iOS Swift - CLGeocoder completionHandler block

I'm trying to parse a location (CLLocation) into a String.

    func locationToString (currentLocation: CLLocation) -> String? {
    var whatToReturn: String?
    CLGeocoder().reverseGeocodeLocation(currentLocation, completionHandler: { (placemarks: [AnyObject]!, error: NSError!) in
        if error == nil && placemarks.count > 0 {
            let location = placemarks[0] as CLPlacemark
            whatToReturn = "\(self.validateString(location.locality)) \(self.validateString(location.thoroughfare)) \(self.validateString(location.subThoroughfare))"

        }
    })
    return whatToReturn
}

Obviously, whatToReturn always returns null, because completionHandler runs in the background. I'm having a hard time understanding how do I update my String when completionHandler finishes?

Thanks.