Merge commit 'be3e8236086165e5e45a5a10783823874b3f3ebd' as 'lib/vscode'

lib/vscode/src/vs/base/common/lazy.ts

@@ -0,0 +1,70 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+/**
+ * A value that is resolved synchronously when it is first needed.
+ */
+export interface Lazy<T> {
+
+	hasValue(): boolean;
+
+
+	getValue(): T;
+
+
+	map<R>(f: (x: T) => R): Lazy<R>;
+}
+
+export class Lazy<T> {
+
+	private _didRun: boolean = false;
+	private _value?: T;
+	private _error: Error | undefined;
+
+	constructor(
+		private readonly executor: () => T,
+	) { }
+
+	/**
+	 * True if the lazy value has been resolved.
+	 */
+	hasValue() { return this._didRun; }
+
+	/**
+	 * Get the wrapped value.
+	 *
+	 * This will force evaluation of the lazy value if it has not been resolved yet. Lazy values are only
+	 * resolved once. `getValue` will re-throw exceptions that are hit while resolving the value
+	 */
+	getValue(): T {
+		if (!this._didRun) {
+			try {
+				this._value = this.executor();
+			} catch (err) {
+				this._error = err;
+			} finally {
+				this._didRun = true;
+			}
+		}
+		if (this._error) {
+			throw this._error;
+		}
+		return this._value!;
+	}
+
+	/**
+	 * Get the wrapped value without forcing evaluation.
+	 */
+	get rawValue(): T | undefined { return this._value; }
+
+	/**
+	 * Create a new lazy value that is the result of applying `f` to the wrapped value.
+	 *
+	 * This does not force the evaluation of the current lazy value.
+	 */
+	map<R>(f: (x: T) => R): Lazy<R> {
+		return new Lazy<R>(() => f(this.getValue()));
+	}
+}