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[Call for Contributions] Haskell Communities and Activities Report, November 2018 edition (35th edition)
by Mihai Maruseac 07 Oct '18

07 Oct '18

Dear all, We would like to collect contributions for the 35th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 4 November 2018 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac FAQ: Q: What format should I write in? A: Any format is ok, I will transpose the submission to the format in use at the time of publication. Previous editions have used a LaTeX format, with the template that is available at: http://haskell.org/communities/11-2018/template.tex And the associated LaTeX style file at http://haskell.org/communities/11-2018/hcar.sty You can use those to edit and preview your entry, but it is very likely that by the time of publication the entire HCAR pipeline would be changed to a more modern one. You will receive a copy of the draft, prior to publication, to ensure that everything looks ok. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. If using LaTeX, you can use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then. PNG is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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optimisation of code
by PICCA Frederic-Emmanuel 02 Oct '18

02 Oct '18

Hello, I would like to have your advice in order to optimize this code. The purpose is to trigg an action 'a' if a list of files (thousands) exists. A process copy files from one directory to another. allFilesThere :: MonadIO m => [Path Abs File] -> m Bool allFilesThere fs = liftIO $ allM (doesFileExist . fromAbsFile) fs trigOnAllFiles :: MonadIO m => m r -> [Path Abs File] -> m r trigOnAllFiles a fs = go where go = do r <- allFilesThere fs if r then a else ( do liftIO $ threadDelay 1000000 go) It works, but it consums a lot's of resources when all the files does not exists yet. So I would like your advice in order to optimize it :) thanks for your help. Frederic

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Re: [Haskell-beginners] FreeBSD port build 8.4.3_1 - StackOverflowHook
by Sumit Raja 29 Sep '18

29 Sep '18

> From: Roger Mason <rmason(a)mun.ca> > It isn't clear from your message how you are building ghc. I have > ghc-8.4.3 installed from the port on FreeBSD 11.1 and it works without > issue. > > These two lines: > > > /usr/local/lib/ghc-8.4.2/rts/libHSrts.a(RTS.o): In function `StackOverflowHook': > > /home/ben/bin-dist-8.4.2-FreeBSD/ghc/rts/hooks/StackOverflow.c:16:0: error: > > refer to 8.4.2, which looks a bit fishy to me. It was a standard port build using portsnap and make && make install. Clean install of the ports tree using a fresh ghc to bootstrap worked correctly - I think the ports tree was very broken. -Sumit

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FreeBSD port build 8.4.3_1 - StackOverflowHook
by Sumit Raja 22 Sep '18

22 Sep '18

Hi, I'm unable to get a consistent build of the FreeBSD 8.4.3 port on a 1.1-RELEASE-p11-amd64. I had success with a one build (in a jail) but am now seeing this problem on a VM: gmake --no-print-directory -f ghc.mk phase=1 phase_1_builds HC [stage 0] ghc/stage1/build/tmp/ghc-stage1 Warning: -rtsopts and -with-rtsopts have no effect with -no-hs-main. Call hs_init_ghc() from your main() function to set these options. /usr/local/lib/ghc-8.4.2/rts/libHSrts.a(RTS.o): In function `StackOverflowHook': /home/ben/bin-dist-8.4.2-FreeBSD/ghc/rts/hooks/StackOverflow.c:16:0: error: multiple definition of `StackOverflowHook' ghc/stage1/build/hschooks.o:hschooks.c:(.text+0x0): first defined here collect2: error: ld returned 1 exit status `gcc7' failed in phase `Linker'. (Exit code: 1) gmake[3]: *** [ghc/ghc.mk:111: ghc/stage1/build/tmp/ghc-stage1] Error 1 Thanks Sumit

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Haskell interpretation of names produces error?
by trent shipley 21 Sep '18

21 Sep '18

data Prop = Const Bool | Var Char | Not Prop | And Prop Prop | If Prop Prop | Or Prop Prop | Yff Prop Prop -- Why do I get errors if "Yff" is replaced with "Iff"? | Xor Prop Prop type Assoc k v = [(k, v)] -- Hutton, Graham. Programming in Haskell (p. 93). Cambridge University Press. Kindle Edition. find :: Eq k => k -> Assoc k v -> v find k t = head [v | (k', v) <- t, k == k'] -- Hutton, Graham. Programming in Haskell (p. 93). Cambridge University Press. Kindle Edition. type Subst = Assoc Char Bool -- Hutton, 2016, Ch 8.6 eval :: Subst -> Prop -> Bool eval _ (Const b) = b eval s (Var x) = find x s eval s (Not p) = not (eval s p) eval s (And p q) = eval s p && eval s q eval s (If p q) = eval s p <= eval s q eval s (Or p q) = eval s p || eval s q eval s (Yff p q) = eval s p == eval s q -- Iff produces error here eval s (Xor p q) = eval s p /= eval s q -- Hutton 2016 Ch 8.6 vars :: Prop -> [Char] vars (Const _) = [] vars (Var x) = [x] vars (Not p) = vars p vars (And p q) = vars p ++ vars q vars (If p q) = vars p ++ vars q vars (Or p q) = vars p ++ vars q vars (Yff p q) = vars p ++ vars q -- Iff produces error here vars (Xor p q) = vars p ++ vars q -- Hutton 2016 Ch 8.6

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Re: [Haskell-beginners] Need better explanation of the 'flipThree' example in LYAH
by Francesco Ariis 19 Sep '18

19 Sep '18

Hello Olumide, On Mon, Sep 17, 2018 at 12:15:58AM +0100, Sola Aina wrote: > flatten( fmap (\a -> ( flatten( fmap ( \b -> return [a,b] ) coin ) ) coin ) > > Is this how to proceed and how am I to simplify this expression? I get a slightly different final expression: flatten (fmap (\a -> flatten (fmap (\b -> return [a,b]) coin)) coin) -- notice the lack of `(` before the second `flatten` This could be rewritten as: flatMap (\a -> flatMap (\b -> return [a,b]) coin) coin -- flatMap :: (a1 -> Prob a2) -> Prob a1 -> Prob a2 -- flatMap f x = flatten (fmap f x) which is a bit easier on the eyes. This expression cannot be simplified any further if we don't bring to the table the definition of `coin`!

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Need better explanation of the 'flipThree' example in LYAH
by Olumide 17 Sep '18

17 Sep '18

Dear List, I'm trying to understand the following example from LYAH import Data.List (all) flipThree :: Prob Bool flipThree = do a <- coin b <- coin c <- loadedCoin return (all (==Tails) [a,b,c]) Where import Data.Ratio newtype Prob a = Prob { getProb :: [(a,Rational)] } deriving Show and data Coin = Heads | Tails deriving (Show, Eq) coin :: Prob Coin coin = Prob [(Heads,1%2),(Tails,1%2)] loadedCoin :: Prob Coin loadedCoin = Prob [(Heads,1%10),(Tails,9%10)] The result: ghci> getProb flipThree [(False,1 % 40),(False,9 % 40),(False,1 % 40),(False,9 % 40), (False,1 % 40),(False,9 % 40),(False,1 % 40),(True,9 % 40)] See http://learnyouahaskell.com/for-a-few-monads-more#making-monads. My understanding of what's going on here is sketchy at best. One of several explanations that I am considering is that all combination of a, b and c are evaluated in (==Tails) [a,b,c] but I cannot explain how the all function creates 'fuses' the list [f a, f b, f c]. I know that all f xs = and . map f xs (the definition on hackage is a lot more complicated) but, again, I cannot explain how the and function 'fuses' the list [f a, f b, f c]. If I'm on the right track I realize that I'm going to have to study the list the between list comprehensions and the do-notation in order how all the return function create one Prob. Regards, - Olumide

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Hutton Ex 8.9.7
by trent shipley 15 Sep '18

15 Sep '18

I couldn't get close on my own. From: https://github.com/pankajgodbole/hutton/blob/master/exercises.hs {- 7. Complete the following instance declarations: instance Eq a => Eq (Maybe a) where ... instance Eq a => Eq [a] where ... -} -- suggested answer instance Eq a => Eq (Maybe a) where -- Defines the (==) operation. Nothing == Nothing = True Just == Just = True -- why isn't this Just a == Just a ? -- My guess is that a and Just a are different types and can't be == in Haskell _ == _ = False instance Eq a => Eq [a] where -- Defines the (==) operation. [] == [] = True [x] == [y] = x == y (x:xs) == (y:ys) = x==y && xs==ys -- I assume this is implicitly recursive. _ == _ = False

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Hutton Ex 8.9.5 & Ex 8.9.6
by trent shipley 14 Sep '18

14 Sep '18

Pankaj Godbole's code examples for 8.9.6 are sufficiently more complex than mine,that I am worried I missed an obvious bug I'm not experience enough to know about or test for. Correction or reassurances would be appreciated. Regards, Trent {-- 5. Given the type declaration data Expr = Val Int | Add Expr Expr define a higher-order function folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a such that folde f g replaces each Val constructor in an expression by the function f, and each Add constructor by the function g. Hutton, Graham. Programming in Haskell (Kindle Locations 3364-3371). Cambridge University Press. Kindle Edition. --} data Expr = Val Int | Add Expr Expr -- Trent's stuff folde :: (Int -> a) -> (a -> a -> a) -> Expr -> a folde f g (Val x) = f x folde f g (Add left right) = g (folde f g left) (folde f g right) {-- 6. Using folde, define a function eval :: Expr -> Int that evaluates an expression to an integer value, and a function size :: Expr -> Int that calculates the number of values in an expression. Hutton, Graham. Programming in Haskell (Kindle Locations 3372-3374). Cambridge University Press. Kindle Edition. --} -- Trent's stuff eval :: Expr -> Int eval = folde id (+) size :: Expr -> Int size = folde (\_ -> 1) (+) -- hlint suggests \_ as const 1 test0 :: Expr test0 = Val 1 test1 :: Expr test1 = Add (Val 1) (Val 2) test3 :: Expr test3 = Add (Val 1) (Add (Val 2) (Val 3)) -- the below are from: -- Pankaj Godbole -- https://github.com/pankajgodbole/hutton/blob/master/exercises.hs evalE :: Expr -> Int evalE (Val n) = n evalE (Add e1 e2) = folde toEnum (+) (Add e1 e2) -- doesn't a Val have to prefix an int? Then why is f "toEnum"? numVals :: Expr -> Int numVals (Val n) = 1 numVals (Add e1 e2) = numVals e1 + numVals e2

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Why Integral makes a function much slower than Int?
by Jaakko Luttinen 08 Sep '18

08 Sep '18

Hi all, I implemented a simple function to calculate the length of a list. However, I noticed that the speed of the function changes significantly if I make I minor change to the type signature but keep the implementation the same otherwise. I was wondering why this happens. Contents of mylength.hs: module MyLength where -- Here is the first implementation: myLength :: Integral n => [a] -> n myLength xs = run xs 0 where run [] n = n run (_:ys) n = run ys (n+1) -- Then, the other implementation just fixes the return type without -- modifying the actual implementation: myLength' :: [a] -> Int myLength' = myLength When I compile these implementations with: ghc -dynamic -O2 mylength.hs Then, launch ghci: ghci -fobject-code And in ghci: :load mylength.hs :set +s (myLength [1..10000000]) :: Int 10000000 (2.30 secs, 1,612,737,944 bytes) (myLength' [1..10000000]) :: Int 10000000 (0.38 secs, 720,077,536 bytes) So, the first implementation is much worse, although I've fixed the return type to Int too. What's going on? Does this mean one shouldn't use Integral but Int instead most of the time? Thanks for help! Regards, Jaakko

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