Hi,

I am new to Happstack. Not so new to Haskell, but I have always had a big
trouble designing monadic data types. It always seems simple, but I have
some kind of mind block there.

I have some server settings (for now, a database connection pool), and I
want to put them into a ReaderT monad, that is then wrapped in the
ServerPartT. Specifically, I have these two functions:

withMongo :: (MongoPool -> IO a) -> IO ()
withMongo f = do
pool <- dbPool
f pool
killAll pool

runDB :: (MonadIO m) => MongoPool -> Action IO a -> m (Either Failure a)
runDB pool f = liftIO $ do
pipe <- runIOE $ aResource pool
access pipe master dbName f

I would love to change "withMongo" in a way that I can drop it in before my
"simpleHTTP" call. The type of runDB could be something like "Action IO a
-> ServerPartT m (Either Failure a)" so it can be used in any handler
without having to carry the pool with me all the time.

I know this is not exactly a Happstack question, but I'd thought the
chances are high someone on here has already done this specifically.

Thanks.

We have a solution for exactly this problem in the Snap Framework.
It's called snaplets and it makes it really easy to do what you're
talking about.  In fact, there are already two different snaplets
available on hackage that provide MongoDB support, so you probably
wouldn't even need to do it yourself.  Snap's low-level API is very
similar to that of Happstack, so you shouldn't have too much trouble
converting if you already have code.

On 1 April 2012 10:58, Lambda Dusk <lambdad...@gmail.com> wrote:

<goog_1591544991>
http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/Con...

Sorry, I misread your question... Though bracket is still useful here. :)

You want something like this:

runServer :: MongoPool -> IO ()
runServer pool =
    simpleHTTP nullConf $
      mapServerPartT (`runReaderT` pool) handlers

-- all handlers should have this type
type Handler = ServerPartT (ReaderT MongoPool IO) Response

handlers :: Handler
handlers = msum [...]

withMongo :: (MongoPool -> IO ()) -> IO ()
withMongo = bracket dbPool killall

main :: IO ()
main = withMongo runServer

I haven't tested this particular code, but it's similar to what I do in
most apps.