Implement futurify callback-last to future-returning

Closes: #431

lib/adapters.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const { Future } = require('./future');
+
 // Convert Promise to callback-last
 //   promise <Promise>
 //   callback <Function>
@@ -67,10 +69,23 @@ const promisifySync = fn => (...args) => {
   return Promise.resolve(result);
 };
 
+// Convert callback contract to Future-returning function
+//   fn <Function> callback-last function
+//
+// Returns: <Function> Future-returning function
+const futurify = fn => (...args) =>
+  new Future((resolve, reject) => {
+    fn(...args, (err, data) => {
+      if (err) reject(err);
+      else resolve(data);
+    });
+  });
+
 module.exports = {
   callbackify,
   asyncify,
   promiseToCallbackLast,
   promisify,
   promisifySync,
+  futurify,
 };

test/future.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const { Future } = require('..');
+const { Future, futurify } = require('..');
 const metatests = require('metatests');
 
 metatests.test('Future map/fork', async test => {
@@ -90,3 +90,37 @@ metatests.test('Future stateless', async test => {
 
   test.end();
 });
+
+metatests.test('Future futurify success', async test => {
+  const f1 = (a, b, callback) => {
+    if (typeof a !== 'number' || typeof b !== 'number') {
+      callback(new Error('Arguments must be numbers'));
+      return;
+    }
+    callback(null, a + b);
+  };
+
+  const f2 = futurify(f1);
+
+  f2(10, 20).fork(value => {
+    test.strictSame(value, 30);
+    test.end();
+  });
+});
+
+metatests.test('Future futurify fail', async test => {
+  const f1 = (a, b, callback) => {
+    if (typeof a !== 'number' || typeof b !== 'number') {
+      callback(new Error('Arguments must be numbers'));
+      return;
+    }
+    callback(null, a + b);
+  };
+
+  const f2 = futurify(f1);
+
+  f2('10', '20').fork(test.mustNotCall, error => {
+    test.strictSame(error.message, 'Arguments must be numbers');
+    test.end();
+  });
+});