I finally finished reading the book Functional Programming in Swift. I have to say it is a pretty good book. I wouldn’t recommend it for a beginner in Swift, but after you familiarize yourself a bit with the language, it is definitely worth reading.
After this I am seriously considering learning Haskell. I think it would make my functional programming skill much better.
I found an interesting Swift tutorial for beginners. It focuses on the way of thinking you should have when programming in Swift. To learn a new syntax isn’t a big deal after all. So this is the hardest part to do. Here is the first post of the series: http://alisoftware.github.io/swift/2015/09/06/thinking-in-swift-1/. If you’re interested you’ll probably find the rest of the articles from there. Enjoy.