// Gradebook: Student records each holding a list of Score records. Shows
// records-within-records, averaging a nested list, and assigning a derived
// letter grade with a guarded match.

cell Gradebook {
    face {
        signal roster() -> List
        signal gpa(name: String) -> Int
        signal letter(name: String) -> String
        signal honor_roll() -> List
    }

    on roster() {
        return list(
            Student { name: "Ada", scores: list(
                Score { subject: "math", pts: 95 },
                Score { subject: "cs",   pts: 99 }) },
            Student { name: "Ben", scores: list(
                Score { subject: "math", pts: 70 },
                Score { subject: "cs",   pts: 60 }) },
            Student { name: "Cal", scores: list(
                Score { subject: "math", pts: 88 },
                Score { subject: "cs",   pts: 82 }) }
        )
    }

    on gpa(name: String) {
        let s = roster() |> find(x => x.name == name)
        if s == () { return -1 }
        return avg(s.scores |> pluck("pts"))
    }

    on letter(name: String) {
        let g = gpa(name)
        return match g {
            n if n >= 90 -> "A"
            n if n >= 80 -> "B"
            n if n >= 70 -> "C"
            _            -> "F"
        }
    }

    on honor_roll() {
        return roster() |> filter(s => avg(s.scores |> pluck("pts")) >= 90) |> pluck("name")
    }
}

cell test GradebookTest {
    rules {
        assert gpa("Ada") == 97
        assert letter("Ada") == "A"
        assert letter("Ben") == "F"
        assert letter("Cal") == "B"
        assert honor_roll()[0] == "Ada"
        assert len(honor_roll()) == 1
    }
}