在haskell中使用mysql简单的Scotty api

Question

我正在关注 this 教程，该教程使用 scotty 和持久性来创建一个简单的 API。

但是，我正在尝试使用 scotty 和 mysql 简单库创建一个简单的 api。

现在我卡在代码中的某一点上。

在下面的代码中，我无法将 getUser 函数转换为类型 "ActionT Error ConfigM"，因为我的代码失败了。

谁能帮助我了解如何转换 getUser 函数以实现所需的类型签名？

代码

type Error = Text
type Action = ActionT Error ConfigM ()

config :: Config
config = Config
    { environment = Development 
     ,db1Conn = connect connectionInfo
    }

main :: IO ()
main = do
  runApplication config

runApplication :: Config -> IO ()
runApplication c = do
  o <- getOptions (environment c)
  let r m = runReaderT (runConfigM m) c
  scottyOptsT o r application

application :: ScottyT Error ConfigM ()
application = do
  e <- lift (asks environment)
  get "/user" getTasksA

getTasksA :: Action
getTasksA = do
    u <- getUser
    json u 

getUser :: IO User
getUser = do
  e <- asks environment
  conn <- db1Conn config 
  [user]<- query_ conn "select login as userId, email as userEmail from member limit 1"
  return user 

错误

• Couldn't match type ‘IO’ with ‘ActionT Error ConfigM’
      Expected type: ActionT Error ConfigM User
        Actual type: IO User
    • In a stmt of a 'do' block: u <- getUser
      In the expression:
        do { u <- getUser;
             json u }
      In an equation for ‘getTasksA’:
          getTasksA
            = do { u <- getUser;
                   json u }

Answer 1

您遗漏了大量代码（导入和编译指示以及User 的定义，请下次添加 - 请参阅MCVE。

但现在你的问题：

我会将Action 类型更改为以下

type Action a = ActionT Error ConfigM a

那么getTasksA 具有以下类型签名

getTasksA :: Action ()
getTasksA = do
    u <- getUser
    json u 

（或者你也可以写成getTasksA = getUser >>= json）

和getUser

getUser :: Action User
getUser = do
  e <- asks environment
  conn <- db1Conn config 
  [user] <- liftIO $ query_ conn "select login as userId, ..."
  return user

几点说明

• [user] <- liftIO $ query .. 是个坏主意——如果找不到用户，这会使你的应用程序崩溃——尝试编写总函数和模式匹配。最好返回Maybe User。

  getUser :: Action (Maybe User)
  getUser = do
    e <- asks environment
    conn <- db1Conn config 
    fmap listToMaybe . liftIO $ query_ conn "select login as userId, ..."
  

1234563 /p>

• 你 ask 多次为 environment 但后来不要使用它。使用-Wall 甚至-Werror 进行编译以获得警告，甚至提升警告以编译错误（这对于生产设置来说是个好主意。