/**
 * A (lazy) Singleton class that will store a single value.
 * The value will only be loaded when asked (lazy).
 *
 * It also works with promised values as they are only fulfilled once.
 *
 * @see {@link https://en.wikipedia.org/wiki/Singleton_pattern}
 */
export declare class Singleton<T> {
    private readonly loader;
    /**
     * Has the Singleton been called ?
     */
    private called;
    /**
     * The singleton value.
     * It can not be used to determine if it is initialized: The final value could be falsy
     */
    private value?;
    /**
     * @returns If the Singleton loader has been called
     */
    get hasBeenCalled(): boolean;
    /**
     * Creates a Singleton
     *
     * @param loader The function called to load the singleton value
     */
    constructor(loader: () => T);
    /**
     * Runs the loader if it has not already been called, then returns the value.
     * Only returns the value if it has already been called
     *
     * @returns The value of this singleton
     */
    get(): T;
}