title: Can I use an NSPredicate in Swift with a nil argument?
body: <p>I'm trying to convert a project that uses Core Data from Objective-C to Swift.</p>
  
  <p>The data model is structured so that I have one master folder which contains other folders - and those folders can also contain other folders, via a "parentFolder" relationship.</p>
  
  <p>Currently, I do this in Objective-C to retrieve the master folder (it finds the only folder without a "parentFolder", and works as expected):</p>
  
  <pre><code>NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:"Folder"];
  request.predicate = [NSPredicate predicateWithFormat:@"parentFolder == %@", nil];
  </code></pre>
  
  <p>In converting to Swift, I'd like to do the same thing:</p>
  
  <pre><code>let request = NSFetchRequest(entityName: "Folder")
  request.predicate = NSPredicate(format: "parentFolder == %@", nil)
  </code></pre>
  
  <p>... but the compiler complains with "Missing argument label 'argumentArray:' in call. (I seem to be confusing it into thinking I need to use the NSPredicate(format: argumentArray:) method instead...)</p>
  
  <p>Is there a correct way to do this in Swift?</p>