Syntactic support, immutability semantics, and compile-time limitations on side-effects. Of course, it's POSSIBLE to do everything with a library + discipline.

Syntactic support, immutability semantics, and compile-time limitations on side-effects. Of course, it's POSSIBLE to do everything with a library + discipline. For example, I can import the Clojure jar and use Clojure's data structures and STM in a Java program. But it is _very_ verbose, and you have to know that certain variables are immutable, or that certain operations are illegal in certain places, etc, rather than having that information encoded into the language.