http://hackage.haskell.org/packages/archive/mtl/1.1.0.2/doc/html/src/Control-Monad-State-Lazy.html
instance (Monad m) => MonadState s (StateT s m) where
get = StateT $ \s -> return (s, s)
put s = StateT $ \_ -> return ((), s)
() 在 put 的定义中做了什么?