Cannot convert empty array of arrays to JSON with Swift 2 - ios

Question

What is wrong with this piece of code (which was inspired by this example)? It currently prints JSON string "(<5b5d>, 4)" instead of the expected "[]".
var tags: [[String]] = []
// tags to be added later ...
do {
let data = try NSJSONSerialization.dataWithJSONObject(tags, options: [])
let json = String(data: data, encoding: NSUTF8StringEncoding)
print("\(json)")
}
catch {
fatalError("\(error)")
}

Answer 1

Short answer: The creation of the JSON data is correct. The problem is in the
conversion of the data to a string, what you want is the NSString method:
let json = NSString(data: data, encoding: NSUTF8StringEncoding) as! String
which produces the expected [].
Slightly longer answer:
Your code
let json = String(data: data, encoding: NSUTF8StringEncoding)
calls the String init method
/// Initialize `self` with the textual representation of `instance`.
/// ...
init<T>(_ instance: T)
and the result is the textual representation of the tuple
(data: data, encoding: NSUTF8StringEncoding):
(<5b5d>, 4)
Actually you can call String() with arbitrary arguments
let s = String(foo: 1, bar: "baz")
print(s) // (1, "baz")
in Swift 2. This does not compile in Swift 1.2, so I am not sure if
this is intended or not. I have posted a question in the
Apple Developer Forums about that:
String init accepts arbitrary arguments.