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Nullable values

Represent absence explicitly.

nullable.voidDownload source ↓
func double(value: i32) -> i32:
    return value * 2
func apply<t>(callback: func(t) -> t, value: t) -> t:
    return callback(value)
func entry(value: i32) -> i32:
    var callback: (func(i32) -> i32)?
    if callback is Ok(var absent):
        return absent(value)
    callback = double
    return match callback:
        Ok(var present): apply(present, value)
        null: -1
Input 5Expected result 10

Make absence part of the type

A question mark marks a nullable type. Parentheses distinguish a nullable function value from a function with a nullable result: (func(i32) -> i32)? is a callback that may be null.

Check before using the value

Ok(var present) matches a present nullable value and captures a nonnullable view. This use of Ok does not convert nullable values into result values. null matches absence.

Follow a callback from absent to present

The callback starts absent, then receives double. The successful match passes it to apply and returns 10 for input 5. Invoking a null callback would be a runtime failure; the match makes the intended case explicit.