This project includes some common and useful extensions for swift
Extension class is in the Extensions folder. Just download it and insert it into your project, then you can use it as usual.
Because Playgrounds are self-contained, so I copy it into Playground's subfolder 'Sources'. Maybe you would find that extension class in 'Sources' is different from class in 'Extensions'. In Playgrounds, you must add public to the class definition. See this example.