Errors and Option

Vow does not have null. Absence is Option<T>; fallible work is Result<T, E>.

Hands-on: Option & Result tutorial · Syntax ergonomics.

Option

fn unwrap_or(opt: Option<int>, fallback: int) -> int {
    return opt ?? fallback
}

Or exhaustive match on Some / None:

return match caps.fs_read {
    Some(c) => with c { read_poc("/tmp/vow_poc.txt") },
    None => 0 - 100
}

Some / None answer “was this grant passed at launch?” — not “did the call fail?” For failures, match Result (Ok / Err). See Option and Result tutorial.

Result

I/O builtins return Result<…, int> (integer error codes for v1).

Match

// fragment-only
return match read_file(path) {
    Ok(s) => len(s),
    Err(_) => 0 - 1
}

try / catch (sugar)

// fragment-only
let ok_val = try fallible(true) catch (e) {
    e
}

try {
    let x = fallible(false)
} catch (e) {
    print("caught err=", e)
}

Warning

try/catch desugars to matching Result. It is not stack-unwinding exceptions and does not bypass the type system.

? propagate

Propagate Err to the caller in one character when the enclosing function returns Result.

Caps fields are Option<Cap> — no grant means None.

Unused Result is an error

Discarding a Result without matching / try / ? is a type error.

vow check tests/lang/ignored_result.vow --json

Sugar that ships

Form	Example
??	none_i ?? 99
?.	Some("abc")?.len()
try / catch	Local Result handling
?	Propagate Err

Next

• Tutorial: Option & Result
• Syntax ergonomics
• Contracts

Note

See tests/lang/option.vow, tests/lang/ergonomics.vow, tests/lang/ignored_result.vow.