为什么bracket需要` `release` `才能产生被忽略的结果？

Question

bracket
        :: IO a         -- ^ computation to run first (\"acquire resource\")
        -> (a -> IO b)  -- ^ computation to run last (\"release resource\")
        -> (a -> IO c)  -- ^ computation to run in-between
        -> IO c         -- returns the value from the in-between computation
bracket before after thing =
  mask $ \restore -> do
    a <- before
    r <- restore (thing a) `onException` after a
    _ <- after a
    return r

这是否与某些API设计模式或约定有关？为什么不使用签名的以下部分？

        -> (a -> IO ())  -- ^ computation to run last (empty result)

或

        -> (a -> IO a)  -- ^ computation to run last (`a` cannot be ignored)