Scotty Using MongoDB - mongodb

Question

I'm relatively new to Haskell, and this is my first time working with monad transformers. I'd really appreciate some help.
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
...
I'm trying to query a mongodb database under the scotty web framework, but I'm getting an error about MonadBaseControl. Do I really have to make an instance of this to connect to a database with scotty, and how would I go about doing it?
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 is generic enough to work in any monad that is instance of MonadBaseControl IO and MonadIO.
For example, you can choose IO monad. In this case you need liftIO . runQuery inside scotty's action:
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
After #Sebastian Philipp added MonadBaseControl instance for Scotty.ActionT, there is no need to lift anything. You can transparently work with mongoDB form scotty. Just change type signature and drop liftIOs:
runQuery :: Pipe -> Query -> ActionM [Document]
...
get "/" $ do
res <- runQuery pipe (select [] "stock_foods")
text $ T.pack $ show res