Retrieving error status from Haskell MongoDB driver - mongodb

Question

I have a MongoDB with one collection user that contains a unique index on email:
import Data.Bson (Value (Int32))
import Database.MongoDB (Index (..), createIndex, (=:))
createIndex $ Index "user" [ "email" =: Int32 1 ] "email" True False
I have written a function that inserts a new user if the email address it is not already in use, and should fail if the email address is already taken:
import Data.Bson (Value (ObjId))
import Database.MongoDB (Action, ObjectId, (=:))
import qualified Database.MongoDB as M (insert)
data User = User
{ email :: Text
, firstName :: Text
, lastName :: Text
} deriving Show
data MongoEntity a = MongoEntity ObjectId a
createIfNotExists :: User
-> Action Handler (Either Text (MongoEntity User))
createIfNotExists (user#User {..}) = do
value <- M.insert "user" [ "email" =: email
, "firstName" =: firstName
, "lastName" =: lastName
]
case value of
ObjId objectId -> return (Right $ MongoEntity objectId user)
_ -> return $ Left "no document"
I would like to detect any errors throw by the M.insert (such as a duplicate key) and return an
error Text message.
Since the M.insert runs in the Action monad, I assume it sets an error status if it fails, but I can't figure out how to retrieve it. I assume that I need something like
error <- getErrorStatus -- or whatever it is called
immediately after the value <- M.insert ... line and then test the error in the case ... of expression.
By the way, I tried writing createIfNotExists using findAndModify but I couldn't capture a failure with that either.

Answer 1

I've never used this module, but reading the documentation, the best I could think of is :
Change the type for createIfNotExists
createIfNotExists :: MonadIO m =>
User -> Action m (Either String (MongoEntity User))
Use catchJust from Control.Exception.Base. The author seems to discourage the usage of Control.Monad.Error.Class.
main :: IO ()
main = do
pipe <- connect (host "127.0.0.1")
let user = User "jd#bar.baz" "John" "Doe"
eitherEntity <- catchJust writeFailureErrorStr
(access pipe master "user" $ createIfNotExists user)
(return . Left)
close pipe
writeFailureErrorStr :: Failure -> Maybe String
writeFailureErrorStr (WriteFailure _err str) = Just str
writeFailureErrorStr _other = Nothing
You probably want to check the _err code value too, since there could be other reasons for WriteFailure.
My understanding of the documentation is that it is not possible to handle errors out of IO.
I'm so confused by this way of doing that I think I'm probably wrong. Please show me a more elegant way if there is one.
More generally, I think that checking existence by try and fail is not very nice. I'd rather do a find request and insert if no result.

Answer 2

From Jean-Baptiste Potonnier's answer and after reading two articles by Michael Snoyman (http://www.yesodweb.com/blog/2014/06/exceptions-transformers and https://www.fpcomplete.com/user/snoyberg/general-haskell/exceptions/exceptions-and-monad-transformers), I rewrote my code as follows:
import Control.Exception.Lifted (handleJust)
import Data.Bson (Value (ObjId))
import qualified Data.Bson as B (Value)
import qualified Data.Text as T (pack)
import Database.MongoDB (Action, Failure (..), (=:))
import qualified Database.MongoDB as M (insert)
createIfNotExists :: User -> Action Handler (Either Failure (MongoEntity User))
createIfNotExists user =
handleJust writeFailureHandler (return . Left) $ do
value <- insertUser user
return $
case value of
ObjId objectId -> Right $ MongoEntity (T.pack $ show objectId) user
_ -> error "Unexpected missing document"
where
writeFailureHandler :: Failure -> Maybe Failure
writeFailureHandler writeFailure#WriteFailure{} = Just writeFailure
writeFailureHandler _ = Nothing
insertUser :: User -> Action Handler B.Value
insertUser User{..} =
M.insert "users" [ "email" =: email
, "firstName" =: firstName
, "lastName" =: lastName
]
using the lifted-base package.