It's a conscious decision not to use a hash. Why? Because it's considerably slower. You may say, "bah! who cares?" or some other variant of the "performance doesn't matter, and if it does switch to C" school of thought, but the POE designers disagree (and I tend to agree with them.) The point is that in writing library code, you don't want to arbitrarily limit the performance of the code. Put another way, you never want your framework to be the performance bottleneck for your application.