执行块解析类型

我最近刚开始在Haskell中编写代码,我开始使用 . 我来自Scala world,在读了一本书和一些博客后我看到了 do块 是我们从理解中得到的灵感。但我仍在努力处理作为输入传递给每个函数的参数->输出

在我的代码中,我使用scotty http服务器库来获取请求,并尝试将该请求持久化到mySQL中。

但在这一行中,我尝试从Maybe获取值,并将其发送到要持久化的另一个函数

user <- (fmap (\user -> insertUser user) maybeUser)

它从不编译,因为函数不返回所需的类型

操作文本IO(可能是用户)

但是

IO用户

这里是我的洞程序

createUser :: ActionM ()
createUser =  do maybeUser <- getUserParam
                          -- Persist the user
                          _ <- persistUser
                          json maybeUser

getUserParam :: ActionT Text IO (Maybe User)
getUserParam = do requestBody <- body
                  return (decode requestBody)

persistUser :: Maybe User -> ActionT Text IO (Maybe User)
persistUser _maybeUser = let maybeUser = _maybeUser in do
                           user <- maybeUser
                           user <- (fmap (\user -> insertUser user) maybeUser)
                           return maybeUser

insertUser :: User -> IO User
insertUser _user = let user = _user in do
    conn <- createConnection
    status <- execute conn insertUserQuery [MySQLInt32 (intToInt32 $ getUserId user), MySQLText "hello_haskell_world"]
    return user