Previous discussion: https://news.ycombinator.com/item?id=9052129 (312 points, 14 days ago, 103 comments)

It has a few interesting comments about why you can't try the example in the "try" box. It's because the "try" box only accepts expressions, not whole programs. In that thread, there are more details and how to rewrite the example as a onliner to try it in the box.