REPL

REPL stands for read-eval-print-loop. Swift REPL is an interactive Swift code interpreter that executes code immediately. To launch Swift REPL, open Terminal. app and execute this command:

$ xcrun swift

Now, you can start typing Swift code and see the result. A nice thing about evaluating code in REPL is that if you make an error that would eventually stop the application execution if you compile and run it, you can still continue evaluating the code and preserve all of the progress. Let's play around and try this code in Swift REPL:

let a = 10
let b = a + "c"
let b = a + 10

Writing code in the REPL console is not as convenient as in the modern Xcode IDE, but being familiar with it is a useful skill. On top of the Swift REPL, Apple has built more powerful tools, such as Playgrounds, which has a nice source code editor and flexibility of Swift REPL.