export class DiscardablePromise<T> implements PromiseLike<T> {
	constructor(private promise: Promise<T>) { }

	static discarded = Symbol("discarded");

	isDiscarded: boolean;

	then<TResult1 = T, TResult2 = never>(onfulfilled?: (value: T) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>): PromiseLike<TResult1 | TResult2> {
		return this.promise.then(x => {
			if (this.isDiscarded) {
				return Promise.reject(DiscardablePromise.discarded);
			}
			else {
				return Promise.resolve(x);
			}
		}).then(onfulfilled, onrejected);
	}

	discard() {
		this.isDiscarded = true;
	}
}

/**
 * Sets the internal state of a promise to discarded
 * @param discardable A promise to discard
 */
export function discard<T>(discardable: DiscardablePromise<T> | null | undefined) {
	if (discardable) {
		discardable.discard();
	}
}