Use values through a box

Call collection and string methods through a box, including nested boxes. Methods borrow its content; mutation requires a mut binding or an exclusive reference:

def main() -> Result[(), AllocError | IoError]:
    mut items = Box(Box([1, 2]?)?)?
    items.append(3)?
    match items.get(2):
        case Some(value):
            print(value)?
        case Nothing:
            print("missing item")?
    text = Box("  hello  ")?
    print(text.strip()?)?
    print(text.startswith("  "))?
    Ok(())

3
hello
True

Read-only methods also work on temporary boxes. Bind a box with mut before calling a mutating method.

The recursive-data lesson explains when a box transfers its contents and when it keeps ownership of them.