Comparing

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

to

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draw_line :: CGFloat -> CGFloat -> CGFloat -> CGFloat -> IO ()
draw_line x y x' y' = do
  c <- _UIGraphicsGetCurrentContext
  _CGContextBeginPath c
  _CGContextMoveToPoint c x y
  _CGContextAddLineToPoint c x' y'
  _CGContextStrokePath c

, the Haskell one isn't terribly different from the original Objective-C code. By building FFI agains the iPhone SDK, Objective-C code can be kept both minimal and functional.

I really began to feel that Cocoa is indeed very nicely designed, and those design patterns for Objective-C are just about right for building GUI.