March 2015 - Beginners

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!

3 4

"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

1 0

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

1 1

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 -

7 18

trees on Haskell : Do I understand it right ?
by Roelof Wobben 01 Mar '15

01 Mar '15

Hello, Suppose we have this definition of a tree : data MessageTree = Leaf | Node MessageTree LogMessage MessageTree deriving (Show, Eq) let Message = LogMessage "E 1 this is a test error" let Message = LogMessage "e 2 this is the second test error " As I understand it right I can make the first entry like this : first_entry = Node Messagetree Message Messagetree And the second one like this second_entry = Node Message Messagetree Message2 Messagetree ?? Roelof

4 10

Using findIndex and then splitAt in Data.List
by Geoffrey Bays 28 Feb '15

28 Feb '15

Hi. An elementary question here about two functions in Data.List: how to use a value from findIndex which returns a Maybe Int, and then use that result in splitAt which takes a regular Int? This is in an IO() do Like so: (This is in an IO() do block) let gradeItemIndex = findIndex (\g -> (itemName g) == (itemName gradeItemP)) gradeItemList let twoListsTuple = splitAt gradeItemIndex gradeItemList // does not compile obviously Many Thanks, Geoffrey

4 8