Syncronous data fetcher in swift

gistfile1.swift

//@auto_closure hack to prevent `LLVM ERROR: unimplemented IRGen feature! non-fixed multi-payload enum layout`
//as in https://github.com/maxpow4h/swiftz/blob/master/swiftz/Either.swift
enum Either<L,R> {
    case Left(@auto_closure () -> L)
    case Right(@auto_closure () -> R)
    
    func flatMapLeft<L1>(f: (L) -> Either<L1, R>) -> Either<L1, R> {
        switch self {
            case let .Left(l): return f(l())
            case let .Right(r): return .Right(r)
        }
    }

    func flatMapRight<R1>(f: (R) -> Either<L, R1>) -> Either<L, R1> {
        switch self {
            case let .Left(l): return .Left(l)
            case let .Right(r): return f(r())
        }
    }
    
    func flatMap<R1>(f: (R) -> Either<L, R1>) -> Either<L, R1> {
        return self.flatMapRight(f)
    }

    func mapLeft<L1>(f: (L) -> L1) -> Either<L1, R> {
        return self.flatMapLeft {
            l -> Either<L1, R> in
            return .Left(f(l))
        }
    }
    
    func mapRight<R1>(f: (R) -> R1) -> Either<L, R1> {
        return self.flatMapRight {
            r-> Either<L, R1> in
            return .Right(f(r))
        }
    }
    
    var description:String {
        switch self {
            case let .Left(l):  return "Left(\(l()))"
            case let .Right(r): return "Right(\(r()))"
        }
    }
}

//TODO: timeout param
func syncFetch(url: NSURL) -> Either<NSError, NSData> {
    let session = NSURLSession.sharedSession()
    let semaphore = dispatch_semaphore_create(0);
    var result: Either<NSError, NSData>?
    
    let task = session.dataTaskWithURL(url, completionHandler:
        { data, response, error -> Void in
            if let e = error {
                result = Either.Left(e)
            } else {
                result = Either.Right(data)
            }
            
            dispatch_semaphore_signal(semaphore)
        }
    )
    
    task.resume()
    
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)

    return result!
}