import Foreign hiding (newForeignPtr)
import Foreign.Concurrent
import System.Mem (performGC)

type A = ForeignPtr Int

type B = ForeignPtr Bool

newA :: IO A
newA = do
  (pa::Ptr Int) <- malloc
  newForeignPtr pa $ do
    putStrLn "before finalizing A"
    free pa
    putStrLn "after finalizing A"
  
newB :: A -> IO B
newB fpa =
  withForeignPtr fpa $ \pa -> do
    (pb::Ptr Bool) <- malloc
    newForeignPtr pb $ do
      putStrLn "before finalizing B"
      free pb
      putStrLn "after finalizing B"
      touchForeignPtr fpa

main = do
  a <- newA
  b <- newB a
  getChar
  performGC
  getChar