Scotty使用MongoDB

Question

我对Haskell比较陌生，这是我第一次使用monad transformers。如果能帮上忙我会很感激的。

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 web框架下的mongodb数据库，但是我得到了一个关于MonadBaseControl的错误。我真的需要创建一个这样的实例才能用scotty连接到数据库吗?我该怎么做呢？

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，您需要在scotty的操作中使用liftIO . runQuery：

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 added MonadBaseControl Scotty.ActionT实例之后，不需要抬起任何东西。你可以透明地使用mongoDB form scotty。只需更改类型签名并删除liftIO：

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