UPDATE 9/11/18: This issue has been fixed in Xcode 11 GM, so if you use that (or the Xcode 11 final release) you should be set! :]
Heads up for those who preordered the Combine book: use Xcode beta 5 if you want to try out the sneak peek chapter.
In the current beta 6 release of Xcode 11, playgrounds are unstable. The Combine book uses playgrounds in several of its chapters, including chapter 2 “Publishers and Subscribers.” If you have access to and can use Xcode beta 5, you should encounter no problems.
However, because many readers may no longer have access to beta 5 — and beta 6 is unusable — we have decided to postpone the EA #2 release of Combine: Asynchronous Programming with Swift. We will closely monitor things on the Xcode release front, and will move forward with the release as soon as we can verify Xcode playgrounds work as expected.
If you have any questions or concerns about this, feel free to reply here. Thanks!