The Ultimate Playground - Coderunner 3
I prototype a lot. In many different languages. I personally work with Swift, but my university focuses on Java, C, Go and so on, and on my workplace as a part-time consultant, I work in a bunch of other languages.
On a personal plan, when i work with Swift, i like the thought of using Playgrounds to prototype different things - but let's be real: Playgrounds sucks (in xCode). Adding to that, it only works for Swift.
That is why i just love CodeRunner 3, it has a lot of built in language support, and it allows one just to pick a language and prototype. No setup, none. Just typing code and run, did it work? Great!
CodeRunner 3 is a beautiful native Mac app, and I highly recommend it.