在Haskell中自动转换FFI调用的类型

import Foreign.Storable
import Foreign.Ptr

int add(int a, int b) {
    return a + b;
}

foreign import ccall "add" c_add :: CInt -> CInt -> CInt

data Point = Point { x :: Double, y :: Double }

instance Storable Point where
    sizeOf _ = sizeOf (undefined :: CDouble) * 2
    alignment _ = alignment (undefined :: CDouble)
    peek ptr = do
        x' <- peekElemOff (castPtr ptr) 0
        y' <- peekElemOff (castPtr ptr) 1
        return $ Point x' y'
    poke ptr (Point x' y') = do
        pokeElemOff (castPtr ptr) 0 x'
        pokeElemOff (castPtr ptr) 1 y'

withPoint :: Point -> (Ptr Point -> IO a) -> IO a
withPoint point action = allocaBytesAligned (sizeOf point) (alignment point) $ \ptr -> do
    poke ptr point
    result <- action ptr
    return result