mongodb - 斯科蒂使用 MongoDB

Question

我对 Haskell 比较陌生，这是我第一次使用 monad 转换器。非常感谢您的帮助。

runQuery :: Pipe -> Query -> ActionM (Either Failure [Document])
runQuery pipe query = access pipe master "nutrition" (find query >>= rest) 

main = do
pipe <- runIOE $ connect $ host "127.0.0.1"  

scotty 3000 $ do
post "/" $ do
        b <-  body
        let user :: Either String User = eitherDecode b
        case user of 
            Left err -> text . pack $ "Could not decode the user:" ++ err ++ ":\n" ++ (show b)
            Right u -> do 
            let query::Query = (select ["i_name" =: ["$in" =: map (unpack . name) (foods u)]] "stock_foods")
            results <- runQuery pipe query  
            ...

我正在尝试查询 scotty 网络框架下的 mongodb 数据库，但我收到有关 MonadBaseControl 的错误。我真的必须为此创建一个实例才能使用 scotty 连接到数据库吗？我将如何去做？

No instance for (MonadBaseControl
                   IO (scotty-0.7.2:Web.Scotty.Types.ActionT Text IO))
  arising from a use of `find'
Possible fix:
  add an instance declaration for
  (MonadBaseControl
     IO (scotty-0.7.2:Web.Scotty.Types.ActionT Text IO))

Answer 1

mongoDB足够通用，可以在作为 MonadBaseControl IO 和 MonadIO 实例的任何 monad 中工作。

例如，您可以选择IO monad。在这种情况下，您需要 liftIO 。在 scotty 的操作中运行查询:

import Web.Scotty
import Database.MongoDB
import qualified Data.Text.Lazy as T
import Control.Monad.IO.Class

runQuery :: Pipe -> Query -> IO [Document]
runQuery pipe query = access pipe master "nutrition" (find query >>= rest) 

main = do
  pipe <- connect $ host "127.0.0.1"
  scotty 3000 $ do
    get "/" $ do
      res <- liftIO $ runQuery pipe (select [] "stock_foods")
      text $ T.pack $ show res

在 @Sebastian Philipp 之后added MonadBaseControl Scotty.ActionT 的实例，不需要解除任何东西。您可以透明地使用 mongoDB 表单 scotty。只需更改类型签名并删除 liftIOs:

runQuery :: Pipe -> Query -> ActionM [Document]
...
    get "/" $ do
      res <- runQuery pipe (select [] "stock_foods")
      text $ T.pack $ show res