[pointed functor](%E7%AC%AC%E4%B9%9D%E7%AB%A0Monad/pointedfunctor.md) [chain](%E7%AC%AC%E4%B9%9D%E7%AB%A0Monad/chain.md) [练习](%E7%AC%AC%E4%B9%9D%E7%AB%A0Monad/%E5%AE%9E%E4%BE%8B.md)