T13311.hs:9:3: error: [GHC-83865]
    • Couldn't match expected type: IO a0
                  with actual type: Maybe a1 -> Maybe b0
    • Probable cause: ‘f’ is applied to too few arguments
      In a stmt of a 'do' block: f
      In the expression:
        do f
           putChar 'a'
      In an equation for ‘g’:
          g = do f
                 putChar 'a'