core data - Can I use an NSPredicate in Swift with a nil argument?

Question

I'm trying to convert a project that uses Core Data from Objective-C to Swift.

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.

Currently, I do this in Objective-C to retrieve the master folder (it finds the only folder without a "parentFolder", and works as expected):

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:"Folder"];
request.predicate = [NSPredicate predicateWithFormat:@"parentFolder == %@", nil];

In converting to Swift, I'd like to do the same thing:

let request = NSFetchRequest(entityName: "Folder")
request.predicate = NSPredicate(format: "parentFolder == %@", nil)

... 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...)

Is there a correct way to do this in Swift?

Answer 1

It looks as if it is (with the current Xcode 6 beta 3 release) generally difficult to pass nil in a variable argument list.

This seems to work:

let predicate = NSPredicate(format: "parentFolder == %@", 0)
print(predicate)
// Output: parentFolder == nil

But the easiest solution would be to simply write the predicate as

NSPredicate(format: "parentFolder == nil")