Errors and absence

Resid has no null and no exceptions. Absence is Option(T); failure that a caller should handle is Result(T, E).

Option

Option(Int) find_index(List(Str) xs, Str want, Int i) {
    if (i >= xs.len()) { return None; }
    if (xs[i] == want) { return Some(i); }
    return find_index(xs, want, i + 1);
}

Int main() {
    List(Str) xs = ["a", "b", "c"];
    println(f"{find_index(xs, "c", 0)} {find_index(xs, "z", 0)}");
    Int at = find_index(xs, "b", 0) else { -1 };
    println(f"{at}");
    return 0;
}

Output

Some(2) None
1

value else { default } unwraps an Option or Result, running the block when there is no value.

Result and ?

Result(Int, Str) parse_port(Str s) {
    if (!str_is_int(s)) { return Err(f"not a number: {s}"); }
    Int p = str_parse_int(s);
    if (p < 1 || p > 65535) { return Err(f"out of range: {p}"); }
    return Ok(p);
}

Result(Int, Str) next_port(Str s) {
    Int p = parse_port(s)?;        // an Err returns from next_port at once
    return Ok(p + 1);
}

Int main() {
    println(f"{next_port("8080")} {next_port("http")} {next_port("99999")}");
    return 0;
}

Output

Ok(8081) Err("not a number: http") Err("out of range: 99999")

e? unwraps a value or returns the None / Err from the enclosing function, so it needs a function that returns an Option or a Result. In main, handle errors with else or match.

Aborts are for bugs

Some failures are not values but program errors, and they abort the process with a message: arithmetic overflow, division by zero, an index out of range, a checked conversion that does not fit, a broken constraint, assert(false, "..."), todo(). Inside a spawn region an abort becomes an Err(RegionError) for the parent instead.