add promote function and tests

README.md

@@ -62,3 +62,13 @@ const readAndWrite = oath.co(function*(suffix) {
 
 readAndWrite('foo').then(console.log, console.err);
 ```
+### promote
+```javascript
+const fs = require('fs');
+
+const readFile = oath.promote(fs.readFile);
+
+readFile('foo.txt', {encoding: 'utf8'})
+  .then(filecontents => console.log('Here is foo.txt: ', filecontents))
+  .catch(error => console.error('Got an error!', error));
+```

index.js

@@ -37,3 +37,23 @@ exports.co = function(generator) {
     });
   });
 };
+
+// promote :: (a -> (Error -> b -> ()) -> ()) -> (a -> Promise b)
+exports.promote = function(nodeFunction) {
+  return arity(nodeFunction.length - 1, () => {
+    const $functionArgs = arguments;
+    return new Promise((resolve, reject) => {
+      Array.prototype.push.call(
+        $functionArgs,
+        error => {
+          if (error != null) {
+            reject(error);
+          } else {
+            resolve.apply(null, Array.prototype.slice.call(arguments, 1));
+          }
+        }
+      );
+      nodeFunction.apply(null, $functionArgs);
+    });
+  });
+};

test/index.js

@@ -75,3 +75,52 @@ describe('co', () => {
     });
   });
 });
+
+describe('promote', () => {
+  it('returns a function with the appropriate length', () => {
+    const f = oath.promote(callback => {
+      setTimeout(() => {
+        callback(null, 'foo');
+      }, 100);
+    });
+    assert.strictEqual(f.length, 0);
+    const g = oath.promote((a, callback) => {
+      setTimeout(() => {
+        callback(null, a);
+      }, 100);
+    });
+    assert.strictEqual(g.length, 1);
+    const h = oath.promote((a, b, callback) => {
+      setTimeout(() => {
+        callback(null, a, b);
+      }, 100);
+    });
+    assert.strictEqual(h.length, 2);
+  });
+
+  it('returns a function which returns a promise', () => {
+    const f = oath.promote(callback => {
+      setTimeout(() => {
+        callback(null, 'foo');
+      }, 100);
+    });
+    const p = f();
+    assert(p instanceof Promise);
+  });
+
+  it('returns a function which returns a promise for a value', (done) => {
+    const f = oath.promote(callback => {
+      setImmediate(() => {
+        callback(null, 'foo');
+      });
+    });
+    const p = f();
+    assert(p instanceof Promise);
+    p.then(value => {
+      assert.strictEqual(value, 'foo');
+      done();
+    }, () => {
+      assert.fail();
+    }).catch(err => console.err(err));
+  });
+});