输入值为期望值的IO Monad

我在哈斯凯尔是个新手,但我仍然无法理解关于monads的do块是如何工作的。

例如,考虑使用我返回整数的do块 IO Integer

getNumber :: IO Integer -- A IO monad of type Integer
getNumber = return 100

但是,例如,有一个返回可能用户的do块,do块的类型是 Maybe User 但我一直在期待 IO (Maybe User)

persistUser :: IO (Maybe User)
persistUser = do return Just(User { userId = 100, userName = "Paul" })

相反,代码甚至没有编译。我无法计算错误编译。

• Couldn't match type ‘a0 -> Maybe a0’ with ‘IO (Maybe User)’
  Expected type: User -> IO (Maybe User)
    Actual type: User -> a0 -> Maybe a0
• The function ‘return’ is applied to two arguments,
  but its type ‘(a0 -> Maybe a0) -> User -> a0 -> Maybe a0’
  has only three
  In a stmt of a 'do' block:
    return Just (User {userId = 100, userName = "Paul"})
  In the expression:
    do { return Just (User {userId = 100, userName = "Paul"}) }

似乎期望用户作为输入,但甚至添加签名

User -> IO (Maybe User) 或 User IO (Maybe User) 不起作用。

也许我对文件中的某些内容感到怀疑。澄清一下这件事会很好!!

当做。