Draft parallel implementation.

runner.js

@@ -71,9 +71,58 @@ function series(list, cb) {
   next();
 }
 
+/**
+ *  Execute task functions in parallel.
+ *
+ *  @function parallel
+ *  @member Runner
+ *  @param {Array} list of task functions.
+ *  @param {Number} [concurrent] number of concurrent calls.
+ *  @param {Function} cb callback function.
+ */
+function parallel(list, concurrent, cb) {
+  if(concurrent instanceof Function) {
+    cb = concurrent;
+    concurrent = 0;
+  }
+
+  var items = list.slice()
+    , scope = this.scope
+    , complete = 0;
+
+  concurrent = concurrent || items.length;
+
+  function run() {
+    for(var i = 0;i < concurrent;i++) {
+      items[i].call(scope, next);
+    }
+    items = items.slice(concurrent);
+  }
+
+  function next(err) {
+    if(err) {
+      return cb(err); 
+    }
+
+    complete++;
+
+    if(complete >= list.length) {
+      return cb(); 
+    }
+
+    if(items.length && (complete % concurrent === 0)) {
+      run(); 
+    }
+  }
+
+  run();
+}
+
 /**
  *  Execute a task by name identifier.
  *
+ *  Dependencies are run in parallel before task execution.
+ *
  *  @function exec
  *  @member Runner
  *  @param {Function|String} id task identifier.
@@ -88,13 +137,24 @@ function exec(id, cb) {
     throw new Error('task not found: ' + id); 
   }
 
-  this.series(task.tasks, cb);
+  function onDependencies(err) {
+    if(err) {
+      return cb(err); 
+    }
+    this.series(task.tasks, cb);
+  }
 
+  if(task.deps.length) {
+    this.parallel(task.deps, onDependencies.bind(this));
+  }else{
+    this.series(task.tasks, cb);
+  }
   return task;
 }
 
 Runner.prototype.get = get;
 Runner.prototype.exec = exec;
 Runner.prototype.series = series;
+Runner.prototype.parallel = parallel;
 
 module.exports = runner;

test/spec/run.js

@@ -85,6 +85,42 @@ describe('mktask:', function() {
     });
   });
 
+  it('should exec task functions dependencies in parallel', function(done) {
+    var mk = mktask()
+      , called = 0
+      , docsCalled
+      , apiCalled
+      , readmeCalled;
+
+    function docs(cb) {
+      called++;
+      docsCalled = 1;
+      cb();
+    }
+
+    function api(cb) {
+      called++;
+      apiCalled = 1;
+      cb();
+    }
+
+    function readme(cb) {
+      called++;
+      readmeCalled = 1;
+      cb();
+    }
+
+    mk.task([docs, api], readme);
+
+    var runner = mk.run();
+    runner.exec(readme, function() {
+      expect(called).to.eql(3); 
+      expect(docsCalled).to.eql(1); 
+      expect(apiCalled).to.eql(1); 
+      expect(readmeCalled).to.eql(1); 
+      done();
+    });
+  });
 
   it('should callback with error on task function exec', function(done) {
     var mk = mktask()