Suppose that we have a code statement that is dependent on another code statement, as shown in the following code:

func sendRequest() { 
    let sessionConfig = URLSessionConfiguration.default 
    let session = URLSession(configuration: sessionConfig, 
                             delegate: nil, 
                             delegateQueue: nil) 
    var url: URL? 
    var request: URLRequest 

    /* First request block starts: */ 
    url = URL(string: "https://httpbin.org/get") 
    request = URLRequest(url: url! as URL) 
    request.httpMethod = "GET" 

    let task = session.dataTask(with: request) { 
        (data: Data?, response: URLResponse?, error: Error?) -> Void in 

        if (error == nil) { 
            let statusCode = (response as! HTTPURLResponse).statusCode 
            print("URL Session Task Succeeded: HTTP (statusCode)") 
        } else { 
            print("URL Session Task Failed: %@",     
            error!.localizedDescription); 
        } 
    }
    task.resume() 
    /* First request block ends */ 

    /* Second request block starts */ 
    url = URL(string: "http://requestb.in/1g4pzn21") 
    // replace with a new requestb.in 
    request = URLRequest(url: url! as URL) 

    let secondTask = session.dataTask(with: request) { 
        (data: Data?, response: URLResponse?, error: Error?) -> Void in 

        if (error == nil) { 
            let statusCode = (response as! HTTPURLResponse).statusCode 
            print("URL Session Task Succeeded: HTTP (statusCode)") 
        } else { 
            print("URL Session Task Failed: %@",    
            error!.localizedDescription); 
        } 
    } 
    secondTask.resume() 
} 

In the preceding code example, we set two different HTTP requests. Suppose that we do not need our first request anymore and we delete the following code block:

url = URL(string: "https://httpbin.org/get") 
request = URLRequest(url: url! as URL) 
request.httpMethod = "GET" 

The compiler will not complain but as we deleted request.httpMethod = "GET", our second request is not going to work. This situation is called temporal coupling. If we had immutable definitions with let, we would avoid temporal coupling.