How to create a Type-Safe custom function in SQLite.swift?

I would like to create a simple distance function to order objects while fetched from a SQLite database in Swift2. I’m using the awesome SQLite.swift framework.

With the following I could fetch the nearest objects:

db.createFunction("distance") { (args) -> Binding? in
    assert(args.count == 4)
    if let lat1 = args[0] as? Double, let lon1 = args[1] as? Double, let lat2 = args[2] as? Double, let lon2 = args[3] as? Double {

        let deltaLat = lat1 - lat2
        let deltaLon = lon1 - lon2
        return deltaLat * deltaLat + deltaLon * deltaLon * 0.46512281898705
    }
    return nil
}

let queryString = "SELECT * FROM objects where lat != \"\" and lng != \"\" ORDER BY distance(lat, lng, \(lat), \(lng)) ASC LIMIT \(fetchLimit)"

let stmt = db.prepare(queryString)
for row in stmt {
    print(row)
}

But I would like to use a Type-Safe SQL Expression without using a query String. How can I add a function to be able to make it work like this:

for row in db.order(distance(lat, lng).limit(fetchLimit) {
    print(row)
}