async는 delay 개념이다. use!, let!, do!는 force 개념이다. 예제: open System.Net open System.IO let wgetAsync url = async { let request = WebRequest.Create (url:string) use! response = request.AsyncGetResponse() use responseStream = response.GetResponseStream() use reader = new StreamReader(responseStream) return reader.ReadToEnd() } let urls = ["http://www.rosettacode.org/"; "http://www.yahoo.com/"; "http:/..