1. Create

• wrap an Objective-C method in C
• import the C function into Haskell through FFI
• create a function that utilize this imported function in Haskell

2. Export

• create an empty function pointer in C
• create a setter function to initialize this function pointer in C
• import the setter function into Haskell
• Use the setter to initialize a Haskell function

3. Use

• somewhere in Objective-C, call into Haskell through this initialized function

Step 2 contains some boilerplates: the function pointer and initializer, so I wrote

OMG

my first C Macro! ><

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/* 
 H(draw_rect) ->

 void (*draw_rect)(void);
 void set_draw_rect(void (*cb)() {
   draw_rect = cb;
 }
*/

#define H(f) \
void (*f)(void);\
void set_##f(void (*cb)()) {\
f = cb;\
}

So my first experiment allows Haskell to draw a line in the view, here's the relavent code:

ScoreView.m

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@implementation ScoreView

void drawLine(double x0, double y0, double x1, double y1)
{
  CGContextRef _context = UIGraphicsGetCurrentContext();
  CGContextBeginPath(_context);
  CGContextMoveToPoint(_context, x0, y0);
  CGContextAddLineToPoint(_context, x1, y1);
  CGContextStrokePath(_context);
}

H(draw_rect)

- (void)drawRect:(CGRect)rect {
  if (draw_rect != nil) draw_rect();
}

@end

Main.hs

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foreign import ccall safe "drawLine" drawLine :: CDouble -> CDouble -> CDouble -> CDouble -> IO ()
foreign import ccall safe "wrapper" mkPlain :: IO () -> IO (FunPtr (IO ()))
foreign import ccall safe "set_draw_rect" set_draw_rect :: FunPtr (IO ()) -> IO ()


main = do
  draw_rect <- mkPlain $ drawLine 0 0 220 220
  set_draw_rect draw_rect

  kon

  free draw_rect

  where
    free = freeHaskellFunPtr