Swift extensions are cool. You can add special support to all SequenceTypes for your types.

SequenceType+CategorySupport.swift

extension SequenceType where Generator.Element == Category {
    func joinNamesWithSeparator(separator: String) -> String {
        return self.reduce("", combine: { ( combinedString: String, cat: Category) in
            guard let name = cat.name else {
                return combinedString
            }
            return combinedString + (combinedString.isEmpty ? name : "\(separator)\(name)")
        })
    }
}

I have an NSManagedObject subclass called Category and now if I have an Array or Set of Category objects I can call categorySet.joinNamesWithSeparator(", ") and get a comma separated list of the category names, where name is a String? property.

How very cool!

You could also write that reduce call like this:

return self.reduce("") {
    if let name = $1.name {
        return $0 + ($0.isEmpty ? name : "\(separator)\(name)")
    } else {
        return $0
    }
}

If you don't mind the $0, $1