In Swift, unlike most other languages, the curly bracket is required after conditional or loop statements. This is one of the safety features that are built into Swift. Arguably, there have been numerous security bugs that may have been prevented if the developer would have used curly braces. These bugs could also have been prevented by other means such as unit testing and code reviews, but requiring developers to use curly braces, in my opinion, is a good security standard.
The following Playground shows you the error you get if you forget to include curly braces: