Introduce option type (#4150)

* Introduce option type

* Improve test naming

src/prelude/maybe.ts

@@ -0,0 +1,20 @@
+export interface Maybe<T> {
+	isJust(): this is Just<T>;
+}
+
+export type Just<T> = Maybe<T> & {
+	get(): T
+};
+
+export function just<T>(value: T): Just<T> {
+	return {
+		isJust: () => true,
+		get: () => value
+	};
+}
+
+export function nothing<T>(): Maybe<T> {
+	return {
+		isJust: () => false,
+	};
+}

test/prelude/maybe.ts

@@ -0,0 +1,28 @@
+/*
+ * Tests of Maybe
+ *
+ * How to run the tests:
+ * > mocha test/prelude/maybe.ts --require ts-node/register
+ *
+ * To specify test:
+ * > mocha test/prelude/maybe.ts --require ts-node/register -g 'test name'
+ */
+
+import * as assert from 'assert';
+import { just, nothing } from '../../src/prelude/maybe';
+
+describe('just', () => {
+	it('has a value', () => {
+		assert.deepStrictEqual(just(3).isJust(), true);
+	});
+
+	it('has the inverse called get', () => {
+		assert.deepStrictEqual(just(3).get(), 3);
+	});
+});
+
+describe('nothing', () => {
+	it('has no value', () => {
+		assert.deepStrictEqual(nothing().isJust(), false);
+	});
+});