What does it mean in Swift: 'case .success(let dict):'

Question

In Swift PromiseKit library there's Alamofire example using a bit strange syntax:

func login(completionHandler: (NSDictionary?, ErrorProtocol?) -> Void    {
    Alamofire.request(.GET, url, parameters: ["foo": "bar"])
        .validate()
        .responseJSON { response in
            switch response.result {
            case .success(let dict):
                completionHandler(dict, nil)
            case .failure(let error):
                completionHandler(nil, error)
            }
        }
}

response is an Alamofire enum describing two cases with associated values:

public enum Result<Value> {
    case success(Value)
    case failure(Error) 
(...)

What I don't get is what does let mean in each case: line and where does the dict (or error) come from? Is this syntactic sugar for something more verbose but less confusing?

Answer 1

In Swift, enums can have associated values (docs). This means, that you can associate an object with cases. The part (let dict) simply means - take the associated value, and put in in a let constant named dict.