Asynchronous F#：异步下载数据

我是编程新手，F#是我的第一语言

以下是我的代码的相关部分：

open System.IO
open System.Net

let downloadHtmlFromUrlAsync (url: string) =
    async { 
        let uri = new System.Uri(url)
        let webClient = new WebClient()
        let! html = webClient.AsyncDownloadString(uri)
        return html
        }

let downloadHtmlToDisk (url: string) (directoryPath: string) = 
    if isValidUrl url then
        let name = getNameFromRedirectedUrl url
        let id = getIdFromUrl url
        let html = downloadHtmlFromUrlAsync url
        let newTextFile = File.Create(directoryPath + "\\" + id.ToString("00000") + " " + name.TrimEnd([|' '|]) + ".html")
        use file = new StreamWriter(newTextFile) 
        file.Write(html) 
        file.Close()

let downloadEntireDatabase (baseUrl: string) (totalNumberOfPeople: int) = 
    let allIds = [ for i in 1 .. totalNumberOfPeople -> i ]

    allIds
    |> Seq.map (fun id -> baseUrl + string(id))
    |> Seq.filter isValidUrl
    |> Seq.map downloadHtmlToDisk
    |> Async.Parallel 
    |> Async.RunSynchronously

我在F#interactive中测试了函数isValidUrl、getNameFromRedirectedUrl和getIdFromUrl。他们工作得很好

我的问题是：当我尝试运行上面粘贴的代码时，会产生以下错误消息：

fs（483,8）：错误FS0193:类型约束不匹配。类型

seq unit）>

与类型不兼容

seq

与类型

string->unit

---

出了什么问题？我应该做哪些更改？

问题可能是这一行（您能告诉我们下载FighterHTMLODisk的定义吗？）：

---

根据错误消息，此函数似乎有签名

string->string->unit

，但您确实需要

string->asynchwhere至少是：
downloadfightrhtmltodisk
？谢谢，卡斯滕。我有一个后续问题：当我调用Async.Parallel时，如何指定要使用的最大线程数？您不能直接-基本上您必须将工作分组到所需的块中，并将其传递给Async.Parallel-或信任线程池；）
  allIds
    ...
    |> Seq.map downloadFighterHtmlToDisk
    ...

let downloadHtmlToDisk (directoryPath: string) (url: string) = 
    async {
        if isValidUrl url then
            let name = getNameFromRedirectedUrl url
            let id = getIdFromUrl url
            let! html = downloadHtmlFromUrlAsync url
            let newTextFile = File.Create(directoryPath + "\\" + id.ToString("00000") + " " + name.TrimEnd([|' '|]) + ".html")
            use file = new StreamWriter(newTextFile) 
            file.Write(html) 
    }

 let downloadEntireDatabase (baseUrl: string) (totalNumberOfPeople: int) = 
        let allIds = [ for i in 1 .. totalNumberOfPeople -> i ]

        allIds
        |> Seq.map (fun id -> (id, baseUrl + string(id)))
        |> Seq.filter (fun (_,url) -> isValidUrl url)
        |> Seq.map (fun (id,url) -> downloadHtmlToDisk (getFighterPath id) url)
        |> Async.Parallel 
        |> Async.RunSynchronously