wscript: pattern matching & errors

Option[T] and Result[T,E] are built in; let-else, match (exhaustive), and ? are the idioms vmlab scripts live on.

Option[T] and Result[T, E] are built in. All fallible vmlab API calls return Result[..., string].

rust

// let-else: bail early (block must diverge — return/break)
let Ok(dc) = lab.vm("dc01") else {
    lab.log("dc01 is not defined")
    return
}

// match (exhaustiveness-checked at compile time)
match dc.wait_ready(600) {
    Ok(_)  => lab.log("ready"),
    Err(e) => { lab.log("not ready: " + e); return }
}

// ? propagates Err/None out of a function with a matching return type
fn step(lab: Lab) -> Result[unit, string] {
    let dc = lab.vm("dc01")?
    dc.wait_ready(600)?
    Ok(())
}

Methods: is_some is_none unwrap unwrap_or expect (Option) / is_ok is_err unwrap unwrap_or unwrap_err expect (Result).