Indeed the intent, though not expressed in the Objective-C declaration through the type system, is that the format string is a string literal.

However the intent of the Swift call described in the article is indeed that of a string literal with 2 data "parameters".

So is the correct use of NSLog in this case is this:

    NSLog("%@", "Failed to get modification date at URL: \(url) - \(error)")

This should really be the behavior of NSLog in Swift, that it expands not to the variadic

    NSLog(format-string-parameter, objc-object-parameters..)

but to

    NSLog(@"%@", string-parameter)