T9612.hs:16:9: error: [GHC-18872]
    • Couldn't match type: [(Int, a)]
                     with: (Int, a)
        arising from a functional dependency between:
          constraint ‘MonadWriter (Int, a) (WriterT [(Int, a)] Identity)’
            arising from a use of ‘tell’
          instance ‘MonadWriter w (WriterT w m)’ at T9612.hs:20:10-59
    • In a stmt of a 'do' block: tell (n, x)
      In the expression:
        do tell (n, x)
           return (1, y)
      In an equation for ‘f’:
          f y (n, x)
            = do tell (n, x)
                 return (1, y)
    • Relevant bindings include
        x :: a (bound at T9612.hs:14:8)
        y :: a (bound at T9612.hs:14:3)
        f :: a -> (Int, a) -> Writer [(Int, a)] (Int, a)
          (bound at T9612.hs:14:1)