const number = try parseU64(str, 10); // returns the error
    const number = parseU64(str, 10) catch 13; // default
    // more complex stuff
    if (parseU64(str, 10)) |number| {
        doSomethingWithNumber(number);
    } else |err| switch (err) {
        error.Overflow => {
            // handle overflow...
        },
        // we promise that InvalidChar won't happen (or crash in debug mode if it does)
        error.InvalidChar => unreachable,
    }