Haskell ScottyM() 函数内的数据库查询

Question

我正在尝试编写简单的休息 API。我使用 Database.SQLite.Simple、Scotty 和 Aeson。我在 ScottyM() 路由函数中遇到数据库查询问题。当我遇到这样的事情时

routes :: [User] -> ScottyM ()
routes allUsers = do
    get "/users/:id" $ do
        id <- param "id"
        json ([x | x <- allUsers, userId x == id] !! 0)

main = do
    conn <- open "data.db"
    users <- ( query_ conn "select id, first_name, second_name, team from users" :: IO [User] )
    scotty 3000 (routes users)

一切正常，但在这种情况下，allUsers 只会在服务器启动时更新一次。每次有人问起我都想问问。所以我写了这个：

routes :: Connection -> ScottyM ()
routes conn= do
   get "/users/:id" $ do
        id <- param "id"
        users <- ( query_ conn "select id, first_name, second_name, team from users" :: IO [User] )
        json (users !! id)

main = do
    conn <- open "data.db"
    scotty 3000 (routes conn)

我收到错误

Couldn't match expected type ‘Web.Scotty.Internal.Types.ActionT
                                Text IO [a0]’
            with actual type ‘IO [User]’
In a stmt of a 'do' block:
  users <- (query_
              conn "select id, first_name, second_name, team from users" ::
              IO [User])
In the second argument of ‘($)’, namely
  ‘do { id <- param "id";
        users <- (query_
                    conn "select id, first_name, second_name, team from users" ::
                    IO [User]);
        json (users !! id) }’
In a stmt of a 'do' block:
  get "/users/:id"
  $ do { id <- param "id";
         users <- (query_
                     conn "select id, first_name, second_name, team from users" ::
                     IO [User]);
         json (users !! id) }

如何解决？另外，如果我想要 select * from users where id = id from parameters?

，如何将参数传递给 sql 查询

Answer 1

get function's second argument is of type ActionM. If you investigate further, you'll see this is just shorthand for the more complicated looking ActionT，e为Text，m为IO。

因为你的do块是这种类型的，你不能直接调用IO函数。您需要使用 liftIO 来获得正确的类型。因此，只需在 query_ 调用之前添加一个 liftIO 即可修复它。

项目 wiki 中有一个名为 using scotty with IO 的示例显示了这一点。

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