Nested self executing closures-CodePudding

I'm trying to nest some self executing closures but I'm getting a strange compiler error ('nil' is incompatible with return type 'NSPredicate') that I'm thinking it's because of the nested self executing closures but I'm not sure and I couldn't find anything relevant when searching for it.

public func fetch(lastFetchedTimestamp: Date?) async throws {
    let predicate: NSPredicate = {
            
            let isApprovedPredicate = NSPredicate(format: "isApproved == 1")
            let preferredLanguagePredicate: NSPredicate? = {
                guard let preferredLanguageCode = Locale.preferredLanguageCode else {
                    return nil
                }
                return .init(format: "language == %@", preferredLanguageCode)
            }()
            let modificationDatePredicate: NSPredicate? = {
                guard let lastFetchedTimestamp else {
                    return nil // <-- 'nil' is incompatible with return type 'NSPredicate' 
                }
                return .init(format: "modificationDate > %@", lastFetchedTimestamp)
            }()
            
            let predicates = [isApprovedPredicate, preferredLanguagePredicate, modificationDatePredicate].compactMap { $0 }
            
            return NSCompoundPredicate(andPredicateWithSubpredicates: predicates)
    }()
}

CodePudding user response：

Try breaking it to different part. For instance:

let modificationDatePredicate: NSPredicate? = {
    guard let lastFetchedTimestamp else {
        return nil
    }
    return .init(format: "modificationDate > %@", lastFetchedTimestamp)
}()

You can change nil to .none, and you will see a pretty clear error:

return .init(format: "modificationDate > %@", lastFetchedTimestamp)
                                              ^^^^^^^^^^^^^^^^^^^^
// Argument type 'Date' does not conform to expected type 'CVarArg'

Similarly, within preferredLanguagePredicate, does Locale.preferredLanguageCode actually exist?

CodePudding user response：

The error code is kind of confusing, this is not about the nil value. If you use Date in NSPredicate you need to cast it to NSDate.

return .init(format: "modificationDate > %@", lastFetchedTimestamp as NSDate)