The Xcode release notes are pretty clear on bugs that exists for a playground’s Sources folder:
Description: Sometimes Xcode will crash after editing a supporting source file for a playground and then viewing the playground itself. This happens when using the Project navigator to move back and forth between the playground and its enclosed files. (20094959)
Workaround: Open and edit the playground supporting source files in their own windows
Description: Playgrounds with supporting source files sometimes fail to execute, showing an error in the Console Output like: “Playground execution failed: error: Couldn’t lookup symbols: __TF16EnrichMe_Sources2hiFT_T_ __TF16EnrichMe_Sourcesau2piSd”. (20100043)
Workaround: View the supporting source file, then view and re-execute the playground
Description: Playgrounds with supporting source files sometimes fail to execute after changing the name of a function in the support file. You may see an error in the Console Output like: “Playground execution failed: MyPlayground.playground:6:1: error: use of unresolved identifier ‘myFunc’”. (20109247)
Workaround: Quit and relaunch Xcode. You may then need to view the supporting file, then view and re-execute the playground
There’s also a problem where new files are simply not recognized, and any material you add is ignored. When this happens quit and relaunch Xcode. This keeps biting me when I develop code in the playground and then create a new Swift source file, paste the code in, and try to access it from the playground.
Hopefully this post will help you avoid the wasted time I spent.