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Today's Topics:
1. FFI Problems (Thomas Jakway)
2. Re: Using findIndex and then splitAt in Data.List
(Michael Orlitzky)
3. Re: Using findIndex and then splitAt in Data.List (Geoffrey Bays)
4. Re: FFI Problems (Brandon Allbery)
----------------------------------------------------------------------
Message: 1
Date: Sat, 28 Feb 2015 08:20:05 -0500
From: Thomas Jakway <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] FFI Problems
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"; Format="flowed"
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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Message: 2
Date: Sat, 28 Feb 2015 09:08:46 -0500
From: Michael Orlitzky <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Using findIndex and then splitAt in
Data.List
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
On 02/27/2015 03:51 PM, Geoffrey Bays wrote:
> 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?
Once you understand why you can't feed a (Maybe Int) to a function that
takes an Int, throw it all away and use `splitOn` from the "split"
package instead =)
https://hackage.haskell.org/package/split-0.2.2/docs/Data-List-Split.html
------------------------------
Message: 3
Date: Sat, 28 Feb 2015 10:02:27 -0500
From: Geoffrey Bays <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Using findIndex and then splitAt in
Data.List
Message-ID:
<cad8ukx7cehtogkfht9sgo7c2pzjdmbmjj5nqhwie6vd6_ub...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Michael:
Thanks, split on would work nicely.
I probably should have backed up in my reasoning process to say that my
real task is to replace a data item in a list with another altered data
item in the same position, or rather in FP, create a new list with the
altered item in the same place. And I need a predicate to find the name of
the item I want to replace.
Any thoughts?
Thanks,
Geoffrey
On Feb 28, 2015 9:08 AM, "Michael Orlitzky" <[email protected]> wrote:
> On 02/27/2015 03:51 PM, Geoffrey Bays wrote:
> > 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?
>
> Once you understand why you can't feed a (Maybe Int) to a function that
> takes an Int, throw it all away and use `splitOn` from the "split"
> package instead =)
>
> https://hackage.haskell.org/package/split-0.2.2/docs/Data-List-Split.html
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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------------------------------
Message: 4
Date: Sat, 28 Feb 2015 10:08:43 -0500
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] FFI Problems
Message-ID:
<CAKFCL4UQFP6DYd+OYYk-nHXKnRf88C=e3gvyuvbyceuxcxw...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sat, Feb 28, 2015 at 8:20 AM, Thomas Jakway <[email protected]> wrote:
> 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 raw FFI is unfriendly, but also not really intended for direct use
except in very simple cases. You probably want to look into c2hs (or
gtk2hsc2hs which is a fork that supports more complex C headers) to
generate the FFI declarations, rather than try to manipulate structs
directly.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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