Playing around with no-parameter typeclasses. These can pull values out of the "App" environment so that the `run` function can use them.

dsl.hs

{-# LANGUAGE MultiParamTypeClasses, MonoLocalBinds #-}

import Control.Monad.Reader

class HasFoo where
    foo :: App String

class HasBar where
    bar :: String

instance HasFoo where
    foo = asks _foo

instance HasBar where
    bar = "BAR"

data Ctx = Ctx
    { _foo :: String
    , _bar :: String
    }

type App a = Reader Ctx a

run :: (HasFoo, HasBar) => String -> App String
run s = do
    f <- foo
    return $ f ++ s ++ bar

m' :: String
m' = runReader (run "|") ctx
  where ctx = Ctx "CTX:FOO" "CTX:BAR"