In your other thread you wrote:
I have now installed Xcode 7.3 and now it is running. To continue
learning, I downloaded the files from the "Everyone Can Program" book
for Xcode 8.
Swift changed quite a bit between Xcode 7 and Xcode 8. Trying to use an Xcode 8 playground with Xcode 7 is unlikely to end well.
Operating system High Sierra
If you’re on 10.13, you can run Xcode 8. If you’re deep into the Xcode 8 tutorial, it would make more sense to do that using Xcode 8 rather than Xcode 7.
On the other hand, 10.13.x can also run Xcode 9 and 10, so it might make sense to use a more modern tutorial.
Share and Enjoy
Quinn “The Eskimo!” @ DTS @ Apple