import type { Schema } from "./Schema.js";
import { ThroughSchema, type ThroughSchemaOptions } from "./ThroughSchema.js";
/** Allowed options for `NullableSchema` */
export interface NullableSchemaOptions<T> extends ThroughSchemaOptions<T | null> {
    /** Default value (defaults to `null`). */
    readonly value?: T | null;
}
/** Validate a value of a specific type or `null`. */
export declare class NullableSchema<T> extends ThroughSchema<T | null> {
    readonly value: T | null;
    constructor({ value, ...options }: NullableSchemaOptions<T>);
    validate(unsafeValue?: unknown): T | null;
    format(value: T | null): string;
}
/** Create a new nullable schema from a source schema. */
export declare const NULLABLE: <T>(source: Schema<T>) => NullableSchema<T>;