specialized extension of Dictionary - swift

Question

How can I declare an extension that will work only for a particular type?
I tried this:
extension Dictionary where
Key : CustomStringConvertible,
Value: CustomStringConvertible
{
func queryString() -> String {
var paramArray = Array<String>()
for (key, value) in self {
paramArray.append("\(key.description)=\(value.description)")
}
return "&".join(paramArray)
}
}
And it compiles fine. But when I try to use it
var d = Dictionary<String, String>()
var q = d.queryString() // <-- ERROR
I get the error:
Cannot invoke 'queryString' with no arguments
What is wrong here? I want to be able to call queryString on a Dictionary but only when it is Dictionary<String, String>
Any help is highly appreciated.
Edit
As #jtbandes said, String does not conform to CustomStringConvertible. CustomStringConvertible Protocol Reference suggests to use String() constructor to get a string rather than using the protocol as a constrain.
NOTE: String(instance) will work for an instance of any type, returning its description if the instance happens to be CustomStringConvertible. Using CustomStringConvertible as a generic constraint, or accessing a conforming type's description directly, is therefore discouraged.
extension Dictionary {
public func queryString() -> String {
var paramArray = Array<String>()
for (key, value) in self {
paramArray.append("\(String(key))=\(String(value))")
}
return "&".join(paramArray)
}
}
Edit2
This is my final version.
extension Dictionary {
public func queryString() -> String {
var queryItems = Array<NSURLQueryItem>()
for (key, value) in self {
queryItems.append(NSURLQueryItem(name: String(key), value: String(value)))
}
let comps = NSURLComponents();
comps.queryItems = queryItems
return comps.percentEncodedQuery!
}
}

Answer 1

String is not CustomStringConvertible. You can use:
extension String: CustomStringConvertible {
public var description: String { return self }
}
Or, I would recommend making your own protocol for this case:
protocol QueryStringConvertible {
var queryStringRepresentation: String { get }
}
extension String: QueryStringConvertible {
var queryStringRepresentation: String { return self /*probably should escape the string here*/ }
}
extension Dictionary where
Key : QueryStringConvertible,
Value : QueryStringConvertible ...
But in reality, I think you might want to take a look at NSURLQueryItem and NSURLComponents :)