small example for Writer vs WriterT error message

Main.hs

module Main where

import Control.Monad.Trans.Writer
import Data.Monoid ( Sum(..) )

f :: Writer (Sum Int) ()
f = undefined

g :: Writer (Sum Int) Int
g = do
  tell $ Sum 1
  f

main = return ()

{-
Main.hs:10:3:
    Couldn't match type ‘()’ with ‘Int’
    Expected type: WriterT (Sum Int) Data.Functor.Identity.Identity Int
      Actual type: Writer (Sum Int) ()
    In a stmt of a 'do' block: f
    In the expression:
      do { tell $ Sum 1;
           f }
Failed, modules loaded: none.
-}