grading in coursera programming languages

grading.sml

datatype note = ProgrammingAssignments of int list
       | PeerAssignments of int list
       | Exam of int list
       | Note of note list

fun grading n =
    case n of
        ProgrammingAssignments pl => (case pl of
                                          [] => 0.0
                                        | x::xs => 0.9 * Real.fromInt(x) + grading (ProgrammingAssignments xs))
     |  PeerAssignments pa => (case pa of
                                   [] => 0.0
                                 | x::xs => 0.1 * Real.fromInt(x) + grading (PeerAssignments xs))
     | Exam e => (case e of
                      [] => 0.0
                    | x::xs => 0.15 * Real.fromInt(x) + grading (Exam xs))
     | Note n => (case n of
                      [] => 0.0
                    | x::xs => grading x + grading (Note xs));