type grade = Pass | No_pass

module type STUDENT = sig
    type student
    val make : string -> student
    val get_tests : student -> int list
end

module Student : STUDENT = struct
    type student = {
        name : string;
        tests : int list;
        course_grade : grade option;
    }

    let make name = {
        name = name;
        tests = [];
        course_grade = None
    }

    let get_tests s = s.tests

    let get_test_score s i = List.nth s.tests i

end

module type GRADSTUDENT = sig
    include module type of Student
    val compute_course_grade : student -> int
end

module GraduateStudent : GRADSTUDENT = struct
    include Student
    let compute_course_grade s =
        let tests = s.tests in  (* <-- THIS WON'T COMPILE *)
        let total = List.fold_left (+) 0 tests in
        let grade = match total / List.length tests with
            | q when q > 80 -> Pass
            | _ -> No_pass
        in
        10
end