October 2014 - Beginners

abstracting parametrized record types
by Alia 29 Apr '15

29 Apr '15

Hi Folks, Here's a problem that's frustrating me: I have defined a parametrized record type which looks like this: data Job a = Job { jobID :: String , jobName :: String , jobTable :: String , jobSource :: IO [a] , jobProcessors :: [a -> a] } where a is a data model type that maps on to a database table, for example: data Person = Person { name :: String , age :: Int } deriving (Show) data Car = Car { brand :: String , value :: Double , year :: Int } deriving (Show) I would like to define a higher-level record type which contains, for configuration purposes, a certain set of jobs for execution. Let's say we call it a JobSet and which could possibly look like this: data JobSet = JobSet { jobsetID :: String , jobsetName :: String , jobs :: [Job] <-- yes I know this is not legal } Is there a legal haskell way to achieve the above objective without having to do something like this which hardcodes the model type into the jobset schema? data JobSet = JobSet { jobsetID :: String , jobsetName :: String , personJobs :: [Job Person] , carJobs :: [Job Car] } Many thanks for any enlightenment on this front. Alia

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latest Haskell Platform build fails
by Michael L Martin 12 Nov '14

12 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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Installation problems
by Jeffrey Brown 28 Oct '14

28 Oct '14

Hi, In trying to install Tidal, following something that worked for someone else who was getting an error similar to one I got, I ran ghc-clang-wrapper <https://gist.github.com/mzero/7245290#file-ghc-clang-wrapper>. I believe it was after that that cabal install stopped working, and I lost a lot of libraries that used to exist, such as Sound.MIDI: GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> import Sound.MIDI <no location info>: Could not find module ‘Sound.MIDI’ It is not a module in the current program, or in any known package. Prelude> When I try to reinstall a library that used to exist -- e.g. "hosc", below -- via cabal install, I now get a giant slew of failures. Any ideas? Many thanks, Jeff sh-3.2# cabal install hosc Resolving dependencies... In order, the following will be installed: binary-0.7.2.2 (new version) data-binary-ieee754-0.4.4 (reinstall) changes: base-4.7.0.1 added, binary-0.7.2.2 added network-2.6.0.2 (new package) text-1.2.0.0 (new package) blaze-builder-0.3.3.4 (reinstall) changes: base-4.7.0.1 added, bytestring-0.10.4.0 added, text-1.2.0.0 added hosc-0.15 (new version) Warning: Note that reinstalls are always dangerous. Continuing anyway... Downloading binary-0.7.2.2... Configuring network-2.6.0.2... Configuring text-1.2.0.0... Failed to install network-2.6.0.2 Build log ( /var/root/.cabal/logs/network-2.6.0.2.log ): Failed to install text-1.2.0.0 Build log ( /var/root/.cabal/logs/text-1.2.0.0.log ): Configuring binary-0.7.2.2... Failed to install binary-0.7.2.2 Build log ( /var/root/.cabal/logs/binary-0.7.2.2.log ): cabal: Error: some packages failed to install: binary-0.7.2.2 failed during the configure step. The exception was: user error (<command line>: cannot satisfy -package-id Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff: Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff is unusable due to missing or recursive dependencies: base-4.7.0.1-df210ede1eb79477fef5662549c32927 bytestring-0.10.4.0-4aa78c8ca7b6b65993eefc131f7d94fa containers-0.5.5.1-0d8db9193d3e3371e0142bcc8a4a0721 deepseq-1.3.0.2-8f63133c1b77f3b3190f04893cf340e4 directory-1.2.1.0-af5afa2b9b551d3fec1a32d6bfd8bd72 process-1.2.0.0-3a472e9c66165e506513ea8f145681a0 time-1.4.2-d6766dce59812a4b19375d9595549a8b unix-2.7.0.1-8adde3f1f2079286da56b30898c2d703 (use -v for more information) ) blaze-builder-0.3.3.4 depends on text-1.2.0.0 which failed to install. data-binary-ieee754-0.4.4 depends on binary-0.7.2.2 which failed to install. hosc-0.15 depends on binary-0.7.2.2 which failed to install. network-2.6.0.2 failed during the configure step. The exception was: user error (<command line>: cannot satisfy -package-id Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff: Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff is unusable due to missing or recursive dependencies: base-4.7.0.1-df210ede1eb79477fef5662549c32927 bytestring-0.10.4.0-4aa78c8ca7b6b65993eefc131f7d94fa containers-0.5.5.1-0d8db9193d3e3371e0142bcc8a4a0721 deepseq-1.3.0.2-8f63133c1b77f3b3190f04893cf340e4 directory-1.2.1.0-af5afa2b9b551d3fec1a32d6bfd8bd72 process-1.2.0.0-3a472e9c66165e506513ea8f145681a0 time-1.4.2-d6766dce59812a4b19375d9595549a8b unix-2.7.0.1-8adde3f1f2079286da56b30898c2d703 (use -v for more information) ) text-1.2.0.0 failed during the configure step. The exception was: user error (<command line>: cannot satisfy -package-id Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff: Cabal-1.20.0.2-b862897337f86101d0e1a35135984eff is unusable due to missing or recursive dependencies: base-4.7.0.1-df210ede1eb79477fef5662549c32927 bytestring-0.10.4.0-4aa78c8ca7b6b65993eefc131f7d94fa containers-0.5.5.1-0d8db9193d3e3371e0142bcc8a4a0721 deepseq-1.3.0.2-8f63133c1b77f3b3190f04893cf340e4 directory-1.2.1.0-af5afa2b9b551d3fec1a32d6bfd8bd72 process-1.2.0.0-3a472e9c66165e506513ea8f145681a0 time-1.4.2-d6766dce59812a4b19375d9595549a8b unix-2.7.0.1-8adde3f1f2079286da56b30898c2d703 (use -v for more information) )

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Haskell typing question
by Cui Liqiang 28 Oct '14

28 Oct '14

Thanks for your help, your suggestion works. But I still don’t quite understand. In the following line: caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)), After applying digitToInt, the type of ‘x’ in the expression above is Int indeed, but Haskell consider the ’10.0’ to be a Int, is it? ---------------------------------------------------------------------------------------------------------------------------- Hi, I am doing an exercise in Haskell, which is converting a string like ?$123.312? to double value. Below is my code: module Main where import Data.Char caluInt l = foldl1 (\acc x -> acc * 10 + x) (map digitToInt l) caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)) convert(x:xs) = let num = [e | e <- xs, e /= ','] intPart = takeWhile (/='.') num decimalPart = tail(dropWhile (/='.') num) in (caluInt intPart) + (caluDecimal decimalPart) And I got an error in this line: caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)), which says: No instance for (Fractional Int) arising from a use of `/' Possible fix: add an instance declaration for (Fractional Int) In the first argument of `(+)', namely `acc / 10.0' In the expression: acc / 10.0 + x In the first argument of `foldr1', namely `(\ x acc -> acc / 10.0 + x)' Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell I want a Fractional? -- Cui Liqiang > Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell > I want a Fractional? Because digitToInt means exactly what it says. If you want it to become something other than Int, apply fromIntegral to its result.

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Cabal-install of local package
by Stefan Höck 28 Oct '14

28 Oct '14

Dear list Like many other I seem to be struggling with some of the concepts of cabal and cabal-install. For the records: I use cabal-install 1.20.0.3 and ghc 7.8.3 on arch linux. For starters, I implemented my own idea of an improved prelude, called the package efa-prelude and the module it exports Efa.Prelude. I then built the package and installed it using `cabal install --user`. This worked and a folder for the library was created at "$HOME/.cabal/lib/x86_64-linux-ghc-7.8.3/efa-prelude-0.1.0". I also registered the library using `cabal register --user` and now cabal prints some information about the package when I type `cabal info efa-prelude`. I am also able to import Efa.Prelude into ghci. However, when I now try to use this library in another local project using a sandbox this time, I can build the project but when I try to install it into the sandbox I get the following error message: Resolving dependencies... cabal: Could not resolve dependencies: trying: test-0.1.0 (user goal) next goal: efa-prelude (dependency of test-0.1.0) Dependency tree exhaustively searched. I just tried and the install works if I do it outside of a sandbox. Obviously I am missing something crucial here. Could somebody please point me in the right direction what is needed to make my own packages available in a sanboxed install? Thanks for your time Stefan

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error compiling haskell-src-1.0.1.4
by Jason T. Slack-Moehrle 27 Oct '14

27 Oct '14

I am trying to compile haskell-platform on an AWS instance. The version: haskell-platform-2011.2.0.0 It was going OK, but now I am getting an error I dont know how to solve: Building haskell-src-1.0.1.4... [1 of 6] Compiling Language.Haskell.Syntax ( Language/Haskell/Syntax.hs, dist/build/Language/Haskell/Syntax.o ) [2 of 6] Compiling Language.Haskell.Pretty ( Language/Haskell/Pretty.hs, dist/build/Language/Haskell/Pretty.o ) [3 of 6] Compiling Language.Haskell.ParseMonad ( Language/Haskell/ParseMonad.hs, dist/build/Language/Haskell/ParseMonad.o ) [4 of 6] Compiling Language.Haskell.ParseUtils ( Language/Haskell/ParseUtils.hs, dist/build/Language/Haskell/ParseUtils.o ) [5 of 6] Compiling Language.Haskell.Lexer ( Language/Haskell/Lexer.hs, dist/build/Language/Haskell/Lexer.o ) [6 of 6] Compiling Language.Haskell.Parser ( dist/build/Language/Haskell/Parser.hs, dist/build/Language/Haskell/Parser.o ) dist/build/Language/Haskell/Parser.hs:0:36: The function `runParserWithMode' is applied to 8 arguments, but its type `ParseMode -> P a0 -> String -> ParseResult a0' has only three In the expression: runParserWithMode mode parse 1 3 4 3 4 2 In an equation for `parseModuleWithMode': parseModuleWithMode mode = runParserWithMode mode parse 1 3 4 3 4 2 Error: Building the haskell-src-1.0.1.4 package failed make: *** [build.stamp] Error 2 Can anyone help me get past this? Best, Jason

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can somebody explain the type of this expression?
by Ovidiu Deac 27 Oct '14

27 Oct '14

Prelude> let f x = x * 2 Prelude> :t f f :: Num a => a -> a The typeclass Num is defined like this: class Num a where (+), (-), (*) :: a -> a -> a ... ...which means that the operator (*) expects two parameters of type a and returns a value of type a. Since the definition of expr looks like this: Prelude> let f x = x * 2 ...and 2 is an Int, I would expect that the type inferred for (*) is (Int -> Int -> Int) and thus f should be (Int -> Int) Can somebody explain this? Thanks!

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Lazy state + IO not lazy?
by Jan Snajder 27 Oct '14

27 Oct '14

Dear Haskellers, I'm confused with the behaviour of a lazy state monad when combined with an IO monad. Look at this: ghci> Control.Monad.State.Lazy.evalState (head `fmap` mapM return (1:undefined)) 0 1 ghci> Control.Monad.State.Lazy.evalStateT (head `fmap` mapM return (1:undefined)) 0 *** Exception: Prelude.undefined I would have expected the second case to behave identically to the first case. Why is this not the case? Cheers, Jan

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Haskell typing question
by Cui Liqiang 26 Oct '14

26 Oct '14

Hi, I am doing an exercise in Haskell, which is converting a string like “$123.312” to double value. Below is my code: module Main where import Data.Char caluInt l = foldl1 (\acc x -> acc * 10 + x) (map digitToInt l) caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)) convert(x:xs) = let num = [e | e <- xs, e /= ','] intPart = takeWhile (/='.') num decimalPart = tail(dropWhile (/='.') num) in (caluInt intPart) + (caluDecimal decimalPart) And I got an error in this line: caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)), which says: No instance for (Fractional Int) arising from a use of `/' Possible fix: add an instance declaration for (Fractional Int) In the first argument of `(+)', namely `acc / 10.0' In the expression: acc / 10.0 + x In the first argument of `foldr1', namely `(\ x acc -> acc / 10.0 + x)' Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell I want a Fractional? -- Cui Liqiang

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Use mysql-simple (OS X and dymanic library trouble)
by Christian Sperandio 26 Oct '14

26 Oct '14

Hi, I’d like to use Haskell for some processing on a MySQL database. Currently, I use Java program to do that but I’d like to change.And an important information, I’m working on OS X Maverick and I use EclipseFP. I installed mysql-simple package. I had some troubles. First, I had to download pcre to get a pcre.h I put into the /usr/include dir. After, to link the mysql-simple package I did an export LD_LIBRARY_PATH=/usr/local/mysql-5.6.11-osx10.7-x86_64/lib. Thus, the library libmysqlclient.18.dylib was found. All stuff above were done by command line. Now, I want to code a simple program: module Main where import Database.MySQL.Simple main::IO() main = undefined hello :: IO Int hello = do conn <- connect defaultConnectInfo [Only i] <- query_ conn "select 2 + 2" return i But when I try to launch a repl from EclipseFP, I’ve got this error: Loading package mysql-0.1.1.7 ... <command line>: can't load .so/.DLL for: /Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib (dlopen(/Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib, 9): Library not loaded: libmysqlclient.18.dylib Referenced from: /Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib Reason: image not found) How can I set the place of this library ? I read some stuff on the web but I don’t understand everything :( Thanks for your help. Chris

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