T6001.hs:8:18:
    Couldn't match type ‘Integer’ with ‘Int’
    Expected type: Integer -> DayKind
      Actual type: Int -> DayKind
    When checking that instance signature for ‘fromInteger’
      is more general than its signature in the class
      Instance sig: Int -> DayKind
         Class sig: Integer -> DayKind
    In the instance declaration for ‘Num DayKind’