Indexer

// Square brackets are an operator too. A type that declares `func []` can be read as `week[day]`,
// and one that also declares `func []=` can be written as `week[day] = 21`. They are declared in
// an `extend` block like `+` and `==` were.
//
// The index need not be a number. Here a week of temperatures is indexed by day, so a reader sees
// `week[Day::Friday]`, and an index the week does not have, such as 9, cannot even be written:
// a plain number is not a `Day`.
import Io::PrintLine;

enum Day {
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

// The highest temperature of each day, stored in an ordinary array.
struct Week {
    highs: int32[7];
}

extend Week {
    // Reading: `week[day]` calls this and gives back a copy of the value.
    func [](self: &Week, day: Day) -> int32 {
        return self.highs[day as uint];
    }

    // Writing: `week[day] = value` calls this. It changes the week, so it takes `&var` self.
    func []=(self: &var Week, day: Day, value: int32) {
        self.highs[day as uint] = value;
    }
}

func Main() -> int {
    var week = Week { highs: [0; 7] };

    week[Day::Monday] = 18;
    week[Day::Tuesday] = 21;
    week[Day::Friday] = 25;
    PrintLine("Monday {}, Tuesday {}, Friday {}", week[Day::Monday], week[Day::Tuesday],
              week[Day::Friday]);

    // Reading and writing are two separate functions, so changing a value in place takes both.
    week[Day::Friday] = week[Day::Friday] - 3;
    PrintLine("Friday, corrected: {}", week[Day::Friday]);

    // A day nobody wrote still holds the zero it started with.
    PrintLine("Sunday {}", week[Day::Sunday]);

    // Writing `week[Day::Friday] -= 3` is refused, because no single function both reads and
    // writes: "operator '-=' cannot read and write through the '[]' operator on 'Week' at once".
    return 0;
}