B Ohr writes:

@ericasadun just successfully created a symbolic link in the Sources folder of a playground to a swift file outside with Xcode 6.3.2

— B. Ohr (@J7zz) May 23, 2015

So I put it to the test.

Single File Test

First test: a single file at the top level of a playground.

1. Create a playground
2. Navigate to its Sources (with the playground closed)
3. Create symbolic link

% cd Sources/
% lns ~/Desktop/test.swift test.swift
Made test.swift -> /Users/ericasadun/Desktop/test.swift

Next, I launched the playground, opened the project navigator, edited test.swift, and attempted to save it.

Also this:

Every time I then tried to close the playground, I got this:

I had to force-quit Xcode. The edits never made it to the file. I’d call that a fail.

Folder Test

Next up, symlinks to source folders. Same steps but the source is enclosed in a top-level folder.

% cd ~/Desktop/SymTest2.playground/
% cd Sources
% lns ~/Desktop/External\ Sources/ External\ Sources
Made External Sources -> /Users/ericasadun/Desktop/External Sources

This time, I was able to load, tweak, and save without incident. So, that’s a success.

Dangerous Waters

That said, I did encounter  issues with otherwise “behaving” symlink playgrounds. There was this:

And sometimes there was this:

and

which did ever not resolve, even after I left it for a quarter an hour to spin its wheel. This happened specifically after I closed a symlinked playground, created an additional symlink and then re-launched.

Conclusions

Although you’re likely to have greatest success using symlinks to enclosing folders, the entire symlink system remains pretty fragile. Caveat codeur.