Implement Future stateless and lazy Promise analogue

Refs: #431

lib/future.js

@@ -0,0 +1,45 @@
+'use strict';
+
+class Future {
+  constructor(executor) {
+    this.executor = executor;
+  }
+
+  static of(value) {
+    return new Future(resolve => resolve(value));
+  }
+
+  chain(fn) {
+    return new Future((resolve, reject) =>
+      this.fork(
+        value => fn(value).fork(resolve, reject),
+        error => reject(error)
+      )
+    );
+  }
+
+  map(fn) {
+    return this.chain(
+      value =>
+        new Future((resolve, reject) => {
+          try {
+            resolve(fn(value));
+          } catch (error) {
+            reject(error);
+          }
+        })
+    );
+  }
+
+  fork(successed, failed) {
+    this.executor(successed, failed);
+  }
+
+  promise() {
+    return new Promise((resolve, reject) => {
+      this.fork(value => resolve(value), error => reject(error));
+    });
+  }
+}
+
+module.exports = { Future };

metasync.js

@@ -12,6 +12,7 @@ const submodules = [
   'control', // Control flow utilities
   'do', // Simple chain/do
   'fp', // Async utils for functional programming
+  'future', // Future stateless and lazy Promise analogue
   'memoize', // Async memoization
   'poolify', // Create pool from factory
   'queue', // Concurrent queue

test/future.js

@@ -0,0 +1,92 @@
+'use strict';
+
+const { Future } = require('..');
+const metatests = require('metatests');
+
+metatests.test('Future map/fork', async test => {
+  Future.of(3)
+    .map(x => x ** 2)
+    .fork(value => {
+      test.strictSame(value, 9);
+      test.end();
+    });
+});
+
+metatests.test('Future lazy', async test => {
+  Future.of(3).map(test.mustNotCall());
+  test.end();
+});
+
+metatests.test('Future resolve', async test => {
+  new Future(resolve => {
+    setTimeout(() => {
+      resolve(5);
+    }, 0);
+  }).fork(value => {
+    test.strictSame(value, 5);
+    test.end();
+  }, test.mustNotCall());
+});
+
+metatests.test('Future reject', async test => {
+  new Future((resolve, reject) => {
+    reject(new Error('msg'));
+  }).fork(test.mustNotCall(), error => {
+    test.strictSame(error.message, 'msg');
+    test.end();
+  });
+});
+
+metatests.test('Future promise', async test => {
+  const value = await Future.of(6)
+    .map(x => ++x)
+    .map(x => x ** 3)
+    .promise();
+
+  test.strictSame(value, 343);
+  test.end();
+});
+
+metatests.test('Future catch', async test => {
+  Future.of(6)
+    .map(() => {
+      throw new Error('msg');
+    })
+    .fork(test.mustNotCall, error => {
+      test.strictSame(error.message, 'msg');
+      test.end();
+    });
+});
+
+metatests.test('Future stateless', async test => {
+  const f1 = Future.of(3);
+  const f2 = f1.map(x => ++x);
+  const f3 = f2.map(x => x ** 3);
+  const f4 = f1.map(x => x * 2);
+
+  f1.fork(value => {
+    test.strictSame(value, 3);
+  });
+
+  f1.fork(value => {
+    test.strictSame(value, 3);
+  });
+
+  f2.fork(value => {
+    test.strictSame(value, 4);
+  });
+
+  f2.fork(value => {
+    test.strictSame(value, 4);
+  });
+
+  f3.fork(value => {
+    test.strictSame(value, 64);
+  });
+
+  f4.fork(value => {
+    test.strictSame(value, 6);
+  });
+
+  test.end();
+});