- Beginners - Haskell.org

Record types with multiple constructors
by Derek McLoughlin 08 Dec '14

08 Dec '14

Hi, Record types usually have a single constructor. I've even seen blog posts that suggest that they must have a single constructor. However, multiple constructors are allowed: data Employee = RegularEmployee { name :: String } | Supervisor { name :: String, salesTarget :: Double } Manager { name :: String, salesTarget :: Double budget :: Double } I don't see this used much in Haskell code - either in explanatory books/tutorials or in code I've examined on GitHub. Are there drawbacks to using multiple constructors in this way? Derek.

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Use Lens to manipulate Data.Tree
by Zach Moazeni 08 Dec '14

08 Dec '14

Hello, I'm trying to understand how I would manipulate a Data.Tree in Haskell. I came across this StackOverflow answer and accompanying blog post, but it doesn't run locally for me: * http://stackoverflow.com/a/15489761/410759 * https://www.fpcomplete.com/user/davorak/code-snippets/zipper-tree-examples Here's a gist with the code and the error I'm seeing: https://gist.github.com/zmoazeni/0049f3ab365ae600b131 I just want to start slow and be able to do three actions: * add +10 to all values in the tree * add a node to an existing node's subForest * remove a node (along with its subForest) from the tree I understand that this won't actually "edit in place" and my code would need to call a function which would return an updated version of the original tree. Finally, Lens wasn't my first choice. I was trying to understand Zippers from http://learnyouahaskell.com/zippers and my brain seized. I'm just falling back to Lens because it seems like the simplest/dsl-ish way to manipulate structures like this. Thanks for any help! -Zach

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haskell Time
by Claudia Weber 02 Dec '14

02 Dec '14

Hi guys, I'm new to Haskell and I'm just trying out some easy functions right now, but I'm having some trouble here... I want to display a clock type Time = (Int, Int) (hoursx,minutesy) x ={0,1,...,23} y={0,1,...,59} where I can add hours and minutes or set back time. If its 23,58 for example and I want to add 35 minutes, the clock has to set to 0,33 not 24,23. Because I'm just getting started with Haskell, I have nooo idea what to do. Would be great if someone could help me out with this :) --- Alle Postfächer an einem Ort. Jetzt wechseln und E-Mail-Adresse mitnehmen! Rundum glücklich mit freenetMail

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building ghc for cross-platform raspberry pi (ubuntu)
by Miro Karpis 25 Nov '14

25 Nov '14

Hi, I'm trying to build ghc for raspberry pi, but I'm having problem with .configure. I'm following these instructions from ( https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/RaspberryPi) I have the tools installed and they should be also in my PATH,..... Please any ideas? ghc> ./configure --target=arm-linux-gnueabihf --enable-unregisterised checking for gfind... no checking for find... /usr/bin/find checking for sort... /usr/bin/sort checking for GHC version date... inferred 7.9.20141121 checking for ghc... /usr/bin/ghc checking version of ghc... 7.6.3 checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... arm-unknown-linux-gnueabihf Build platform inferred as: x86_64-unknown-linux Host platform inferred as: x86_64-unknown-linux GHC build : x86_64-unknown-linux GHC host : x86_64-unknown-linux GHC target : arm-unknown-linux configure: Building in-tree ghc-pwd checking for path to top of build tree... /home/parallels/code/ghc checking for gcc... gcc checking whether the C compiler works... yes checking for C compiler default output file name... a.out checking for suffix of executables... checking whether we are cross compiling... no checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking how to run the C preprocessor... gcc -E checking for grep that handles long lines and -e... /bin/grep checking for egrep... /bin/grep -E checking for ANSI C header files... yes checking for sys/types.h... yes checking for sys/stat.h... yes checking for stdlib.h... yes checking for string.h... yes checking for memory.h... yes checking for strings.h... yes checking for inttypes.h... yes checking for stdint.h... yes checking for unistd.h... yes checking for arm-linux-gnueabihf-gcc... /home/parallels/code/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-gcc checking for arm-linux-gnueabihf-ld... /home/parallels/code/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-ld checking for arm-linux-gnueabihf-nm... /home/parallels/code/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-nm checking for arm-linux-gnueabihf-ar... /home/parallels/code/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-ar checking for arm-linux-gnueabihf-ranlib... /home/parallels/code/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian/bin/arm-linux-gnueabihf-ranlib checking for llc... no checking for opt... no checking whether #! works in shell scripts... yes checking for perl... /usr/bin/perl checking version of gcc... configure: error: Need at least gcc version 3.0 (3.4+ recommended) Regards, Miro

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IntelliJ IDEA Haskell
by Andrew Bernard 20 Nov '14

20 Nov '14

Greetings List, I’d like to try IntelliJ IDEA for Haskell. There seem to be many plugins in various stages of maturity. Which one is recommended? Is this a good environment for Haskell development? Andrew

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[Haskell] ANNOUNCE: Haskell Communities and Activities Report (27th ed., November 2014)
by Mihai Maruseac 18 Nov '14

18 Nov '14

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (27th edition, November 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2014/report.pdf http://haskell.org/communities/11-2014/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of April 2015: target deadline for contributions to the May 2015 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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Unable to sequence using 'do' operator
by Rohit Sharma 18 Nov '14

18 Nov '14

type Parser a = String -> [(a, String)] Hi, I am learning haskell and wondering why my definition of firstAndThird does not work with the do operator, however when i try to use the same using bind (firstandThird2) it works as expected. I basically want to return a tuple of first and third character of a string. Can someone please correct me what i might be overseeing? I have pasted the same code on codetidy in case if the email lose formatting. http://codetidy.com/5726/ Many Thanks, Rohit zero :: Parser a zero = \inp -> [] result :: a -> Parser a result x = \inp -> [(x, inp)] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] bind :: Parser a -> (a -> Parser b) -> Parser b p `bind` f = \inp -> concat [ ((f x) inp') | (x, inp') <- p inp] sat :: (Char -> Bool) -> Parser Char sat predicate = item `bind` (\x -> if predicate x then result x else zero ) lower :: Parser Char lower = sat (\x -> 'a' <= x && x <= 'z') firstAndThird :: Parser (Char, Char) firstAndThird = do x <- item item y <- item return (x,y) firstAndThird2 :: Parser (Char, Char) firstAndThird2 = item `bind` \x -> item `bind` \y -> item `bind` \z -> result (x,z)

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Bind parser returning wrong return type
by Rohit Sharma 17 Nov '14

17 Nov '14

Hi All, Sorry if this is a very lame question but i am a beginner and much appreciate if some one could correct me? I am reading haskell book and curious why the return type of the bind operator look odd to me For the given definitions * type Parser a = String -> [(a, String)]* * item :: Parser Char* * item = \inp -> case inp of * * [] -> []* * (x:xs) -> [(x,xs)]* * bind :: Parser a -> (a -> Parser b) -> Parser b* * p `bind` f = \inp -> concat [ f x inp' | (x, inp') <- p inp]* when I define z in GHCI as * let z = item `bind` (\x -> (\y -> result (x,y))) "Rohit"* the return type is * >> :t z* * z :: Parser ([Char], Char)* Question: (1) Shouldn't the return type of (Char, [Char])? looking at the list comprehension, "(x, inp') <- p inp" should yield -> "('r', "ohit")". Next f x inp' is left associative, so f x should yield character 'r' and pass to the lambda that should return result tuple ('r', "ohit"), but why is it that z type is ([Char], char) :: (x,y) (2) How can i print the value of z in the above case on the ghci Many thanks, Rohit

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latest Haskell Platform build fails
by Michael L Martin 13 Nov '14

13 Nov '14

Hello all, I am trying to install the latest version of Haskell Platform and am having a problem. I created a new VM and installed Ubuntu 14.04 server and applied the latest updates. Also installed packages "build-essential" and "zlib1g-dev". I then downloaded and installed ghc-7.8.3 from https://www.haskell.org/ghc/download_ghc_7_8_3#x86_64linux and then downloaded and installed cabal 1.20.0.3 from http://hackage.haskell.org/package/cabal-install. After installing cabal, I did a "cabal update" followed by "cabal install hscolour". Then downloaded the latest Haskell Platform tarball (haskell-platform-2014.2.0.0-srcdist.tar.gz) and attempted to install like this: ./platform.sh ../ghc-7.8.3-x86_64-unknown-linux-deb7.tar.bz2 This fails like this: *** *** Building hptool *** Building hptool-0.1... Preprocessing executable 'hptool' for hptool-0.1... *** *** Running hptool *** # rm (for build/.markers/rel/build/target/usr/local/haskell/ghc-7.8.3-x86_64/lib/happy-1.19.4) # rm (for build/.markers/rel/build/package/happy-1.19.4/build) happy-1.19.4 needs mtl-2.1.3.1 >>> Building happy-1.19.4 # cp (for build/.markers/rel/build/package/happy-1.19.4/build) # rm (for build/.markers/rel/build/package/happy-1.19.4/build) # ghc-pkg (for build/.markers/rel/build/package/happy-1.19.4/build) # ghc-pkg (for build/.markers/rel/build/package/happy-1.19.4/build) Reading package info from "build/package/mtl-2.1.3.1/inplace.conf" ... done. mtl-2.1.3.1: package(s) with this id already exist: mtl-2.1.3.1 Error when running Shake build system: * build-all * build-product * build/product/haskell-platform-2014.2.0.0-unknown-posix-x86_64.tar.gz * build/target * build/.markers/rel/build/target/usr/local/haskell/ghc-7.8.3-x86_64/lib/happy-1.19.4 * build/.markers/rel/build/package/happy-1.19.4/build Development.Shake.command, system command failed Command: /home/mmartin/Downloads/haskell-platform-2014.2.0.0/build/ghc-bindist/local/bin/ghc-pkg register --package-db=build/package/happy-1.19.4/package.conf.d --verbose=1 build/package/mtl-2.1.3.1/inplace.conf Exit code: 1 Stderr: mtl-2.1.3.1: package(s) with this id already exist: mtl-2.1.3.1 Any suggestions on how to get past this? Thanks, Michael Martin

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Finding Keith Numbers with Haskell
by Jack Mott 12 Nov '14

12 Nov '14

I'm on day 2 of learning Haskell, and decided to write a program to find Keith Numbers as my first project. So far so good, I have some code that does the job (see below). First I would just like to ask others to look it over and let me know if I am doing anything particularly wrong or not advised here. Also, I am using the "isKeithHelper" function because I didn't know how to make use of the filter function directly with isKeith, because isKeith takes two parameters. I'm pretty sure there is a better way to handle that, any tips? Lastly, I'd like to make this as efficient as possible. At the moment I am adding [s] to the end of the list in the isKeith function, which I know is O(n) instead of O(1). I don't think there is a way to avoid either that or having to use drop which would have the same downside. Is this a case where a Sequence or Vector might be more useful? How would I convert this to use a Sequence instead? import Data.Time.Clock.POSIX import Data.Digits isKeith :: Integer -> [Integer] -> Bool isKeith n l | s == n = True | s > n = False | otherwise = isKeith n x (l ++ [s]) where s = sum l isKeithHelper :: Integer -> Bool isKeithHelper n = isKeith n (digits 10 n) main :: IO () main = do start <- getPOSIXTime print (filter isKeithHelper [1000..9999]) end <- getPOSIXTime print (end - start)

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