Merge commit 'be3e8236086165e5e45a5a10783823874b3f3ebd' as 'lib/vscode'

lib/vscode/extensions/markdown-language-features/src/util/lazy.ts

@@ -0,0 +1,39 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+export interface Lazy<T> {
+	readonly value: T;
+	readonly hasValue: boolean;
+	map<R>(f: (x: T) => R): Lazy<R>;
+}
+
+class LazyValue<T> implements Lazy<T> {
+	private _hasValue: boolean = false;
+	private _value?: T;
+
+	constructor(
+		private readonly _getValue: () => T
+	) { }
+
+	get value(): T {
+		if (!this._hasValue) {
+			this._hasValue = true;
+			this._value = this._getValue();
+		}
+		return this._value!;
+	}
+
+	get hasValue(): boolean {
+		return this._hasValue;
+	}
+
+	public map<R>(f: (x: T) => R): Lazy<R> {
+		return new LazyValue(() => f(this.value));
+	}
+}
+
+export function lazy<T>(getValue: () => T): Lazy<T> {
+	return new LazyValue<T>(getValue);
+}