Some of the issues are in https://devforums.apple.com/community/tools/languages/swift

For example, when the REPL is invoked with /Applications/Xcode6-Beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift, is possible to write:

let a= 1

This create a constant integer. According to the docs, is a constant and can't change.

However, you can write:

a=2

And it work. But if is write in XCode it show that that is a error