Haskell:MonadState如何运作？

Question

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？

Answer 1

()是动作的返回值.因为put它的副作用(改变状态),它不会返回任何有用的东西.