How do I access a global MongoDB connection from a handler? - mongodb

Question

I’m trying to make a simple web application using Snap. I want to have a global MongoDB connection which I can use across handlers.
I open this connection in appInit, which works.
data App = App { _mongoDB :: Pipe -- connection
}
appInit :: SnapletInit App App
appInit = makeSnaplet "site" "My Site" Nothing $ do
db <- liftIO $ do
pipe <- runIOE $ connect (host "127.0.0.1")
return pipe
return $ App db
However, I have no idea how to access this connection (_mongoDB) from a handler. I tried several things, including the following, but I get type errors every time.
watchHandler :: Handler App App ()
watchHandler = do
res <- liftIO $ do
pipe <- gets _mongoDB -- type error
results <- access pipe master "db" (find $ select [] "movies")
return results
writeBS "test"
It gives me the following error:
No instance for (MonadState App IO)
arising from a use of `gets'
Possible fix: add an instance declaration for (MonadState App IO)
In a stmt of a 'do' block: pipe <- gets _mongoDB
In the second argument of `($)', namely
`do { pipe <- gets _mongoDB;
results <- access pipe master "db" (find $ select [] "movies");
return results }'
In a stmt of a 'do' block:
res <- liftIO
$ do { pipe <- gets _mongoDB;
results <- access pipe master "db" (find $ select [] "movies");
return results }
It confuses the hell out of me. How can I access my MongoDB connection from a handler?

Answer 1

The liftIO block is just for IO operations, accessing the application state needs to be done in the Handler monad itself.
watchHandler :: Handler App App ()
watchHandler = do
pipe <- gets _mongoDB
res <- liftIO $ do
results <- access pipe master "db" (find $ select [] "movies")
return results
writeBS "test"
Also, binding a value and then immediately returning it is redundant. You can just write:
watchHandler :: Handler App App ()
watchHandler = do
pipe <- gets _mongoDB
res <- liftIO $ access pipe master "db" (find $ select [] "movies")
writeBS "test"