Validate and transform

Reps.of(n) checks a raw Int against the refinement — here, that it falls in the range 1 to 100 — and returns a Result. This is the bridge from untrusted input to a refined value: the validation happens once, explicitly, and produces either the constrained value or an error.

The ? operator then does the routing. It unwraps a success in place, or returns the error from the enclosing function early, so everything after let r = Reps.of(n)? only ever sees a value that has already passed the check. Open it in the playground to follow the value from raw Int to validated Reps.

commons demo

---
`Reps.of(n)` validates an `Int` against the refinement and returns a `Result`.
The `?` operator unwraps the success or returns the error early, so the rest of
the function only ever sees a value that already passed the check.
---
type Reps = Int where InRange(1, 100)

fn doubled(n: Int) -> Result[Int, ValidationError] {
  let r = Reps.of(n)?
  Ok(r * 2)
}

Open in playground ↗