F# type error: String vs Async<string>

Question

我是编程新手，F# 是我的第一门 .NET 语言。

这是我目前编写的一些代码：

let downloadFromWebsite (url: string) =
    async { 
        let uri = new System.Uri(url)
        let webClient = new WebClient()
        let! html = webClient.AsyncDownloadString(uri)
        printfn "Read %d characters from %s" html.Length url
        return html
        }

let results = downloadFromWebsite @"http://plato.stanford.edu"
printf "%s" results

错误信息如下：

~vs8222.fsx(19,13): error FS0001: This expression was expected to have type string but here has type Async

出了什么问题？我应该做哪些改变？

Answer 1

results 是 Async<string>，而您使用的格式字符串需要 string。您需要运行计算并获得结果，然后才能打印。您可以使用 Async.RunSynchronously:

let results = downloadFromWebsite @"http://plato.stanford.edu" |> Async.RunSynchronously
printf "%s" results

F# type error: String vs Async<string>

F# type error: String vs Async<string>

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