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

[Michael Snoyman]

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 <greg at gregweber.info> 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 <michael at snoyman.com>
> wrote:
>>
>> On Wed, Jun 29, 2011 at 11:17 AM, Kamil Ciemniewski
>> <ciemniewski.kamil at gmail.com> wrote:
>> > 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
>> >
>> > _______________________________________________
>> > web-devel mailing list
>> > web-devel at haskell.org
>> > http://www.haskell.org/mailman/listinfo/web-devel
>> >
>> >
>>
>> 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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>> web-devel at haskell.org
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>
>