[Haskell-beginners] FFI Problems
Sylvain Henry
hsyl20 at gmail.com
Mon Mar 2 15:30:27 UTC 2015
I forgot to comment this line from your code:
readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
primReadMusicMetadata cs)
I would write:
readMusicMetadata a = withCString a $ \cs -> do
p <- primReadMusicMetadata cs
peek p
Or:
readMusicMetadata a = withCString a $ \cs -> (peek =<<
primReadMusicMetadata cs)
Or (with (<=<) from Control.Monad):
readMusicMetadata a = withCString a (peek <=< primReadMusicMetadata)
Best regards,
Sylvain
2015-03-02 16:20 GMT+01:00 Sylvain Henry <hsyl20 at gmail.com>:
> Hi,
>
> 1) You have a copy-paste error on the following line (s/bitrate/track)
> track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>
> 2) By looking at the values you get in hexadecimal, you are reading 64bits
> values when your C struct contains 32 bits values. If you use: peek ... ::
> IO Int32, it should be ok.
>
> 3) From a style point of view, you could use Applicative operators (from
> Control.Applicative) to avoid temporary names:
> peek a = MusicMetaData
> <$> liftM constantToCodec $ (((#peek struct music_metadata, codec)
> a) :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, length) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, bitrate) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, channels) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, track) a) ::
> IO Int32)
> <*> (peekCString =<< (#peek struct music_metadata, title) a)
> ...
>
> 4) In your constantToCodec function, you don't need the temporary names
> for all the constants.
>
> Best regards,
> Sylvain
>
> PS : recently I have been working on rewriting the FFI page on the wiki to
> make it more beginner-friendly ;-). It is not totally finished but I'm open
> to any comment. https://www.haskell.org/wiki/Foreign_Function_Interface
>
> 2015-02-28 14:20 GMT+01:00 Thomas Jakway <tjakway at nyu.edu>:
>
>> 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!
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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