Scotty monad transformer for per-handler Reader

In the question Web, Scotty: connection pool as monad reader it is shown how to use ScottyT to embed a Reader monad in the stack to access a static configuration (in that case, a connection pool).

I have a similar question, but simpler – or at least I thought so…

I want to add a Reader to a single handler (i.e. a ActionT), not the whole app.

I started modifying the program from the question above, but I cannot figure out how to turn an ActionT Text (ReaderT String IO) into the ActionT Text IO the handler needs to be. After fumbling around and trying to use typed holes hoping to see how to construct this I have to give up for now and ask for help. I really feel this should be simple, but cannot figure out how to do this.

Here's the program, with the lines where I'm stuck highlighted:

{-# LANGUAGE OverloadedStrings #-}

import qualified Data.Text.Lazy as T
import           Data.Text.Lazy (Text)
import           Control.Monad.Reader
import           Web.Scotty.Trans

type ActionD = ActionT Text (ReaderT String IO)

main :: IO ()
main = do
  scottyT 3000 id id app

-- Application
app ::  ScottyT Text IO ()
app = do
  get "/foo" $ do
    h <- handler              -- ?
    runReaderT h "foo"        -- ?
--get "/bar" $ do
--  h <- handler
--  runReaderT h "bar"

-- Route action handler
handler ::  ActionD ()
handler = do
  config <- lift ask
  html $ T.pack $ show config

Solution

If you want to run each action in a separate reader, you don't need the more complex Scotty.Trans interface at all. You can just build you monad stack the other way around, with ReaderT on top.

import qualified Data.Text.Lazy as T
import           Control.Monad.Reader
import           Web.Scotty

type ActionD = ReaderT String ActionM

main :: IO ()
main = do
  scotty 3000 app

-- Application
app ::  ScottyM ()
app = do
  get "/foo" $ do
    runReaderT handler "foo"

-- Route action handler
handler ::  ActionD ()
handler = do
  config <- ask
  lift $ html $ T.pack $ show config