interface Optional {
    <T, F>(fn: (x: T) => F, x: T): F | true;
    <T, F>(fn: (x: T) => F): (x: T) => F | true;
}
/**
 * Wraps a one-parameter function,
 * new function returns true if passed parameter is undefined, otherwise returns result of calling `fn`
 *
 * @param {Function} fn function to wrap
 * @returns {Function}
 * @example
 *
 *      const optIsString = optional(isString);
 *      optIsString() //=> true
 *      optIsString({}) //=> false
 *      optIsString('str') //=> true
 */
export declare const optional: Optional;
export default optional;