Everyone says that Swift is a "modern" lenguaje, but which characteristics in specific makes Swift "modern" ??
"Modern" means that something is contemporary. I'm sorry if it sounds obvious, but lots of people confuse "modern" with "futuristic" when it doesn't mean that. It means that it reflects the state of the art today, learning from past experiences and being influenced by current trends. "Modern" doesn't even mean novel - coroutines are quite modern, in that they're being used more than ever nowadays and lots of languages recently added support for them, but they date back to punch-cards.
Sorry to be philosophical about it, but it's really kind of a subjective question - you decide what you expect of a language today.
There are some people who say Swift isn't modern enough (we don't have coroutines yet, for example), and others who say it's too modern (lots of people were upset when C-style
for loops disappeared).
Personally, if I were going to pick a single dividing line between "modern" languages and other languages it would be the security-first mindset brought on by ubiquitously networked computers; at a language-feature level, that would mostly be memory safety.
Some of the "Ancient" Programming languages that were invented in the early days (read prior to 1980) are still serve their purpose very well where it belongs (I am mostly thinking here about C, C++ and their usage in embedded systems)
When technology improves, language features such as
- inherent support for functional programming
- higher level language but still without having to deal with memory management
are expected as they are now "obvious"
IMHO, Swift is modern in these aspects. And it is not the only one, of course.