Q:

How to append a character to a string in Swift?

This used to work in Xcode 6: Beta 5. Now I'm getting a compilation error in Beta 6.
for aCharacter: Character in aString {
    var str: String = ""
    var newStr: String = str.append(aCharacter) // ERROR
    ...
}

Error: Cannot invoke append with an argument of type Character

A:

Update for the moving target that is Swift:
Swift no longer has a + operator that can take a String and an array of characters. (There is a string method appendContentsOf() that can be used for this purpose).
The best way of doing this now is Martin R’s answer in a comment below:
var newStr:String = str + String(aCharacter)

Original answer:
This changed in Beta 6. Check the release notes.I'm still downloading it, but try using:
var newStr:String = str + [aCharacter]

A:

This also works
var newStr:String = str + String(aCharacter)

A:

append append(c: Character) IS the right method but your code has two other problems.
The first is that to iterate over the characters of a string you must access the String.characters property.
The second is that the append method doesn't return anything so you should remove the newStr.
The code then looks like this:
for aCharacter : Character in aString.characters {
    var str:String = ""
    str.append(aCharacter)
    // ... do other stuff
}

