December 2020 - Haskell-Cafe

Re: [Haskell-cafe] Intermediate Modern Haskell
by Johannes Waldmann 17 Dec '20

17 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 16 Dec '20

16 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 15 Dec '20

15 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 14 Dec '20

14 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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