is-narrowing

is checks whether a value matches a pattern and, where it does, narrows the type accordingly. Inside the if r is Ok(n) branch, n is the Int payload of the Ok — already extracted, with no separate unwrap step and no way to reach for a value that is not there.

It is the lightweight counterpart to match: useful when you care about one shape and have a sensible fallback for the rest. Open it in the playground to see how n is in scope only where it is known to exist.

is-narrowing.bynk

commons demo

---
`is` tests a value against a pattern and narrows it. Inside the `if`, `n` is an
`Int` — the `Ok` payload — without an explicit unwrap.
---
fn useValue(r: Result[Int, String]) -> Int {
  if r is Ok(n) {
    n
  } else {
    0
  }
}

Open in playground ↗