/** A validation rule, given the value and name, return the error string if valid, or undefined if valid. */
export type Rule<V> = (opts: {
    value: V;
    key: string;
    originalValue: V;
}) => string | undefined;
/** A rule that validates `value` is not `undefined`, `null`, or empty string. */
export declare const required: <V>({ value: v }: {
    value: V;
}) => string | undefined;