data ResourceVar a =
  ResourceVar
    { freeTVar :: TVar ( IO () )
    , getter   :: IO a
    , setter   :: a -> IO ( IO () )
    }

instance HasGetter ( ResourceVar a ) a where
  get ( ResourceVar { getter } ) = liftIO getter

class HasSetter t a | t -> a where
  ($=) :: MonadResource m => t -> a -> m ()

instance {-# OVERLAPPABLE #-} StateVar.HasSetter t a => HasSetter t a where
 ($=) = (StateVar.$=)

instance HasSetter ( ResourceVar a ) a where
( ResourceVar { freeTVar, setter } ) $= a = do
  freeResource <- liftIO . atomically $ swapTVar freeTVar ( pure () )
  ( key, _ ) <- allocate ( setter a ) id
  liftIO freeResource
  liftIO . atomically $ modifyTVar freeTVar ( *> release key )


{-# NOINLINE iniFilenameTVar #-}
iniFilenameTVar :: TVar ( IO () )
iniFilenameTVar = unsafePerformIO ( newTVarIO ( pure () ) )

iniFilename :: ResourceVar ( Maybe String )
iniFilename = ResourceVar { freeTVar = iniFilenameTVar, getter, setter }
  where
    getter = do
      cStr <- [C.exp| const char* { GetIO().IniFilename } |]
      if cStr == nullPtr then return Nothing else Just <$> peekCString cStr

    setter = \case
      Nothing ->
        [C.block| void { GetIO().IniFilename = $(char* nullPtr); } |]
        pure ( pure () )

      Just str -> do
        strPtr <- newCString str
        [C.block| void { GetIO().IniFilename = $(char* strPtr); } |]
        pure ( free strPtr )