr a -> Expr b at /dev/proj/src/Ex12.hs:66:11-38 ‘b’ is a rigid type variable bound by the type signature for: fmap :: forall a b. (a -> b) -> Expr a -> Expr b at /dev/proj/src/Ex12.hs:66:11-38 • In the expression: coerce y In an equation for ‘fmap’: fmap _ y@(Val _) = coerce y In the instance declaration for ‘Functor Expr’ • Relevant bindings include y :: Expr a (bound at /dev/proj/src/Ex12.hs:69:10) fmap :: (a -> b) -> Expr a -> Expr b (bound at /dev/proj/src/Ex12.hs:67:3) | 69 | fmap _ y@(Val _) = coerce y | ^^^^^^^^ =======================================

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Textbook example of instance Foldable ((,) a)
by Johannes Waldmann 30 Nov '20

30 Nov '20

Dear Cafe - you might find this entertaining: an error in my code that cost me some hours (most Pictures are equal?) until I finally realized instance Eq a => Eq (Picture a) where p == q = dim p == dim q - && all (\ i -> contents p i == contents q i) (bounds p) + && all (\ i -> contents p i == contents q i) (A.range $ bounds p) Henning already called this "Matlab league" behaviour, https://mail.haskell.org/pipermail/libraries/2016-February/026678.html Well it's just a fun observation now, I am not implying anything. Enjoy! - J. Link to full source: https://gitlab.imn.htwk-leipzig.de/autotool/all0/-/issues/660#note_24567

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Amazonka, conduit and sockets not closing
by Magnus Therning 29 Nov '20

29 Nov '20

I've run into a problem with running out of filedescriptors. The following snippet is a trimmed down version of what I'm doing: #+begin_src haskell main :: IO () main = do awsEnv <- newEnv Discover runAWSCond awsEnv $ sqsSource queueUrl .| C.mapC snd .| sqsDeleteSink queueUrl where runAWSCond awsEnv = runResourceT . runAWS awsEnv . within Frankfurt . C.runConduit sqsSource :: MonadAWS m => T.Text -> C.ConduitT () (T.Text, T.Text) m () sqsSource queueUrl = do (_, msgs) <- C.lift $ recvSQS queueUrl C.yieldMany msgs sqsSource queueUrl sqsDeleteSink :: MonadAWS m => T.Text -> C.ConduitT T.Text o m () sqsDeleteSink queueUrl = do C.await >>= \case Nothing -> pure () Just receiptHandle -> do void $ C.lift $ delSQS queueUrl receiptHandle sqsDeleteSink queueUrl recvSQS queueUrl = do let rm = receiveMessage queueUrl & rmMaxNumberOfMessages ?~ 10 rmrs <- send rm let status = rmrs ^. rmrsResponseStatus msgs = rmrs ^. rmrsMessages & traversed %~ extract pure (status, catMaybes msgs) where extract msg = do body <- msg ^. mBody rh <- msg ^. mReceiptHandle pure (body, rh) delSQS queueUrl receiptHandle = do let dm = deleteMessage queueUrl receiptHandle send dm #+end_src This works fine for a while, but given a queue with enough messages it will fail with something like #+begin_example TransportError (HttpExceptionRequest Request { host = "sqs.eu-central-1.amazonaws.com" port = 443 secure = True requestHeaders = [("Host","sqs.eu-central-1.amazonaws.com"),("X-Amz-Date","20201126T101659Z"),("X-Amz-Content-SHA256","2e4bdf20a857a1416f218b1218670cf019ff53268d0adb34fe06402a62f3271d"),("Content-Type","application/x-www-form-urlencoded; charset=utf-8"),("Authorization","<REDACTED>")] path = "/" queryString = "" method = "POST" proxy = Nothing rawBody = False redirectCount = 0 responseTimeout = ResponseTimeoutMicro 70000000 requestVersion = HTTP/1.1 } (ConnectionFailure Network.Socket.getAddrInfo (called with preferred socket type/protocol: AddrInfo {addrFlags = [AI_ADDRCONFIG], addrFamily = AF_UNSPEC, addrSocketType = Stream, addrProtocol = 0, addrAddress = <assumed to be undefined>, addrCanonName = <assumed to be undefined>}, host name: Just "sqs.eu-central-1.amazonaws.com", service name: Just "443"): does not exist (System error))) #+end_example After some detours I found out that it's actually not a network issue, but rather that the process runs out of filedescriptors. Using =lsof= I can see that it doesn't seem to close /any/ sockets at all, instead they get stuck in a =CLOSE_WAIT= state: #+begin_example COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME wd-stats 88674 magnus 23u IPv4 815196 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:60624->52.119.188.213:https (CLOSE_WAIT) wd-stats 88674 magnus 24u IPv4 811362 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:43482->52.119.189.184:https (CLOSE_WAIT) wd-stats 88674 magnus 25u IPv4 811386 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:60628->52.119.188.213:https (CLOSE_WAIT) wd-stats 88674 magnus 26u IPv4 813527 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:43486->52.119.189.184:https (CLOSE_WAIT) ... #+end_example Am I using Amazonka and/or Conduit in a way that results in this? How do I should I use them? Or, is it an issue somewhere "below" my code? What can I do address that? Thanks for any insights or help /M -- Magnus Therning OpenPGP: 0x927912051716CE39 email: magnus(a)therning.org twitter: magthe http://magnus.therning.org/ Action is the foundational key to all success. — Pablo Picasso

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Re: [Haskell-cafe] Textbook example of instance Foldable ((,)a)
by Barak A. Pearlmutter 26 Nov '20

26 Nov '20

On Tue, 24 Nov 2020, Henning Thielemann wrote: > If people would use custom pair types we would not need Foldable on pairs, > at all. To be fair, if people would use custom pair types we would not need *pairs* at all. --Barak Pearlmutter

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network v3.1.2.1
by Kazu Yamamoto 26 Nov '20

26 Nov '20

Hi cafe, network v3.1.2.1 has been released. This version is bug-fixes only: * Increasing base lower bound to 4.9. [#473](https://github.com/haskell/network/pull/473) * Suppressing errors from removeFile in UNIX bind. [#478](https://github.com/haskell/network/pull/478) * Restoring UNIX stub functions on Windows. [#489](https://github.com/haskell/network/pull/489) --Kazu

1 0

Types of partially instantiated polymorphic helper functions
by Todd Wilson 26 Nov '20

26 Nov '20

Dear Cafe, This has got to be a trivial question, but I don't see a solution, nor could I find anything through searching: If I want to write a type declaration for the helper function g here, what do I write? f :: a -> [b] -> [(a,b)] f a bs = g bs where g :: ???? g [] = [] g (x:xs) = (a,x) : g xs Apologies if I'm wasting bandwidth with my ignorance. Todd Wilson

5 6

Looking for idiomatic example of wrapping a Haskell library
by Stuart Hungerford 25 Nov '20

25 Nov '20

Hi Haskellers, In a Haskell learning project I'm looking at using a couple of Haskell packages (from Hackage) for linear algebra and graphics. I've got a strong feeling my choices of these libraries are going to change as the project progresses and to avoid having to change references to these library functions throughout my modules I'd like to "wrap" them in modules of my own. Can anyone point me to a project where a Haskell library has been idiomatically wrapped this way? I can see newtypes and "smart" constructors of the wrapped types will go some way towards this, but I suspect there's more techniques I haven't yet learned. Thanks in advance, Stu

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Re: [Haskell-cafe] What to call Occult Effects
by Olaf Klinke 25 Nov '20

25 Nov '20

> Dear Olaf and everyone, > > the law do {_ <- b; a} = a is equivalent to do {_ <- b; return ()} = > return (), for, assuming the latter, > > do {_ <- b; a} > = do {_ <- do {_ <- b; return ()}; a} > = do {_ <- return (); a} > = a > > and it does actually have some history. It was dubbed > 'discardability' by Thielecke [1] and explored by > Führmann in [2] together with some other important properties of > effects, such as 'centrality' and > 'commutativity'. Combinations of these properties and examples are > explored in the conventional > Haskell-like style in [3] (where 'discardability' is called 'side- > effect freeness' though). > > Cheers, > Sergey > > [1] H. Thielecke, Categorical structure of continuation passing > style, Ph.D. Thesis, University of > Edinburgh, 1997. > > [2] C. Führmann, Varieties of effects, in: M. Nielsen, U. Engberg > (Eds.), Foundations of Software > Science and Computation Structures, FOSSACS 2002, in: Lect. Notes > Comput. Sci., vol. 2303, Springer, > 2002, pp. 144–158. > > [3] L. Schröder, T. Mossakowski, Monad-independent dynamic logic in > HasCasl, J. Logic Comput. 14 (2004) > 571–619 I've got the impression that all three of the above works look at a problem dual to the OP in a certain sense: At least [1] and [2] ask: For a given monad M, what is the subset of computations b with b >> a = a for all a? In contrast, I understood Kim-Ee's question as: What are the monads M where all computations b belong to this class? Assuming this property, the monoid M () is isomorphic to (). Indeed, the monoid M () inherits the property a <> b = b for all a, b :: M () and the only monoid with this property is the one- element monoid. In category theory a monad M with M 1 being isomorphic to the terminal object 1 is called 'affine'. Bart Jacobs in [4] gives a diagram (10) that amounts to return.const = fmap.const and shows that this is equivalent to being affine. Hence we might also use 'affine' for monads with Kim-Ee's property. Bart Jacobs' proof first reduces the general identity return.const = fmap.const :: a -> m b -> m a to the case b = () just as Sergey did above. Then = fmap (const a) = fmap (const a) . (id :: m () -> m ()) = fmap (const a) . return . const () by assumption = return . const a . const () by naturality of return = return . const a property of const I also learned that the school of 'effects' tries to avoid monads (this going back to John Power), so the terms 'discardable' and 'affine' might just be names for the same thing in different communities. Further there is a categorical construction (see [4]) to compute the greatest affine sub-monad of a monad. Interestingly, the affine part of the state monad is the reader monad, that is, functions s -> (a,s) that may use s but not change it. Olaf [4] https://hal.inria.fr/hal-01446033/file/418352_1_En_5_Chapter.pdf

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Virtual Munich Haskell Meeting, 2020-11-25 @ 19:30
by Leah Neukirchen 23 Nov '20

23 Nov '20

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, November 25 at 19:30. **Due to meetup limitations in Bavaria, this meeting will take place online!** For details see here: https://muenchen.haskell.bayern/dates.html 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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xmonad: eyes based navigation between monitors.
by Karel Gardas 20 Nov '20

20 Nov '20

Hello, I'm using xmonad in a setup with 3 monitors and I'm curious if anybody here got kind of eyes-based navigation working between monitors/workspaces. Explanation: I need to type into windows on different monitors but in not specific order. Sometime I got too deep into thinking that I forget to switch workspace and then I type to different window in different workspace than is intended (and which I see with eyes). Basically I'd like some mode where xmonad will switch workspaces based on my eye/head position so only workspace I follow with eyes will be active. I see that such software (following head/eyes) already exist in AI domain (E.g. NV's xavier NX developer kit demo: from 16:00: https://www.youtube.com/watch?v=aUz1bNGm04w) but I'm more interested in amd64/linux setup as this is for my workstation... Any idea highly appreciated! Thanks! Karel

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 207, Issue 16
by Sumit Raja 19 Nov '20

19 Nov '20

> It may be that you have a dependency of this specific test (or the test > itself) that has the following stanza in its Cabal file: > "extra-libraries: gmp" > > Thanks, makes sense. Any shortcuts to try and find which package has gmp enabled? Interestingly this problem doesn't happen with ghc-8.6.5. Does build order decide on if this issue is triggered or not? -Sumit

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ghci module re exporting qualified as
by Immanuel Litzroth 18 Nov '20

18 Nov '20

I'm trying to make a test module to run tests. I want to import the test functionality + imported functionality in the scope of the test module under ghci. I also want to have short names in the Test module for reexported modules. I am using Data.Map and Data.Set ... so lot's of conflicting exports. Is this possible? #+BEGIN_SRC haskell :tangle T1.hs module T1 ( module Data.Map) where import qualified Data.Map #+END_SRC works but no browsing info shows up #+BEGIN_EXAMPLE λ⊢ :show modules T1 .... λ⊢ :show imports import T1 -- added automatically import Prelude -- implicit λ⊢ fromList [] <interactive>:114:1-8: error: Variable not in scope: fromList ∷ [a0] → t λ⊢ Data.Map.fromList [] fromList [] λ⊢ :browse! T1 λ⊢ #+END_EXAMPLE #+BEGIN_SRC haskell :tangle T2.hs module T2 ( module Map) where import qualified Data.Map as Map #+END_SRC #+BEGIN_EXAMPLE λ⊢ :show modules T2 :show imports import T2 -- added automatically import Prelude -- implicit λ⊢ fromList [] <interactive>:119:1-8: error: Variable not in scope: fromList ∷ [a0] → t λ⊢ Map.fromList [] <interactive>:120:1-12: error: Not in scope: ‘Map.fromList’ No module named ‘Map’ is imported. λ⊢ Data.Map.fromList [] fromList [] λ⊢ :browse! T2 λ⊢ #+END_EXAMPLE -- -- Researching the dual problem of finding the function that has a given point as fixpoint.

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Why is my 'text' missing an instance?
by amindfv＠mailbox.org 18 Nov '20

18 Nov '20

tl;dr: I seem to have two versions of 'text' on my system, both of which should have an instance for (Binary Text). However, one version seems to claim not to have that instance. $ cat test.hs import qualified Data.Text as T import qualified Data.Binary as B main = print $ B.encode $ T.pack "hello" $ /usr/local/bin/ghci --version The Glorious Glasgow Haskell Compilation System, version 8.10.2 $ /usr/local/bin/ghci test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:4:16: error: • No instance for (B.Binary T.Text) arising from a use of ‘B.encode’ • In the second argument of ‘($)’, namely ‘B.encode $ T.pack "hello"’ In the expression: print $ B.encode $ T.pack "hello" In an equation for ‘main’: main = print $ B.encode $ T.pack "hello" | 4 | main = print $ B.encode $ T.pack "hello" | ^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. $ /usr/local/bin/ghc-pkg list text /usr/local/lib/ghc-8.10.2/package.conf.d text-1.2.3.2 /home/name/.ghc/x86_64-linux-8.10.2/package.conf.d text-1.2.4.1 $ ghc-pkg-8.10.2 list text [same result] $ /usr/local/bin/ghci -package text-1.2.3.2 test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, one module loaded. $ /usr/local/bin/ghci -package text-1.2.4.1 test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:4:16: error: • No instance for (B.Binary T.Text) arising from a use of ‘B.encode’ • In the second argument of ‘($)’, namely ‘B.encode $ T.pack "hello"’ In the expression: print $ B.encode $ T.pack "hello" In an equation for ‘main’: main = print $ B.encode $ T.pack "hello" | 4 | main = print $ B.encode $ T.pack "hello" | ^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Both text-1.2.3.2 and text-1.2.4.1 have a Binary instance for Text. What's going on? (After writing this out I rebuilt an unmodified clone of text-1.2.4.1 but I see the same results.) Thanks, Tom

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GHC linker error 8.8.4 Windows
by Sumit Raja 17 Nov '20

17 Nov '20

Hi, Suddenly started getting the following linker error after I moved code around in my project. It only seems to occur when I am building one specific cabal test target. When building a dependent build it build successfully but cabal repl test-xxx fails. I have no consistent way to repro but I will keep trying and file a defect.. GHCi, version 8.8.4: https://www.haskell.org/ghc/ :? for help GHC runtime linker: fatal error: I found a duplicate definition for symbol __gmpn_sec_sqr_itch whilst processing object file C:\Users\sumit\Apps\ghc-8.8.4\mingw\lib\libgmp.dll.a The symbol was previously defined in C:\Users\sumit\Apps\ghc-8.8.4\lib\integer-gmp-1.0.2.0\HSinteger-gmp-1.0.2.0.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. ghc.exe: panic! (the 'impossible' happened) (GHC version 8.8.4 for x86_64-unknown-mingw32): loadArchive "C:\\Users\\sumit\\Apps\\ghc-8.8.4\\mingw\\lib\\libgmp.dll.a": failed Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug CallStack (from HasCallStack): die', called at .\\Distribution\\Client\\ProjectOrchestration.hs:1041:55 in main:Distribution.Client.ProjectOrchestration cabal.exe: repl failed for mkr-txn-0.1.0.0-inplace-test-mkr-txn. Regards Sumit

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Re: [Haskell-cafe] Reading Haddock sources on hackage no longer possible with simple browsers?
by M Douglas McIlroy 16 Nov '20

16 Nov '20

> I am pretty sure nobody writes code like this > I confirmed your example, reading with lynx and w3m. > wouldn’t just directly opening the sources (in a text editor) be simpler? Is there a (simple) way to preserve the experience when browsing using Lynx without also holding back features aimed at a regular browsing experience? I second that. I have always been annoyed by the ransom-note appearance of colorized listings. But that's just my taste. I suspect, though, that lots of people have been frustrated when they discover they can't use the code they see because it's festooned with markup. Golang.org offers similar listings, but also provides a button for selecting the underlying plain text. I would like hackage a lot better if it did so, too. Doug

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[ANN] base64-bytestring-1.2.0.1 + future plans
by Emily Pillmore 15 Nov '20

15 Nov '20

Hi Everyone, I'm pleased to announce that we've released a new version of `base64-bytestring` that now supports `bytestring-0.11` in a backwards-compatible manner. I'd also like to make an RFC: we are thinking that we will drop support for GHC versions <7.10 in future releases of the project. The earliest release in which this can be expected will be around 6 months from now. The dev burden for supporting back to 7.0 is high, and unless someone screams in protest, the value does not seem to be there. I'd like to get your thoughts on this. Cheers, Emily

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Redirecting hackage.haskell.org to HTTPS
by Tikhon Jelvis 15 Nov '20

15 Nov '20

Pages on hackage.haskell.org are currently served over both HTTP and HTTPS. Package security on Hackage does not depend on HTTPS, so we keep HTTP endpoints for backwards compatibility with automated systems that depend on Hackage and do not support HTTPS. However, this means that it's possible for users to inadvertently browse Hackage pages on HTTP which is not a great user experience. To address this issue without breaking existing scripts, we are planning to redirect requests to HTTPS based on User-Agent headers: requests with "Mozilla/5.0" in their User-Agent string will be redirected to HTTPS and other requests will remain unchanged. Please contact us at committee(a)haskell.org if this change will cause problems with how you use Hackage. Otherwise, the new behavior will go into effect on 2020-11-23. Thanks! -Tikhon Jelvis, on behalf of the Haskell.org Committee

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