Tagless final and Scotty

As a first simple little experiment I wrote a tiny little web server that would print a string to stdout when receiving the request to GET /route0.

The printing to stdout is the operation I want to make abstract.

class Monad m => MonadPrinter m where
  mPutStrLn :: Text -> m ()

I then created an application type that is an instance of that class.

newtype AppM a = AppM { unAppM :: IO a }
  deriving (Functor, Applicative, Monad, MonadIO)

instance MonadPrinter AppM where
  mPutStrLn t = liftIO $ putStrLn (unpack t)

Then I added a bit of Scotty boilerplate. It's not strictly necessary, but does make the code a bit nicer to read.

type FooM = ScottyT Text AppM
type FooActionM = ActionT Text AppM

foo :: MonadIO m => Port -> ScottyT Text AppM () -> m ()
foo port = scottyT port unAppM

With that in place the web server itself is just a matter of tying it all together.

main :: IO ()
main = do
  foo 3000 $ do
    get "/route0" $ do
      lift $ mPutStrLn "getting /route0"
      json $ object ["route0" .= ("ok" :: String)]
    notFound $ json $ object ["error" .= ("not found" :: String)]

That was simple enough.

In order to try out how to deal with configuration I added a class for doing some simple logging

class Monad m => MonadLogger m where
  mLog :: Text -> m ()

The straight forward way to deal with configuration is to create a monad stack with ReaderT and since it's logging I want to do the configuration consists of a single LoggerSet (from fast-logger).

newtype AppM a = AppM { unAppM :: ReaderT LoggerSet IO a }
  deriving (Functor, Applicative, Monad, MonadIO, MonadReader LoggerSet)

That means the class instance can be implemented like this

instance MonadLogger AppM where
  mLog msg = do
    ls <- ask
    liftIO $ pushLogStrLn ls $ toLogStr msg

Of course foo has to be changed too, and it becomes a little easier with a wrapper for runReaderT and unAppM.

foo :: MonadIO m => LoggerSet -> Port -> ScottyT Text AppM () -> m ()
foo ls port = scottyT port (`runAppM` ls)

runAppM :: AppM a -> LoggerSet -> IO a
runAppM app ls = runReaderT (unAppM app) ls

With that in place the printing to stdout can be replaced by a writing to the log.

main :: IO ()
main = do
  ls <- newStdoutLoggerSet defaultBufSize
  foo ls 3000 $ do
    get "/route0" $ do
      lift $ mLog "log: getting /route0"
      json $ object ["route0" .= ("ok" :: String)]
    notFound $ json $ object ["error" .= ("not found" :: String)]

Not really a big change, I'd say. Extending the configuration is clearly straight forward too.

At work we use correlation IDs¹ and I think that the most convenient way to deal with it is to put the correlation ID into the configuration after extracting it. That is, I want to modify the configuration on each request. Luckily it turns out to be possible to do that, despite using ReaderT for holding the configuration.

I can't be bothered with a full implementation of correlation ID for this little experiment, but as long as I can get a new AppM by running a function on the configuration it's just a matter of extracting the correct header from the request. For this experiment it'll do to just modify an integer in the configuration.

I start with defining a type for the configuration and changing AppM.

type Config = (LoggerSet, Int)

newtype AppM a = AppM { unAppM :: ReaderT Config IO a }
  deriving (Functor, Applicative, Monad, MonadIO, MonadReader Config)

The logger instance has to be changed accordingly of course.

instance MonadLogger AppM where
  mLog msg = do
    (ls, i) <- ask
    liftIO $ pushLogStrLn ls $ toLogStr msg <> toLogStr (":" :: String) <> toLogStr (show i)

The get function that comes with scotty isn't going to cut it, since it has no way of modifying the configuration, so I'll need a new one.

mGet :: ScottyError e => RoutePattern -> ActionT e AppM () -> ScottyT e AppM ()
mGet p a = get p $ do
  withCfg (\ (ls, i) -> (ls, succ i)) a

The tricky bit is in the withCfg function. It's indeed not very easy to read, I think

withCfg = mapActionT . withAppM
  where
    mapActionT f (ActionT a) = ActionT $ (mapExceptT . mapReaderT . mapStateT) f a
    withAppM f a = AppM $ withReaderT f (unAppM a)

Basically it reaches into the guts of scotty's ActionT type (the details are exposed in Web.Scotty.Internal.Types, thanks for not hiding it completely), and modifies the ReaderT Config I've supplied.

The new server has two routes, the original one and a new one at GET /route1.

main :: IO ()
main = do
  putStrLn "Starting"
  ls <- newStdoutLoggerSet defaultBufSize
  foo (ls, 0) 3000 $ do
    get "/route0" $ do
      lift $ mLog "log: getting /route0"
      json $ object ["route0" .= ("ok" :: String)]
    mGet "/route1" $ do
      lift $ mLog "log: getting /route1"
      json $ object ["route1" .= ("bar" :: String)]
    notFound $ json $ object ["error" .= ("not found" :: String)]

It's now easy to verify that the original route, GET /route0, logs a string containing the integer '0', while the new route, GET /route1, logs a string containing the integer '1'.