option¶

Option is a value that is either present or absent. It is part of the prelude, so it needs no import. A lookup that may find nothing answers one: m.get(k), xs.first(), xs.max().

The type¶

Option is an ordinary candela enum with a type parameter for what it holds:

enum Option<T> {
    Some(T),
    None,
}

You construct and match the variants directly:

fn main() {
    let o = Some(5);
    match o {
        Some(v) => { print(v); }
        None => { print("nothing"); }
    }
}

Some(x) needs no type argument: the value it is given decides the option's type, so Some(5) is an Option<int> and the value bound in a Some arm comes back with the type that went in.

struct Point {
    x: int,
    y: int,
}

fn nearest(points: Point[]) -> Option<Point> {
    if points.len() == 0 {
        return None;
    }
    return Some(points[0]);
}

fn main() {
    match nearest([Point { x: 1, y: 2 }]) {
        Some(p) => { print(p.x, p.y); }
        None => { print("no points"); }
    }
}

None names no payload, so on its own it is an Option<any>, which goes wherever an Option<T> is expected. Name the argument (Option<Point>) where you want the check. See enums for the enum and match syntax, and generics for type parameters.

x? reads the value out of a Some and hands a None back to the caller of the function it is written in, which has to return an Option too:

fn total(prices: {string: int}) -> Option<int> {
    let a = prices.get("apple")?;
    let b = prices.get("pear")?;
    return Some(a + b);
}

fn main() {
    print(total({"apple": 2, "pear": 3}));
    print(total({"apple": 2}));
}

See error handling for the rule.

The module is pure candela, so it compiles into a .cdlb artifact and runs under candela-vm with no dynamic library.

Methods¶

The helpers are methods on the option value, defined in an impl Option<T> block.

is_some¶

o.is_some()

• Returns: a bool, true when the option holds a value.

is_none¶

o.is_none()

• Returns: a bool, true when the option is empty.

unwrap¶

o.unwrap()

• Returns: the contained value.
• Raises: called unwrap on a None option when the option is None.

unwrap_or¶

o.unwrap_or(default)

• default: the value to return when the option is None. It has the option's own payload type.
• Returns: the contained value, or default.
• Raises: nothing.

map¶

o.map(f)

• f: takes the contained value, returns the mapped value.
• Returns: Some(f(v)) when the option holds v, and None when it is empty. f is not called on a None.

and_then¶

o.and_then(f)

• f: takes the contained value and returns an option of its own.
• Returns: f(v) when the option holds v, and None when it is empty. f is not called on a None. Use it to chain steps that may each come back empty, where map would give you an option inside an option.

or¶

o.or(other)

• other: the option to fall back on. It has the same payload type.
• Returns: this option when it holds a value, and other when it is empty.

filter¶

o.filter(pred)

• pred: takes the contained value and returns a bool.
• Returns: this option when it holds a value pred answers true for, and None otherwise. pred is not called on a None.

fn describe(x) { return "value " + string(x); }

fn main() {
    let s = Some(5);
    let n = None;
    print(s.map(describe).unwrap());
    print(n.unwrap_or(0));
}

For a value that carries a reason for being absent, use result instead.