import { OptionalRuntype, Runtype } from './runtype';
/**
 * Optional (?), only usable within `record`
 *
 * Marks the key its used on as optional, e.g.:
 *
 *    record({foo: optional(string())})
 *
 *    => {foo?: string}
 */
export declare function optional<A>(t: Runtype<A>): OptionalRuntype<A>;