Re: [web-devel] [ Newbie ] [ MongoDB ] Using MongoDB Persistent backend

OK, simple question: why can't we use a type of MongoDBReader of: newtype MongoDBReader m a = MongoDBReader (Action m a) deriving (Monad, Trans.MonadIO, Functor, Applicative) Michael On Thu, Jun 30, 2011 at 6:55 PM, Greg Weber wrote:

I think it is about time to finally fix this. I am probably the least knowledgeable about Monad mashing, but seems like a good time to learn! Here is a simple app we can use: https://gist.github.com/1056520 But maybe it is best to use the test suite. Here are all the hacks to get the tests to run right now that need to be removed: instance Context DB.MasterOrSlaveOk IO where   context = undefined   push = undefined instance Context DB.Database IO where   context = undefined   push = undefined instance Context DB.WriteMode IO where   context = undefined   push = undefined instance Context DB.Pipe IO where   context = undefined   push = undefined instance Control.Monad.Throw.Throw DB.Failure IO where   throw = undefined   catch = undefined

On Wed, Jun 29, 2011 at 1:25 AM, Michael Snoyman wrote:

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On Wed, Jun 29, 2011 at 11:17 AM, Kamil Ciemniewski wrote:

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Hi all,

I've got some "newbie" problems using MongoDB as backend in Persistent.

Basically I've got my app type defined as: data MyApp = MyApp     { getStatic :: Static -- ^ Settings for static file serving.     , connPool :: ConnPool Host     }

And I made it an instance of YesodPersist by: instance YesodPersist MyApp where   type YesodDB MyApp = MongoDBReader Host   runDB db = liftIOHandler            $ fmap connPool getYesod >>= (\p -> return (p, "localhost"))

...

...

= runMongoDBConn db

And I made it an instance of YesodAuth as well by: instance YesodAuth MyApp where   type AuthId MyApp = UserId

  loginDest _ = RootR   logoutDest _ = RootR

  getAuthId creds = runDB $ do       x <- getBy $ UniqueUser $ credsIdent creds       case x of       Just (uid, _) -> return $ Just uid       Nothing -> do           fmap Just $ insert $ User (credsIdent creds) Nothing

  authPlugins = [ authEmail ]

I've defined User model as yesod scaffold tool defines it.

Now, when I try to compile it i get:

No instances for (Control.Monad.Context.Context                         Database.MongoDB.Connection.MasterOrSlaveOk                         (GGHandler s MyApp IO),                       Control.Monad.Context.Context                         Database.MongoDB.Query.Database (GGHandler s MyApp IO),                       Control.Monad.Context.Context                         Database.MongoDB.Internal.Protocol.Pipe (GGHandler s MyApp IO),                       Control.Monad.Context.Context                         Database.MongoDB.Query.WriteMode (GGHandler s MyApp IO),                       Control.Monad.Throw.Throw                         Database.MongoDB.Query.Failure (GGHandler s MyApp IO))       arising from a use of `insert'     Possible fix:       add instance declarations for       (Control.Monad.Context.Context          Database.MongoDB.Connection.MasterOrSlaveOk          (GGHandler s MyApp IO),        Control.Monad.Context.Context          Database.MongoDB.Query.Database (GGHandler s MyApp IO),        Control.Monad.Context.Context          Database.MongoDB.Internal.Protocol.Pipe (GGHandler s MyApp IO),        Control.Monad.Context.Context          Database.MongoDB.Query.WriteMode (GGHandler s MyApp IO),        Control.Monad.Throw.Throw          Database.MongoDB.Query.Failure (GGHandler s MyApp IO))     In the expression: insert     In the second argument of `($)', namely       `insert $ User (credsIdent creds) Nothing'     In the expression:         fmap Just $ insert $ User (credsIdent creds) Nothing

The problem is probably very trivial.. But I've got no ideas how to fix it.

Best regards Kamil Ciemniewski

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I don't have direct experience with the MongoDB backend, but I can give this a shot if I saw some code. I think one possibility would be to wrap everything in a liftIO, but then you won't have access to the Handler monad features. This would prevent you from doing things like setting messages and sending redirects from the database access itself, but should otherwise be fine.

Michael

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