I read What's new in Xcode 6. The article introduces some new feature about Xcode 6, and it says:
Xcode’s debugger includes an interactive version of the Swift language, known as the REPL (Read-Eval-Print-Loop). Use Swift syntax to evaluate and interact with your running app or write new code in a script-like environment. The REPL is available from within LLDB in Xcode’s console, or from Terminal.
I want to know how to get the REPL?
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
then you can do one of these:
xcrun swift lldb --repl
As of Xcode 6.1 - typing
swift in the terminal launches the REPL as well.