Add parTraverseTask to IList.

lib/src/ilist.dart

@@ -290,6 +290,14 @@ abstract class IList<A> implements TraversableMonadPlusOps<IList, A> {
     return result.map((l) => l.reverse());
   }
 
+  Task<IList<B>> parTraverseTask<B>(Task<B> f(A a)) {
+    return map(f).foldLeft(
+      Task.value(nil()),
+      (previous, next) =>
+        previous.both(next).map((a) => a.value1.appendElement(a.value2))
+    );
+  }
+
   Evaluation<E, R, W, S, IList<B>> traverseEvaluation<B, E, R, W, S>(Monoid<W> WMi, Evaluation<E, R, W, S, B> f(A a)) {
     Evaluation<E, R, W, S, IList<B>> result = new Evaluation(WMi, (r, s) => new Future.value(new Right(new Tuple3(WMi.zero(), s, nil()))));
     var current = this;

test/ilist_test.dart

@@ -176,4 +176,22 @@ void main() {
 
   test("isEmpty", () => qc.check(forall(intILists, (IList<int> il) => (il.length() == 0) == il.isEmpty)));
 
+  test("traverseTask is serial", () async {
+    final t = IList.from([1, 2, 3]).traverseTask(
+      (i) => Task.value(i).delayBy(const Duration(seconds: 1)));
+
+    final result = await t.timed.run();
+
+    expect(result.value1 >= const Duration(seconds: 3), true);
+  });
+
+  test("parTraverseTask is concurrent", () async {
+    final t = IList.from([1, 2, 3, 4, 5]).parTraverseTask(
+      (i) => Task.value(i).delayBy(const Duration(seconds: 1)));
+
+    final result = await t.timed.run();
+
+    expect(result.value1 <= const Duration(milliseconds: 1100), true);
+  });
+
 }