Yes the syntax, but also many of the libraries and conventions are the same. The package management/build tool has a lot of similarity as well. When learning a new language, learning the standard library and the many conventions is a lot of the friction. With Ruby -> Elixir that is a lot less.

Also, I think you underestimate how many people (such as myself) who use Ruby in a very functional way already. Immutability is really not all that different. The only thing is you can't use ! methods and instead have to assign. For example instead of `array.sort!()` it's `array = array.sort()`. It's really not that hard, and in fact I found it easier because I didn't have to remember if `array.concat()` mutated or not.

IMHO the only "hard" things for a rubyist to learn are pattern matching and Actor model (OTP) concepts. Actor model concurrency is definitely different. But for an experienced programmer, that can be learned in a couple of hours (or less).