import Control.Monad.Reader

data Foo m = Foo { action :: m Int }

runFoo :: (MonadReader (Foo m) m) => m Int
runFoo = ask >>= action

bar :: IO Int
bar = runReaderT runFoo (Foo readLn)

{-
error:
    • Couldn't match type ‘IO’ with ‘ReaderT (Foo IO) IO’
        arising from a functional dependency between:
          constraint ‘MonadReader
                        (Foo (ReaderT (Foo IO) IO)) (ReaderT (Foo IO) IO)’
            arising from a use of ‘runFoo’
          instance ‘MonadReader r (ReaderT r m)’ at <no location info>
    • In the first argument of ‘runReaderT’, namely ‘runFoo’
      In the expression: runReaderT runFoo (Foo readLn)
      In an equation for ‘bar’: bar = runReaderT runFoo (Foo readLn)
-}


-- This does though:

runFoo :: (MonadReader (Foo n) m) => m (n Int)
runFoo = action <$> ask

ioFoo :: (MonadIO m) => Foo m
ioFoo = Foo (liftIO readLn)

bar :: (MonadIO m) => m Int
bar = join $ runReaderT runFoo ioFoo


-- This does not:

runFoo :: (MonadReader (Foo m) m) => m Int
runFoo = ask >>= action

ioFoo :: MonadIO m => Foo m
ioFoo = Foo $ liftIO readLn

bar :: MonadIO m => m Int
bar = runReaderT runFoo ioFoo