Swift dictionary bug? - dictionary

Question

So I started a project in Swift, and I've come to this problem:
this code works:
var dictionary = ["a":"valueOfA","b":"valueOfB","c":"valueOfC"]
println(dictionary)
dictionary["c"] = "newValOfC"
println(dictionary)
and this doesn't:
var dictionary = [:]
dictionary = ["a":"valueOfA","b":"valueOfB","c":"valueOfC"]
println(dictionary)
dictionary["c"] = "newValOfC"
println(dictionary)
Gives an error:
Playground execution failed: error: <REPL>:35:17: error: cannot assign to the result of this expression
dictionary["c"] = "newValC"
~~~~~~~~~~~~~~~ ^
Notice that this is not a constant value
So why doesn't the line
dictionary = ["a":"valueOfA","b":"valueOfB","c":"valueOfC"]
give an error?

Answer 1

Since the context does not provide enough information to infer the type, you'll need to explicitly name it as a dictionary, otherwise swift assumes it is an NSDictionary (I'm not clear on why though. I assume for better obj-c compatibility):
The following code all works:
// Playground
import UIKit
var str:NSString = "Hello, playground"
var d0 = [:]
var d1: Dictionary = [:]
d0.setValue(UIWebView(), forKey: "asdf")
d1["asdf"] = 1
d1["qwer"] = "qwer"

Answer 2

Okay, I found it, the problem is that by initializing an empty dictionary, the type inference gets a little crazy.
You'll need this code:
var dictionary = Dictionary<String, String>()
instead of
var dictionary = [:]
but that still does not explain why the line
dictionary = ["a":"valueOfA","b":"valueOfB","c":"valueOfC"]
does not give an error

Answer 3

And referring to Swift Language Guide, the
dictionary = [:]
syntax is correct "if the context already provides type information".

Answer 4

The big difference is that
var dictionary = [:]
doesn't give any chance the compiler to infer the right type, whereas
var dictionary = ["a":"valueOfA","b":"valueOfB","c":"valueOfC"]
does.
Actually it looks like the first line produces a __NSDictionaryI instance, whereas the second one produces a Dictionary<String,String as expected.
From The Swift Programming Language book
If the context already provides type information, create an empty dictionary with an empty dictionary literal, which is written as [:] (a colon inside a pair of square brackets):
which means the first line is ambiguous.

Answer 5

The [:] syntax only works if the current context has enough information to be able to infer what the types are. Here are a few ways where the context is known...
As has been mentioned already, the code below works because it can be inferred that the keys and values are strings:
var dictionary = [ "a" : "valueOfA" , "b" : "valueOfB" , "c" : "valueOfC" ]
A dictionary passed into a function has a known context:
func dictionaryFunc(var dictionary : Dictionary<String, String>) {
// Do stuff with the dictionary
}
In both cases, the key/value types for the dictionary are now known, so you could do this to create a new dictionary with the same variable name...
dictionary = [:]
In the first case, if you used the [:] syntax after the declaration you would be erasing the contents of the original dictionary. In the second case, dictionaries get copied when they get passed into functions, so using [:] would only be erasing the copy.

Answer 6

In Swift 2.0 I'm using this:
var dictionary = [String:String]()
dictionary["key1"] = "value1"