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Haskell FFI and finalizers
by Maxime Henrion 09 Oct '07

09 Oct '07

Hello all, I have recently developed a small set of bindings for a C library, and encountered a problem that I think could be interesting to others. My problem was that the C function I was writing bindings to expects to be passed a FILE *. So, I had basically two possibles routes to take: 1) Mimic the C API and have the haskell function take a Handle. Unfortunately, I can see no way to go from a Handle to a Ptr CFile, at least no portable way, so I discarded this option. 2) Deviate from the C API slightly and have the haskell function take a FilePath instead of a Handle. This is the option I chose, and this is where things get interesting. In order to pass a Ptr CFile (FILE *) to the C function, I had to call fopen() myself, using a usual FFI binding: foreign import ccall unsafe "fopen" fopen :: CString -> CString -> IO (Ptr CFile) That's the easy part. Now my problem was that I had to find a way to automatically close this FILE * when it isn't used anymore, in order not to leak FILE structures (and thus fds, etc). A finalizer is typically what I need, but unfortunately, a finalizer has a very strict shape: type FinalizerPtr a = FunPtr (Ptr a -> IO ()) That is, a finalizer can only be a pointer to a foreign function, and the foreign function itself needs a quite specific shape. And then I discovered Foreign.Concurrent, which allows one to associate a plain Haskell IO action to a pointer. The 'Foreign.Concurrent' name is a bit misleading to me; it seems this module is named so because it needs concurrency itself, rather than providing stuff for concurrency. So, in the end, I've got this code: import Foreign import Foreign.C import qualified Foreign.Concurrent as FC ... data PlayerStruct type Player = ForeignPtr PlayerStruct ... foreign import ccall unsafe "dd_newPlayer_file" dd_newPlayer_file :: Ptr CFile -> Ptr ImageStruct -> IO (Ptr PlayerStruct) foreign import ccall unsafe "&dd_destroyPlayer" destroyPlayerFinal :: FunPtr (Ptr PlayerStruct -> IO ()) foreign import ccall unsafe "fopen" fopen :: CString -> CString -> IO (Ptr CFile) foreign import ccall unsafe "fclose" fclose :: Ptr CFile -> IO CInt ... mkFinalizedPlayer :: Ptr PlayerStruct -> IO Player mkFinalizedPlayer = newForeignPtr destroyPlayerFinal newPlayerFile :: FilePath -> Image -> IO Player newPlayerFile path image = do withCString path $ \cpath -> do withCString "rb" $ \cmode -> do file <- throwErrnoIfNull "fopen: " (fopen cpath cmode) withForeignPtr image $ \ptr -> do player <- dd_newPlayer_file file ptr >>= mkFinalizedPlayer FC.addForeignPtrFinalizer player (fclose file >> return ()) return player So I'm adding the "usual" finalizer, and with the help of Foreign.Concurrent, I can add a second free-form one (fclose file >> return ()), in order to close the file I opened at an appropriate time. I'm looking forward hearing about other people's opinions, and wether this is a correct solution to the initial problem or not. I think there is another way to solve this, which is to provide the finalizer still in haskell code, but export the haskell code using FFI, so that I can use it as a plain, normal finalizer. I'm still unsure about this. Cheers, Maxime

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Some lambda identities
by Yitzchak Gale 09 Oct '07

09 Oct '07

While playing with @pl on #haskell, I noticed some weird and surprising lambda identities. For example: let {c = (.); c4 = c c c c} Then we have: c c4 == c c4 c c c c Proof: Repeatedly apply the identity: (*) x (y z) == c x y z More stuff: c c2 == c4, c c3 == c7, but c cn does not appear to be reducible for n>3. c7 c3 == c3 c7 c c c You get a lot more interesting stuff when you throw flip into the mix. The identity (*) is actually a semi-associativity condition that makes the entire lambda calculus into a semi-monoid. Apparently with very interesting structure. Anyone know more about these things? Thanks, Yitz

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EnableGUI hack changes working directory
by apfelmus 09 Oct '07

09 Oct '07

Hello, the EnableGUI hack for getting a window in GHCi on Mac OS X (10.3.9 for me) unexpectedly changes the working directory for the running ghci [...] apfelmus$ ghci -i../../Hackage/SOE/src [...] GHC Interactive, version 6.6.1, for Haskell 98. *Main> :! pwd /Users/apfelmus/Documents/Programmierung/Haskell/Graphics/Kunst *Main> enableGUI >> run lambda Loading package OpenGL-2.2.1 ... linking ... done. Loading package GLFW-0.1 ... linking ... done. *Main> :! pwd /opt/local/lib/ghc-6.6.1 The effect is that reloading the Main module now fails *Main> :r <no location info>: can't find file: Main.hs Any solution? Btw, using enableGUI exactly once at the beginning of the ghci session doesn't work *Main> enableGUI <interactive>: ../../Hackage/SOE/src/EnableGUI.o: unknown symbol `_CPSEnableForegroundOperation' unless ghci is started with the -framework Carbon option. But then, the working directory remains fine, so I guess the change of working directory bug happens while dynamically loading frameworks/shared libraries. Is there a way to automate ghci ... EnableGUI -framework Carbon and typing enableGUI at the first prompt? Something like a custom shell script ghci+gui or even ghci --with-gui ? Regards, apfelmus

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unsafePerformIO: are we safe?
by Jorge Marques Pelizzoni 09 Oct '07

09 Oct '07

Hi, all! This is a newbie question: I sort of understand what unsafePerformIO does but I don't quite get its consequences. In short: how safe can one be in face of it? I mean, conceptually, it allows any Haskell function to have side effects just as in any imperative language, doesn't it? Doesn't it blow up referential transparency for good? Is there anything intrinsic to it that still keeps Haskell "sound" no matter what unsafePerformIO users do (unlikely) or else what are the guidelines we should follow when using it? Thanks in advance. Cheers, Jorge.

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Type-level arithmetic
by Andrew Coppin 09 Oct '07

09 Oct '07

I've seen quite a few people do crazy things to abuse the Haskell type system in order to perform arithmetic in types. Stuff the type system was never ever intended to do. Well I was just wondering... did anybody ever sit down and come up with a type system that *is* designed for this kind of thing? What would that look like? (I'm guessing rather complex!)

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ANNOUNCE: binary 0.4: high performance, pure binary parsing and serialisation
by Don Stewart 08 Oct '07

08 Oct '07

Binary: high performance, pure binary encoding, decoding and serialisation for Haskell ---------------------------------------------------------------------- The Binary Strike Team is pleased to announce release 0.4 of Data.Binary, the pure, efficient binary serialisation library for Haskell, now available from Hackage: tarball: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary/0.4 darcs: darcs get http://darcs.haskell.org/binary The 'binary' package provides efficient serialisation of Haskell values to and from lazy ByteStrings, and a low level layer for high performance decoding and decoding binary data. ByteStrings constructed this way may then be written to disk, written to the network, or further processed (e.g. stored in memory directly, or compressed in memory with zlib or bzlib). *Very* high performance can be expected, with throughput over 1G/sec observed in practice (good enough for most networking scenarios, we suspect). Encoding and decoding are achieved by the functions: encode :: Binary a => a -> ByteString decode :: Binary a => ByteString -> a which mirror the read/show functions. Convenience functions for serialising to disk are also provided: encodeFile :: Binary a => FilePath -> a -> IO () decodeFile :: Binary a => FilePath -> IO a To serialise your Haskell data, all you need do is write an instance of Binary for your type. For example, suppose in an interpreter we had the data type: import Data.Binary import Control.Monad data Exp = IntE Int | OpE String Exp Exp We can serialise this to bytestring form with the following instance: instance Binary Exp where put (IntE i) = putWord8 0 >> put i put (OpE s e1 e2) = putWord8 1 >> put s >> put e1 >> put e2 get = do tag <- getWord8 case tag of 0 -> liftM IntE get 1 -> liftM3 OpE get get get The binary library has been heavily tuned for performance, particularly for writing speed. On average, Data.Binary is 10x faster than NewBinary, and has the advantage of a pure interface, and bytestring return values. Binary was developed by a team of 8 during the Haskell Hackathon, Hac 07, in January 2007, and this maintainence release has taken place during the second hackathon. Binary is portable, using the foreign function interface and cpp, and has been tested with Hugs and GHC. Happy hacking! -- Don

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Confusing type specialisation in ghci
by David Carter 08 Oct '07

08 Oct '07

Hello, Can someone elucidate to me this behaviour from ghci 6.6.1? Why is the type of sqlist specialised to Integer? Prelude> let sq x = x * x Prelude> :t sq sq :: (Num a) => a -> a Prelude> sq 2.5 6.25 Prelude> :t map sq map sq :: (Num a) => [a] -> [a] Prelude> map sq [2.5] [6.25] This is all as I would expect so far, but: Prelude> let sqlist = map sq Prelude> :t sqlist sqlist :: [Integer] -> [Integer] And indeed, I get Prelude> sqlist [2.5] <interactive>:1:8: No instance for (Fractional Integer) ... etc Thanks! David -- The Wellcome Trust Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.

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Re: [Haskell] Converting Double from String
by Brent Yorgey 08 Oct '07

08 Oct '07

(moving to haskell-cafe...) On 10/8/07, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > On 10/8/07, bjornie <bjorn.wikstrom(a)gmail.com> wrote: > > > > Hi! I'm kind of new to functional programming with Haskell, I'm reading > a > > university course you see. Now I'm having some problems with one of our > lab > > assignments, we're supposed to write a program handling numeric > operations > > (addition, multiplication, and some functionality like sin and cos). > > > > I've made a data type; > > data Expr = Num Double | Add Expr Expr | Mul Expr Expr... > > > > I.e for representing the expression: > > 2.4 * (4.0 + 1.5) > > The Haskell representation is: > > Mul (Num 2.4) (Add (Num 4.0) (Num 1.5)) > > > > My problem is to convert a String to an expression like the one above. > > I.e the String: "2.4*(4.0+1.5)" > > > > If we would use plain Integers there would not be a problem, since the > > Prelude function 'digitToInt' :: Char -> Int does nearly what I would > like > > to do. But I'm stuck on the Doubles! How can I solve this in a smooth > way? > > You can use read. Here's a session from GHC's interpreter: > > $ ghci > Prelude> read "1.0" :: Double > 1.0 > > Since read is polymorphic (i.e. works for different types) you need to > specify the type of the result you want using ":: Double". ...although if you have something like Num (read x), you don't need the :: Double since the typechecker can figure it out (the Num constructor has to take a Double). Note that you would use the 'read' function to read integer values, too; digitToInt is just for dealing with single Chars, not entire integral values. The read function takes a String representation of anything that is "readable" (any type that is an instance of the Read type class) and converts it into a value of the requested type, so it works for Doubles, Ints, Integers, lists, and so on... Also, if I were you I'd use something other than "Num" as the name of the data constructor for numeric values in your Expr type. Num is already used for something else (it's the name of the general numeric type class), and although it's legal to have the same name for both, it might lead to confusion, especially when dealing with error messages. Good luck with learning Haskell and with your course! If you need more help there's also the #haskell IRC channel on freenode.net<http://haskell.org/haskellwiki/IRC_channel>, where you'll find lots of friendly people who would be glad to answer your questions. -Brent

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Data types, opengl display loop and readIORef/writeIORef
by bbrown 08 Oct '07

08 Oct '07

This is more an aesthetics question, I have a simple opengl application that has a singleton like object I need to pass around to the display loop and possibly to the keyboard action handler. (pseudo code) data SimpleMech = SimpleMech { mechPos :: !MVector, mechRot :: !MVector } deriving (Read) mainMech = initMech (0, 0, 0) (0, 0, 0) mainMechRef <- newIORef(mainMech) displayCallback $= displayGameLoop mainMechRef rotateMech :: (Double, SimpleMech) -> SimpleMech rotateMech (angle, mech) = -- Calculate a new rotation let (xrot,yrot,zrot) = (mechRot mech) newyrot = newyrot + angle in SimpleMech { mechRot = (xrot, newyrot, zrot), mechPos = (mechPos mech) } displayGameLoop mechRef = do mech <- get mechRef mechRef $= (rotateMech (0.1, mech)) ... ... For some reason in that code above, the "mechRef" doesnt update the value I want to change. It is like I am getting an instance of the initial valyue back. Also, when I use writeIORef, eg writeIORef mechRef (rotateMech (0.1, mech) I get stack overflow exceptions. The displayGameLoop will get called an infinite amount of times, so I think I have to be careful there. But I wonder why the reference isn't getting updated? Is this an approach you would take, eg passing "mechRef" around, a singleton game object. -- Berlin Brown [berlin dot brown at gmail dot com]

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cabal library dependencies
by Nehir Sonmez 08 Oct '07

08 Oct '07

Hi All, To test some programs written in CABAL, I need to use a server (where I do not have root access) with GHC 6.6.1 (in-place) installed. However, many programs need more libraries that have to be installed, (such as binary, or derive) for which, even when I use ./Setup configure --prefix=something, 1) After the ./Setup build, it can not do ./Setup install correctly because I am not root, ie. Installing: /usr/local/lib/derive-0.1/ghc-6.6.1 & /usr/local/bin derive-0.1... Setup: /usr/local/lib/derive-0.1: createDirectory: permission denied (Permission denied) 2) But the .o and the .a files are created. However possibly due to (1), I can not make the program see the library, so in the end when I try configuring the program, I get (for example): Setup: cannot satisfy dependency derive-any possible solutions I could think of were: 1) LD_LIBRARY_PATH 2) use configure with a --with flag 3) copying the library files to the prefix directory of the program I ultimately want to run 4) The -L flag and still no luck, so here I am asking for your help... Thanks a lot in advance, Sincerely, -Nehir Sonmez

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