Apple released their new programming language, Swift to the world at the WWDC 2014, which can improve writing and debugging code faster and on the fly with the help of playground features. Swift is capable of creating iOS and OS X apps evolving from the likes of C and Objective-C programming languages. Cocoa and Cocoa Touch frameworks structure the Swift language as well.
The playground makes the development realtime with less chances of making errors and improves productivity on the fly. Apple has also released a free eBook on iTunes and from the initial reviews from readers, it is the best book out there to learn swift. But, the book needs an iOS device to read.
Link to the online Developer Library : https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_399
Early achievements of the language include Nate Murray , who is the co-founder of Fullstack.io, cloned “Flappy Bird” within a day!