Wrapping the NSError ** dance

gistfile1.swift

final class Box<T> {
    let value: T
    init(_ v: T) {
        value = v
    }
}

enum Result<T> {
    case Success(Box<T>)
    case Error(NSError)
}

func evaluateCocoaStatementWrappingError<T>(statement: (errorPointer: NSErrorPointer) -> T?) -> Result<T> {
    var error: NSError?
    if let result = statement(errorPointer: &error) {
        return .Success(Box(result))
    } else {
        return .Error(error!)
    }
}

let result = evaluateCocoaStatementWrappingError { url.bookmarkDataWithOptions(NSURLBookmarkCreationOptions.WithSecurityScope, includingResourceValuesForKeys: nil, relativeToURL: nil, error: $0) }