This is Swift’s REPL (Read-Evaluate-Print Loop) which lets us execute code interactively.
Note: You can alternatively set the DEVELOPER_DIR environment variable, or of course add this to your PATH.
This allows such things as:
[peacock@trashcan ~]$ swift Welcome to Swift! Type :help for assistance. 1> let seats = 7 seats: Int = 7 2> let family = 3 family: Int = 3 3> var extraSpace = (seats - family) extraSpace: (Int) = 4 4> :exit