you'll find this example really helpfull
---------- Forwarded message ----------
From: Claude Heiland-Allen <[EMAIL PROTECTED]>
Date: 2008/6/5
Subject: Re: [Haskell-cafe] example of FFI FunPtr
To: "Galchin, Vasili" <[EMAIL PROTECTED]>
Cc: haskell <haskell-cafe@haskell.org>
Galchin, Vasili wrote:
>
> Hello,
>
> I want to model a Haskell function that is a callback from C. I have
> only found one example in the unix package's Semaphore.hsc, which apparently
> is not used. I want to be able to marshall a Haskell function that is a
> first class citizen residing in a Haskell data type and pass to a C function
> via FFI. Are there examples of this?
Attached is a simple example.
The main thing to note is 'foreign import ccall "wrapper"' which gives
you a factory for turning Haskell functions into foreign function
pointers.
More information:
http://www.cse.unsw.edu.au/~chak/haskell/ffi/
Claude
--
http://claudiusmaximus.goto10.org
CallBacker: CallBacker.hs callerback.c callerback.h
ghc -O2 -Wall -fffi -o CallBacker CallBacker.hs callerback.c
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module Main(main) where
import Foreign.C.Types(CDouble)
import Foreign.Ptr(FunPtr, freeHaskellFunPtr)
foreign import ccall "wrapper"
wrap :: (CDouble -> CDouble) -> IO (FunPtr (CDouble -> CDouble))
foreign import ccall "callerback.h twice"
twice :: FunPtr (CDouble -> CDouble) -> CDouble -> IO CDouble
square :: CDouble -> CDouble
square x = x * x
main :: IO ()
main = do
squareW <- wrap square
let x = 4
y <- twice squareW x
z <- twice squareW y
print y
print z
freeHaskellFunPtr squareW
#include "callerback.h"
double twice(d2d f, double x) {
return f(f(x));
}
#ifndef CALLERBACK_H
#define CALLERBACK_H
typedef double (d2d)(double);
double twice(d2d f, double x);
#endif
CallBacker: CallBacker.hs callerback.c callerback.h
ghc -O2 -Wall -fffi -o CallBacker CallBacker.hs callerback.c
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