haskell callback with captured variable

CallBacker.hs

module Main where

import Foreign.C.Types
import Foreign.Ptr 

foreign import ccall "wrapper"
  wrap :: (CDouble -> CDouble) -> IO (FunPtr (CDouble -> CDouble))

foreign import ccall "callerback.h twice"
  twice :: FunPtr (CDouble -> CDouble) -> CDouble -> IO CDouble 

myfunc :: CDouble -> CDouble -> CDouble 
myfunc y x = y * (x * x) 

main :: IO ()
main = do 
  myfuncW <- wrap (myfunc 2)
  let x = 4 
  y <- twice myfuncW x 
  z <- twice myfuncW y
  print y 
  print z
  freeHaskellFunPtr myfuncW 

callerback.c

#include "callerback.h"
double twice(d2d f, double x) { 
  return f(f (x)); 
}

callerback.h

#ifndef CALLERBACK_H
#define CALLERBACK_H
typedef double (d2d)(double); 
double twice(d2d f, double x); 
#endif