Swift Playgrounds is not a real REPL though as it re-evaluates the entire program after every code change†. While it nicely annotates the code with per-line res

Swift Playgrounds is not a real REPL though as it re-evaluates the entire program after every code change†. While it nicely annotates the code with per-line results, program state is not persisted between edits and side-effectful operations are repeated each time. That seriously limits its real-world usefulness to little more than a cute toy. Ironically, Swift does have a true REPL which you can invoke in Terminal (`swift`, unsurprisingly enough). That does persist state over the entire session while showing the output of each line, albeit with old-school CLI line editor. The Print bit really sucks though as it ignores `description` methods and does a full debug dump of complex structures every time. (Plus, of course, users need to be comfortable finding their way around a 1970s shell, which Playgrounds’ audience won’t be.) Two partial “REPL” implementations that could’ve been a single knock-it-out-the-park REPL had they bothered to consolidate and refine. Swift may be many things, but great at joined-up thinking it is not. -- (†Unless they’ve changed that behavior since I last tried it. Probably not.)