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[TFP'21] first call for papers: Trends in Functional Programming 2021, 17-19 February (with Lambda Days 2021 & TFPIE 2021)
by Peter Achten 29 Sep '20

29 Sep '20

------------------------------------------------------------------------- First call for papers 22nd Symposium on Trends in Functional Programming tfp2021.org ------------------------------------------------------------------------- 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 Submission deadline for draft papers: 15th January, 2021 Symposium dates: 17-19th February, 2021 * We strongly encourage authors to submit their work for the first deadline. Authors whose papers are accepted for presentation, but not immediately for the proceedings in this first round, will have almost two months to address the reviewers' concerns. Papers submitted for the first deadline will also have priority for the presentation slots at the symposium. Visit tfp2021.org for more information.

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Proposed "Restructuring" of State Monad page in Haskell Wikibooks
by David James 29 Sep '20

29 Sep '20

Hello – I’m proposing to restructure this<https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State>, and I have a draft here<https://en.wikibooks.org/wiki/Davjam2:Example/StateMonad>. I’ve given my main reasons for the restructuring at the top of the draft page. I’d very much like feedback before updating the real page, especially if people don’t like the new one much. I’m new here, so should probably say a bit about myself. I’ve been learning-by-doing Haskell for about two years. Sometimes I feel I’m starting to get it, but these feelings don’t usually last long. I’m certainly not an expert, and don’t have a PhD in Very Clever Things. I do have quite an extensive IT background, originally programming in Lisp and Prolog, then (sadly) C++, VB and SQL but have had quite a long break from programming (doing dumb things like architecting systems and project managing) before looking at Haskell. I built this website<https://decimos.net/> and this library<https://hackage.haskell.org/package/MapWith>, just for my own amusement. Apologies if this isn’t the right place to send this. (But then where is?) I’ve already put a note on the page itself about the new draft, but I’ve no idea whether anyone will notice it. Thanks very much, David.

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I think I discovered my first Monoid instance
by Mario Lang 29 Sep '20

29 Sep '20

Hi. A common theme of some Haskell talks seems to be to motivate people to go and hunt for instances of standard typeclasses. As the hobbyist Haskell programmer that I am, I kept on wondering if I would ever find some. It feels sort of unrealistic, or at least unlikely, that I should suddenly start to see meaningful structure in code just because I think I know some typeclasses. I think I finally discovered a Monoid instance that helps to express a concept in code pretty well. However, unsure about the soundness of my blundering about, I'd like to confirm (or disprove!) my findings before I fall in love with the approach too much. In my chess library (chessIO) I use bitboards to represent chess positions. data QuadBitboard = QBB { black :: {-# UNPACK #-} !Word64 , pbq :: {-# UNPACK #-} !Word64 , nbk :: {-# UNPACK #-} !Word64 , rqk :: {-# UNPACK #-} !Word64 } deriving (Eq) A quad bitboard is a space optimisation based on the observation that a square can only be occupied by one piece. occupied QBB{pbq, nbk, rqk} = pbq .|. nbk .|. rqk pnr QBB{pbq, nbk, rqk} = pbq `xor` nbk `xor` rqk white = liftA2 xor occupied black pawns = liftA2 (.&.) pnr pbq knights = liftA2 (.&.) pnr nbk bishops = liftA2 (.&.) pbq nbk rooks = liftA2 (.&.) pnr rqk queens = liftA2 (.&.) pbq rqk kings = liftA2 (.&.) nbk rqk We can create a bitboard with a single occupied square: square :: Int -> Word4 -> QuadBitboard square !sq !nb = QBB (f 0) (f 1) (f 2) (f 3) where !b = bit sq f !n = fromIntegral ((nb `unsafeShiftR` n) .&. 1) * b And, as an aside, we can even use pattern synonyms to give these nibbles meaningful names. pattern NoPiece = 0 pattern WhitePawn = 2 pattern WhiteKnight = 4 pattern WhiteBishop = 6 pattern WhiteRook = 8 pattern WhiteQueen = 10 pattern WhiteKing = 12 pattern BlackPawn = 3 pattern BlackKnight = 5 pattern BlackBishop = 7 pattern BlackRook = 9 pattern BlackQueen = 11 pattern BlackKing = 13 But what felt really like a cool discovery, is combination: instance Monoid QuadBitboard where mempty = QBB 0 0 0 0 -- | bitwise XOR instance Semigroup QuadBitboard where QBB b0 b1 b2 b3 <> QBB b0' b1' b2' b3' = QBB (b0 `xor` b0') (b1 `xor` b1') (b2 `xor` b2') (b3 `xor` b3') With this, we can pretty easily define a function to create a quad bitboard from a string: instance IsString QuadBitboard where fromString = go (7, 0) mempty where go _ !qbb "" = qbb go (!r,_) qbb ('/':xs) = go (r - 1, 0) qbb xs go (!r,!f) !qbb (x:xs) | inRange ('1','8') x = go (r, f + (ord x - ord '0')) qbb xs | otherwise = go (r, f + 1) (qbb <> square (r*8+f) nb) xs where nb = case x of 'P' -> WhitePawn 'N' -> WhiteKnight 'B' -> WhiteBishop 'R' -> WhiteRook 'Q' -> WhiteQueen 'K' -> WhiteKing 'p' -> BlackPawn 'n' -> BlackKnight 'b' -> BlackBishop 'r' -> BlackRook 'q' -> BlackQueen 'k' -> BlackKing _ -> error $ "QuadBitBoard.fromString: Illegal FEN character " <> show x standard :: QuadBitboard standard = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR" The rest of the module builds on the Monoid instance of QuadBitboard. Again, this is the first time a instance like this turns up in my Haskell experiments. It feels extremely satisfying having discovered this. But maybe I am violating some laws (I hear instances do that sometimes!) or some other thing is totally wrong. One might say I am not fully trusting the peace. https://hackage.haskell.org/package/chessIO/docs/Game-Chess-QuadBitboard.ht… -- CYa, ⡍⠁⠗⠊⠕

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Status of Haskell on the browser?
by Bryan Richter 29 Sep '20

29 Sep '20

A perennial question, I'm sure. I've been trying out some browser-side programming lately, and I'm curious if anyone in the community has any vision for where things are going. GHCJS looks like it hasn't had any updates in a while. Is it approaching its twilight years, or do people still have plans for it? Asterius looks interesting, but I'm not too clear on where WASM sits in the web space. Would a suitably-advanced Asterius cover all of our Haskell-on-the-browser needs, or would there still be a need for a separate javascript backend? Basically, I'm curious to know what people are working on in this realm! -Bryan

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Compiler Construction (CC) 2021 - Call for Papers
by Delphine Demange 28 Sep '20

28 Sep '20

ACM SIGPLAN 2021 International Conference on Compiler Construction (CC 2021) Co-located with CGO, HPCA and PPoPP Sat 27 February - Wed 3 March 2021 https://conf.researchr.org/home/CC-2021 CALL FOR PAPERS The International Conference on Compiler Construction (CC) is interested in work on processing programs in the most general sense: analyzing, transforming or executing input that describes how a system operates, including traditional compiler construction as a special case. = IMPORTANT DATES = Abstract Submission: November 8, 2020 Full Paper Submission: November 10, 2020 Author Response Period: December 7 - 9, 2020 Author Notification: December 22, 2020 Artifact Submission: January 5, 2021 AE Notification: January 20, 2021 Final Papers due: January 22, 2021 Conference: February 27 - March 3, 2021 Original contributions are solicited on the topics of interest which include, but are not limited to: - Compilation and interpretation techniques, including program representation, analysis, and transformation; code generation, optimization, and synthesis; the verification thereof - Run-time techniques, including memory management, virtual machines, and dynamic and just-in-time compilation - Programming tools, including refactoring editors, checkers, verifiers, compilers, debuggers, and profilers - Techniques, ranging from programming languages to micro-architectural support, for specific domains such as secure, parallel, distributed, embedded or mobile environments - Design and implementation of novel language constructs, programming models, and domain-specific languages CC is an ACM SIGPLAN conference, and implements guidelines and procedures recommended by SIGPLAN (https://www.sigplan.org) Prospective authors should be aware of SIGPLAN’s Copyright policies. Proceedings will be made available online in the ACM digital library from one week before to one week after the conference. Full CfP: https://conf.researchr.org/track/CC-2021/cc-research-papers ARTIFACT EVALUATION Authors of accepted papers will be invited to submit their artifacts for the Artifact Evaluation (AE). The Artifact Evaluation process begins after the acceptance notification, and is run by a separate committee whose task is to assess how the artifacts support the work described in the papers. To ease the organization of the AE committee, we kindly ask authors to indicate at the time they submit the paper, whether they are interested in submitting an artifact. Papers that go through the Artifact Evaluation process successfully will receive a seal of approval printed on the papers themselves. Authors of accepted papers are encouraged, but not required, to make these materials publicly available upon publication of the proceedings, by including them as “source materials” in the ACM Digital Library. CC AE web page: https://conf.researchr.org/track/CC-2021/research-artifacts ORGANIZERS General Chair: Aaron Smith - Microsoft / University of Edinburgh Program Chairs: Delphine Demange - Univ Rennes, Inria, CNRS, IRISA Rajiv Gupta - UC Riverside Artifact Evaluation Chairs: Bruno Bodin - Yale-NUS College Michel Steuwer - University of Glasgow Web Chair: Martin Lücke - University of Edinburgh Steering Committee: Björn Franke (Chair) - University of Edinburgh Jose Nelson Amaral - University of Alberta Christophe Dubach - University of Edinburgh Sebastian Hack - Saarland University Manuel Hermenegildo - IMDEA Software Institute and T.U. of Madrid (UPM) Alexandra Jimborean - University of Murcia Milind Kulkarni - Purdue University Louis-Noël Pouchet - Colorado State University Peng Wu - Futurewei Technologies Jingling Xue - UNSW Sydney Ayal Zaks - Intel Corporation and Technion Program Committee: Guillaume Baudart - IBM Research Walter Binder - University of Lugano Simone Campanoni - Northwestern University Albert Cohen - Google Caroline Collange - Inria, Univ Rennes, CNRS, IRISA Huimin Cui - Institute of Computing Technology, CAS Christophe Dubach - McGill University Benoît Dupont de Dinechin - Kalray Bernhard Egger - Seoul National University Christine Flood - Red Hat Laure Gonnord - University of Lyon and LIP Myoungsoo Jung - KAIST Andrew Kennedy - Facebook Dongyoon Lee - Stony Brook University Christian Lengauer - University of Passau Xavier Leroy - Collège de France and Inria Yun Liang - Peking University Toby Murray - University of Melbourne and Data61 Biswabandan Panda - Indian Institute of Technology Kanpur Santosh Pande - Georgia Tech Louis-Noël Pouchet - Colorado State University Gabriel Rodríguez - Universidade da Coruña Jan Vitek - Northeastern University Jingling Xue - UNSW Sydney Zhijia Zhao - UC Riverside

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Virtual Munich Haskell Meeting, 2020-09-28 @ 19:30
by Leah Neukirchen 25 Sep '20

25 Sep '20

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, September 28 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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Open Kattis Problem Srednji: Hints to improve my algorithm
by Dominik Bollmann 25 Sep '20

25 Sep '20

Hi Haskell-Cafe, I've been trying to solve the Open Kattis Problem called Srednji recently, unfortunately without success. Given a sequence A and a number B within that sequence this problem asks to find all odd sub-sequences of A that, when sorted, have B as their median in the middle. That is, from A we may remove some prefix and/or suffix and if the resulting sub-sequence -- when sorted -- contains B in the middle, then this sub-sequence is a solution. The problem asks to find the number of all solutions. Check out https://open.kattis.com/problems/srednji for the details. My Haskell solution below tries to find the number of odd sub-sequences by first locating the median and then repeatedly moving left and right from that median to find larger and larger sub-sequence candidates. Each found candidate is checked to have B in the middle when sorted in order to become a solution. Moreover, I also extend each such candidate further to the left (and to the right, respectively) to determine whether these leftward or rightward extensions are solutions, too. I think with this approach I systematically enumerate all solutions. Unfortunately, though, this approach is too slow and times out on the 11th hidden test cases. I'd therefore be thankful for hints about different approaches to solving this problem more efficiently. Thanks! Dominik. ==================================================================== My current, slow Haskell code is this: import Data.Maybe import Data.Sequence (Seq, (<|), (|>)) import qualified Data.Sequence as Seq import qualified Data.Vector.Unboxed as Vec data SubSeq = SubSeq { getBalance :: {-# UNPACK #-} !Int , getSubSeq :: Seq Int , from :: {-# UNPACK #-} !Int , to :: {-# UNPACK #-} !Int } balancedSubSeqs :: [Int] -> Int -> [SubSeq] balancedSubSeqs seq med = do candidate <- leftRight [val0] let lefts = leftLeft candidate [] rights = rightRight candidate [] candidate ?: lefts ++ rights where medidx = fromJust (Vec.findIndex (== med) arr) val0 = SubSeq 0 (Seq.singleton med) medidx medidx arr = Vec.fromList seq leftRight cands@(SubSeq balance seq i j : _) | i-1 < 0 || j+1 >= Vec.length arr = cands | otherwise = let v1 = arr Vec.! (i-1) v2 = arr Vec.! (j+1) balance' = newBalance balance v1 v2 seq' = (v1 <| seq) |> v2 in leftRight (SubSeq balance' seq' (i-1) (j+1) : cands) leftLeft cand@(SubSeq balance seq i j) sols | i-2 < 0 = sols | otherwise = let v1 = arr Vec.! (i-2) v2 = arr Vec.! (i-1) balance' = newBalance balance v1 v2 seq' = v1 <| v2 <| seq newCand = SubSeq balance' seq' (i-2) j in leftLeft newCand (newCand ?: sols) rightRight cand@(SubSeq balance seq i j) sols | j+2 >= Vec.length arr = sols | otherwise = let v1 = arr Vec.! (j+1) v2 = arr Vec.! (j+2) balance' = newBalance balance v1 v2 seq' = seq |> v1 |> v2 newCand = SubSeq balance' seq' i (j+2) in rightRight newCand (newCand ?: sols) newBalance old n1 n2 | n1 < med, n2 < med = old - 2 | n1 > med, n2 > med = old + 2 | otherwise = old infixr 5 ?: --(?:) :: SubSeq -> [SubSeq] -> [SubSeq] x@(SubSeq b _ _ _) ?: xs | b == 0 = x : xs | otherwise = xs main :: IO () main = do [len, median] <- fmap read . words <$> getLine seq <- fmap read . words <$> getLine let solutions = balancedSubSeqs seq median print (length solutions)

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Re: [Haskell-cafe] Are explicit exports and local imports desirable in a production application?
by Olaf Klinke 23 Sep '20

23 Sep '20

Dear Ignat, have you seen https://wiki.haskell.org/Import_modules_properly https://wiki.haskell.org/Qualified_names I find the arguments convincing. Even in my own packages I sometimes get lost where a certain function was imported from. When neither exports nor imports are done explicitly, you usually have only two choices: 1. search all sources (e.g. with grep -l) 2. rely on the haddock index Maybe your IDE can do that for you, but you can't expect all downstream users or all your colleagues to do the same. -- Olaf

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Re: [Haskell-cafe] Readable Haskell
by Ignat Insarov 23 Sep '20

23 Sep '20

Hello Ian and Mario. > > We have retina screens, GPU accelerated rendering and magical > > auto-completion. > > Who's "we"? 1 and 2 are constrained by money. 1 and 3 are also > constrained by personal preference. > > I need a _big_ screen because I also do photo processing beside coding, > and I can't afford 2 screens, one big and another high density. > > Command shell is a very old and peculiar human interface — if human at > > all. They had 80 character wide screens and visibly lagging connexion. > > We have retina screens, GPU accelerated rendering and magical > > auto-completion. > > You might have that. I, as a blind braille display user, > have one line of at most 80 characters. When I am traveling, I > typically only have 40 characters. Given these constraints, I still > consider a command line interface superior to any sort of buttonized > nonesense. But thats just me, apparently. This is not how I expected my message to be understood. I did not mean to imply that every Haskell programmer has or should have a retina screen and a high performance GPU — only that, as a profession, we have way better tools now than back then. In humanities, it is usual for there to be either a normal distribution or a Pareto distribution in any large enough sample of data. So, unlike in precise sciences, a counter-example does not refute a proposition. What matters is that there is a trend. And there is a trend associated with larger and finer displays. It dawns on even the most _«old school»_ people by now. See for example a letter on Linux Kernel Mailing List.[1] > When I tile my terminal windows on my display, I can have 6 terminals > visible at one time, and that's because I have them three wide. And I > could still fit 80% of a fourth one side-by-side. > > And guess what? That's with my default "100x50" terminal window (go to > your gnome terminal settings, you'll find that the 80x25 thing is just > an initial default that you can change), not with some 80x25 one. And > that's with a font that has anti-aliasing and isn't some pixelated > mess. I have no specific insight about how to optimize readability for people that listen to code[2] or sense it with their hands. What I am sure of is that there are better ways to go about it than dwelling in the past. [1]: https://lkml.org/lkml/2020/5/29/1038 [2]: https://www.vincit.fi/fi/software-development-450-words-per-minute/

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Readable Haskell
by Misja Alma 21 Sep '20

21 Sep '20

Hi, I have been writing Haskell in my spare time for a couple of years now, but when I showed some code lately to a friend he remarked that he didn't find it very readable. Actually I agree, when I look at my own code of a couple of months old I have trouble figuring out too what exactly it is doing. I'm coming from a Java and Scala background and there, especially for Java, are some generally accepted best practices that make sure that your teammates don't have too much trouble reading your code. E.g. write short functions with a single responsibility, use variable, class and function names that explain what they are meant for, etc. I think some of those best practices, like short functions with single responsibility, are useful for Haskell as well. But Haskell is a different language than Java and has its own strong points and pitfalls regarding readability, so it probably needs different coding standards as well. I have been looking on the Internet if I could find some tips about improving readability but all I could find was http://www.haskellforall.com/. Although there are some useful tips in there, this site seems to be aimed at making Haskell easier to read for newcomers from other languages. What I am interested in are tips from real projects that are built by real teams. Does anybody have any tips, or are there some sites or books that I could read about this topic? Thanks, Misja

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