I am somewhat stuck trying to write a helper function for querying my CoreData store.

For simple things (like string values etc) the following syntax works well:

let predicate = NSPredicate(format:"myProperty == (someString)")

However, it breaks if I try to say something like "self in (arrayOfIDs)".

Here is a small reproducible example which breaks:

let ids = [1,2,3]
let predicate = NSPredicate(format:"self in (ids)")

Running this code fails with libc++abi: terminating due to uncaught exception of type NSException. Clearly, NSPredicate cannot be constructed in such a way.

I have figured out that the issue can be resolved if I actually use CVarArgs like so:

let ids = [1,2,3]
let predicate = NSPredicate(format:"self in %@", args: ids)

However, as I try to employ this in a function where I am going to pass predicate, I would like to avoid passing format and CVarArgs.

So my next logical assumption is that I could construct a String using String(format:String, args: any CVarArg...) and pass this as a predicate format without additional arguments.

Surprisingly, this breaks as well:

let ids = [1,2,3]
let predicate = NSPredicate(format:String(format:"self in %@", args: ids))

Does anyone have any idea how this could be resolved?

For context, this is the function which I am trying to write:

static func fetch<T:FetchDeclaringManagedObject>(_ request:NSFetchRequest<T>, predicate:String?, context:NSManagedObjectContext = defaultContext, sortDescriptors:[NSSortDescriptor]? = nil) -> [T] {
    let request = T.fetchRequest()
    if let p = predicate {
        request.predicate = NSPredicate(format: p)
    }
    // REST OF CODE
}

It would be super convenient if I did not have to rewrite it to specify both format and CVarArgs 🙂