- Beginners - Haskell.org

LTS is good but...
by emacstheviking 04 Mar '15

04 Mar '15

I started an OpenGL project within an LTS sandbox as guided on another post and, somewhat foolishly it would see, I did a "cabal install cabal-install" when prompted that a new version was available. The very next time I tried to work on my code.... I got this: bash-3.2$ pwd /Users/seancharles/Documents/Coding/haskell/lts1 bash-3.2$ cabal build cabal: You need to re-run the 'configure' command. The version of Cabal being used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0). bash-3.2$ cabal configure Resolving dependencies... Configuring lts1-0.1.0.0... cabal: At least the following dependencies are missing: base ==4.7.* bash-3.2$ cabal build cabal: You need to re-run the 'configure' command. The version of Cabal being used has changed (was Cabal-1.20.0.1, now Cabal-1.16.0). bash-3.2$ ls -l total 112 -rw-r--r-- 1 seancharles staff 19 Mar 2 12:53 LICENCSE -rw-r--r-- 1 seancharles staff 552 Mar 3 20:09 Main.hs -rw-r--r-- 1 seancharles staff 477 Mar 3 20:07 Main.hs~ -rw-r--r-- 1 seancharles staff 46 Mar 2 12:52 Setup.hs -rw-r--r-- 1 seancharles staff 30198 Mar 2 12:51 cabal.config -rw-r--r-- 1 seancharles staff 1090 Mar 2 12:51 cabal.sandbox.config drwxr-xr-x 5 seancharles staff 170 Mar 2 12:53 dist -rw-r--r-- 1 seancharles staff 1932 Mar 2 12:52 lts1.cabal bash-3.2$ So, welcome to a different kind of hell, "cabal version hell" perhaps? Can anybody help me get it running again? Thanks. Sean.

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Alfred Programs in Haskell
by Richard Guay 04 Mar '15

04 Mar '15

Hi, I am using Haskell to write Alfred programs, but I have run into an issue I just can not fix. The routine for reading a data file in a application specific location for Alfred never detects the file as non-existant. It then dies on the read. Here is the code: getAlfredCacheFileContents :: String -> IO (String) getAlfredCacheFileContents fileName = do h <- getHomeDirectory fExist <- doesFileExist $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName if fExist then do contents <- readFile $ h ++ cacheDirBasic ++ getBundleID ++ "/" ++ fileName return contents else return "" I just installed the latest version on my Mac Air. It always does the call to readFile, even when the file doesn't exist. -- Sent with Postbox <http://www.getpostbox.com>

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GHC 7.8.4 source build fails on OSX...
by emacstheviking 04 Mar '15

04 Mar '15

ld: couldn't dlopen() /usr/lib/libdtrace.dylib: dlopen(/usr/lib/libdtrace.dylib, 1): Symbol not found: _CGLGetCurrentContext Referenced from: /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo Expected in: /System/Library/Frameworks/OpenGL.framework/Versions/A/OpenGL in /System/Library/Frameworks/CoreVideo.framework/Versions/A/CoreVideo for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) make[1]: *** [rts/dist/build/libHSrts-ghc7.8.4.dylib] Error 1 make: *** [all] Error 2 iMac:ghc-7.8.4 vosabristol$ Perhaps I need to tweak the configure parameters and try again... stackoverflow has this page: http://stackoverflow.com/questions/19480099/compiling-macos-application-wit… There is no CGL framework in OS X (CGL is included in the OpenGL framework, you need only #include OpenGL/OpenGL.h), and you do not use -lXXX to link against a framework either. Now, depending on how your Makefile is structured, you may also have a property known as LD_FLAGS for a separate linking stage - you *also* need to include -framework OpenGL there. This directive does *both*, include path resolution and linker path resolution for frameworks. – Andon M. Coleman <http://stackoverflow.com/users/2423205/andon-m-coleman> Oct 20 '13 at 19:36 <http://stackoverflow.com/questions/19480099/compiling-macos-application-wit…>

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command-line interaction with running process
by Elise Huard 04 Mar '15

04 Mar '15

Hi, I'm trying to do the following: implementing a command-line interface to directly interact with the state or the parameters of a looping/running process (a game, to be specific). My first thought is to fork a thread, passing in a TChan/TVar or similar to enable communication, and to have the command-line interface on the thread (getLine or haskelline ...). Another option would be to have a bona fide server-client interface (with sockets or others), to fork a thread again to run the server and to connect using the client library - which would allow me to use ghci and have transparent serialization under the hood. Are there known use cases? Am I missing something, for instance is it possible to interact with a running process directly using ghci? Any tips? Thank you, Elise

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Google Summer of Code
by Michael Jones 04 Mar '15

04 Mar '15

If any students are interested in Google Summer of Code projects, there is a Minnow Board project here: http://elinux.org/Minnowboard:GSoC2015 There is a Haskell project listed in the SMBus/PMBus section. Mike

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fractal x = fractal [y - head x + z | y <- x, z <- x] with take doesnt end
by '2+ 03 Mar '15

03 Mar '15

hi! since my fractal :: [Int] -> [Int] fractal x = [y - head x + z | y <- x, z <- x] seems to be working fine with something like: take 300 $ cycle $ fractal $ fractal $ [11..17] ++ [55..77] wanted to twist the function to something totally recursive but: fractal x = fractal [y - head x + z | y <- x, z <- x] experimented with: take 300 $ fractal $ [11..17] ++ [55..77] doesnt end its process and eat up the RAM any hints to solve this issue? thank you ----- 2g --- http://sarigama.namaste.jp

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FFI Problems
by Thomas Jakway 03 Mar '15

03 Mar '15

I'm very new to Haskell and am trying to write a "real" program to motivate myself to learn it better (so far I've only gotten through Project Euler problems after reading LYAH and most of RWH). I'm using Taglib (https://github.com/taglib/taglib) to read the metadata from a music file and print it. I have a struct C-side (with C linkage) serving as the bridge between Taglib's C++ and Haskell's FFI. A small demo program (compiled with gcc and linked against the C++ object files) gives the correct results, but Haskell is weirdly only getting /some /of it right. Specifically, the C string fields are working but ints are not. The output from the C demo (what Haskell should be printing): music_metadata title: It's My Life, artist: Bon Jovi, album: Bon Jovi Greatest Hits - The Ultimate Collection comment: , genre: Rock, track: 3, length: 224, bitrate: 256, channels: 2, codec: 768 The output from Haskell: MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate = 8589934848, channels = 12884901890, track = 8589934848, title = "It's My Life", artist = "Bon Jovi", album = "Bon Jovi Greatest Hits - The Ultimate Collection", comment = "", genre = "Rock"} I would have expected it to work or not work at all, but did not anticipate getting only some of it right. I was going to include snippets from my hsc file but given how new I am to Haskell I don't trust myself to assume where the problem is, so sorry if this is way too long: {-# LANGUAGE CPP, ForeignFunctionInterface #-} module MusicReader ( Codec, MusicMetadata, readMusicMetadata ) where import Control.Monad import Foreign import Foreign.C.Types import Foreign.C.String import System.IO.Unsafe as Unsafe #include "CodecDefines.h" #include "MusicReader.h" constantToCodec code | code == mp3 = MP3 | code == flac = FLAC | code == ogg = OGG_VORBIS | code == mp4 = MP4 | code == mpeg = MPEG | code == none = NONE | code == unknown = UNKNOWN | otherwise = UNKNOWN where mp3 = #const MP3_CODEC flac = #const FLAC_CODEC ogg = #const OGG_VORBIS_CODEC mp4 = #const MP4_CODEC mpeg = #const MPEG_CODEC none = #const NO_EXTENSION unknown = #const UNKNOWN_EXTENSION data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN deriving (Show) data MusicMetadata = MusicMetadata { codec :: Codec, length :: Int, bitrate :: Int, channels :: Int, track :: Int, title :: String, artist :: String, album :: String, comment :: String, genre :: String } deriving (Show) instance Storable MusicMetadata where sizeOf _ = (#size struct music_metadata) alignment _ = alignment (undefined::CDouble) peek a = do codec <- liftM constantToCodec $ (((#peek struct music_metadata, codec) a) :: IO Int) length <- ((#peek struct music_metadata, length) a) :: IO Int bitrate <- ((#peek struct music_metadata, bitrate) a) :: IO Int channels <- ((#peek struct music_metadata, channels) a) :: IO Int track <- ((#peek struct music_metadata, bitrate) a) :: IO Int title <- ((#peek struct music_metadata, title) a) :: IO CString artist <- ((#peek struct music_metadata, artist) a) :: IO CString album <- ((#peek struct music_metadata, album) a) :: IO CString comment <- ((#peek struct music_metadata, comment) a) :: IO CString genre <- ((#peek struct music_metadata, genre) a) :: IO CString --FIXME: find replacement for temporary names marshalledTitle <- peekCString title marshalledArtist <- peekCString artist marshalledAlbum <- peekCString album marshalledComment <- peekCString comment marshalledGenre <- peekCString genre return (MusicMetadata codec length bitrate channels track marshalledTitle marshalledArtist marshalledAlbum marshalledComment marshalledGenre) poke a = undefined --This is the "primitive" FFI call--calls the C function and gets a pointer --in return --TODO: write a higher level function this module should export that calls --primReadMusicMetadata and converts the C Pointer into the Haskell data --MusicMetadata foreign import ccall unsafe "read_metadata" primReadMusicMetadata :: CString -> IO (Ptr MusicMetadata) --convert the Haskell string to a CString, call into the FFI then --dereference the resulting pointer readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $ primReadMusicMetadata cs) Here's the struct in MusicReader.h (in an extern C block): struct music_metadata { int codec; int length, bitrate, channels; int track; char *title, *artist, *album, *comment, *genre; }; with the corresponding call: struct music_metadata* read_metadata(char*); I've tried playing around with the alignment but it didn't do anything. I also tried declaring the struct's fields as int32_t which also did nothing. The C demo in question is a very simple: #include <stdio.h> #include "MusicReader.h" #define FILENAME "Its_My_Life.m4a" int main() { struct music_metadata* metadata = read_metadata(FILENAME); printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n", metadata->title, metadata->artist, metadata->album); printf("comment: %s,\tgenre: %s,\ttrack: %d,\n", metadata->comment, metadata->genre, metadata->track); printf("length: %d,\tbitrate: %d,\tchannels: %d,\n", metadata->length, metadata->bitrate, metadata->channels); printf("codec: %d\n"); } It just reads the metadata into the struct and prints the fields. I've gotten the impression of the Haskell FFI being very beginner-unfriendly, which isn't surprising but still disappointing because it would be a great opportunity to replace some projects with Haskell. Any help is appreciated, including general feedback on my code!

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"Template Haskell and higher order meta programming."
by Song Zhang 03 Mar '15

03 Mar '15

I generally have 2 questions. I am writing some functions to derive typeclass instances with a package called derive. I just want to get rid of using Derivation type with a stupid reason. I want use just Name type. How can I defined a function with type derivings :: Name -> Name -> Q [Dec] by using function derive :: Derivation -> Name -> Q [Dec] so that user can just write derivings ''Eq ''D instead of derive makeEq ''D? > {-# LANGUAGE TemplateHaskell #-} > import Data.DeriveTH -- in derive package > import Language.Haskell.TH > import qualified Language.Haskell.TH.Syntax as S > data D = D > derivings :: Name -> String -> Q Exp > derivings cla typ = do > let makeClassName = mkName $ "make" ++ nameBase cla > a <- [| derive makeClassName (mkName typ) |] > return a > instance S.Lift Name where > lift x = varE x Main> :t derivings ''Eq "D" derivings ''Eq "D" :: Q Exp Main> :t $(derivings ''Eq "D") $(derivings ''Eq "D") :: Q [Dec] There is no problem to quote twice in other file in order to get the declaration. Maybe it should called second order meta programming. But the following has problems: > derivings'' :: Name -> Name -> Q Exp > derivings'' cla typ = do > let makeClassName = mkName $ "make" ++ nameBase cla > a <- [| derive makeClassName typ |] > return a Main> :t derivings'' derivings'' :: Name -> Name -> Q Exp Main> :t derivings'' ''Eq ''D derivings'' ''Eq ''D :: Q Exp Main> :t $(derivings'' ''Eq ''D) <interactive>:1:3: Illegal variable name: ‘D’ When splicing a TH expression: Data.DeriveTH.derive makeEq (Language.Haskell.TH.Syntax.mkName Main.D) In the splice: $(derivings'' ''Eq ''D) In the first case I used String, it works. however, it is not workiing if I use D with a Name type. Cannot figure it out. My second question is that if my return type is Q [Exp] while the [Exp] can be further expanded into [Dec]. How can I do it to expand [Exp] into [Dec]? Do we have a function for $ in $(thcode_with_Q_Exp) so that I do not need to splice each Exp value? Best wishes Song

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CodeWarriors -- Spreading the word
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 02 Mar '15

02 Mar '15

I just found out about www.codewarriors.com, and wanted to spread the word. It's a good place for beginners to get questions in increasing order of complexity. Best. -- Regards Sumit Sahrawat

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trying to install libraries for Haskell School of Expression
by DJ 02 Mar '15

02 Mar '15

Trying to install the package soegtk. I am running linux mint 17 with the latest (as of Feb. 28) haskell platform and cabal-install. I did install gtk2hs-buildtools and put the .cabal/bin in my path. Cannot install soegtk: Failed to install glib-0.12.5.4 cabal: Error: some packages failed to install: cairo-0.12.5.3 failed during the configure step. The exception was: ExitFailure 1 gio-0.12.5.3 depends on glib-0.12.5.4 which failed to install. glib-0.12.5.4 failed during the configure step. The exception was: ExitFailure 1 gtk-0.12.5.7 depends on glib-0.12.5.4 which failed to install. pango-0.12.5.3 depends on glib-0.12.5.4 which failed to install. soegtk-0.12.1 depends on glib-0.12.5.4 which failed to install. Confession: I abandoned Haskell two years ago because of frustration with cabal and hackage. I decided to get back to the language today, and to start with Haskell School of Expression. I immediately run into the problem that the first thing I try to install with cabal does not work. Thanks for any help. Please tell me things are not just as bad now as they were when I left ;-) - DJP -

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