Using ReaderT transformer in ScottyT (vs ActionT) - haskell

Question

I'm trying to thread configuration through my Scotty based application using ReaderT monad transformer approach, and having trouble doing so. I have to use configuration both when defining routes (as some of them depend on the config) and when handling actual requests.
The latter works just fine in the ActionT, but no matter what I try I just can't get the types right in ScottyT.
Here's the minimal example I compiled from the ReaderT sample from Scotty GitHub repository:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Applicative
import Control.Monad.Reader (MonadIO, MonadReader, ReaderT, asks, lift, runReaderT)
import Data.Default.Class (def)
import Data.Text.Lazy (Text, pack)
import Prelude
import Web.Scotty.Trans (ScottyT, get, scottyOptsT, text, capture)
data Config = Config
{ environment :: String
} deriving (Eq, Read, Show)
newtype ConfigM a = ConfigM
{ runConfigM :: ReaderT Config IO a
} deriving (Applicative, Functor, Monad, MonadIO, MonadReader Config)
application :: ScottyT Text ConfigM ()
application = do
get "/" $ do
e <- lift $ asks environment
text $ pack $ show e
path <- lift $ asks environment
get (capture path) $ do
text $ pack $ "Hello, custom path"
main :: IO ()
main = scottyOptsT def runIO application where
runIO :: ConfigM a -> IO a
runIO m = runReaderT (runConfigM m) config
config :: Config
config = Config
{ environment = "Development"
}
The error I'm getting is:
• No instance for (Control.Monad.Trans.Class.MonadTrans
(ScottyT Text))
arising from a use of ‘lift’
• In a stmt of a 'do' block: path <- lift $ asks environment
I've looked through the code where ScottyT type is outlined, and indeed there doesn't seem to be an instance of MonadTrans defined for it.
However, I don't feel I have enough both mana and Haskell experience to find a way out of it and would appreciate any help!
Thank you!

Answer 1

With a collective mind we all came to a currently viable solution to the problem.
ScottyT type cased to be a monad transformer with after https://github.com/scotty-web/scotty/pull/167 got merged, therefore there's currently no way of using it this way. There was a PR https://github.com/scotty-web/scotty/pull/181 aimed at bringing that feature back, but as far as I understood it has never got merged.
Since it's not a monad transformer we can only wrap it again:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Applicative
import Control.Monad.Reader (MonadIO, MonadReader, ReaderT, asks, lift, runReaderT)
import Data.Default.Class (def)
import Data.Text.Lazy (Text, pack)
import Prelude
import Web.Scotty.Trans (ScottyT, get, scottyOptsT, text, capture)
data Config = Config
{ environment :: String
} deriving (Eq, Read, Show)
newtype ConfigM a = ConfigM
{ runConfigM :: ReaderT Config IO a
} deriving (Applicative, Functor, Monad, MonadIO, MonadReader Config)
application :: ConfigM (ScottyT Text ConfigM ())
application = do
path <- asks environment
return $
get "/" $ do
e <- lift $ asks environment
text $ pack $ show e
get (capture path) $
text $ pack $ "Hello, custom path"
runIO :: Config -> ConfigM a -> IO a
runIO c m = runReaderT (runConfigM m) c
main :: IO ()
main = do
let config = Config { environment = "/path" }
app <- runIO config application
scottyOptsT def (runIO config) app
Thanks everyone for helping me out, and hopefully this helps another wandering Scotty like me :).