Method at

packages/effect/src/Optic.ts:703–703

* Focuses on a key only if it exists (`Object.hasOwn`). Both * `getResult` and `replaceResult` fail when the key is absent. * * **Details** * * Unlike `.key()`, which always succeeds on the read side, `.at()` is * useful for Records or arrays where the key/index may not be present.

(
    this: Optional<S, A>,
    key: Key,
    ..._err: ForbidUnion<A, "cannot use `at` on a union type">
  )

   * @see `.key()` — when the key is always present
   */
  at<S, A extends object, Key extends keyof A>(
    this: Optional<S, A>,
    key: Key,
    ..._err: ForbidUnion<A, "cannot use `at` on a union type">