Question

我正在学习F＃（这很有趣，不适合工作/学校）而且我正在尝试编写一个简单的解析器来计算Windows Phone应用程序在多个市场中的评论数量。毫无疑问，我到目前为止的代码是丑陋的，但我正在努力改进它并遵循函数式编程范式。由于我来自C，C ++，C＃世界，所以很难。

来自C世界，我喜欢空值。我知道函数式编程/ F＃不鼓励使用null，但我无法找到一种不使用它的方法。例如，在函数解析中有一个空检查。我怎么不这样做？
现在我的代码只计算第一页上的评论数量，但是应用可能有超过10条评论，因此可能会有多个页面。我如何递归浏览所有页面（功能下载评论或解析）。
我们如何将此代码扩展为完全异步？

以下是我到目前为止的代码。除了上面的问题，我真的希望有人能帮助我，并告诉我如何改进我的代码的整体结构。

open System
open System.IO
open System.Xml
open System.Xml.Linq
open Printf

type DownloadPageResult = {
    Uri: System.Uri;
    ErrorOccured: bool;
    Source: string;
}

type ReviewData = {
    CurrentPageUri: System.Uri;
    NextPageUri: System.Uri;
    NumberOfReviews: int;
}

module ReviewUrl = 
    let getBaseUri path =
        new Uri(sprintf "http://cdn.marketplaceedgeservice.windowsphone.com/%s" path)

    let getUri country locale appId =
        getBaseUri(sprintf "/v8/ratings/product/%s/reviews?os=8.0.0.0&cc=%s&oc=&lang=%s&hw=520170499&dm=Test&chunksize=10" appId country locale)

let downloadPage (uri: System.Uri) =
    try
        use webClient = new System.Net.WebClient()
        printfn "%s" (uri.ToString())
        webClient.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8")
        webClient.Headers.Add("Accept-Encoding", "zip,deflate,sdch")
        webClient.Headers.Add("Accept-Language", "en-US,en;q=0.8,fr;q=0.6")
        webClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1482.0 Safari/537.36")
        { Uri = uri; Source = webClient.DownloadString(uri); ErrorOccured = false }
    with error -> { Uri = uri; Source = String.Empty; ErrorOccured = true }

let downloadReview country locale appId =
    let uri = ReviewUrl.getUri country locale appId
    downloadPage uri

let parse(pageResult: DownloadPageResult) =
    if pageResult.ErrorOccured then { CurrentPageUri = pageResult.Uri; NextPageUri = null; NumberOfReviews = 0 }
    else 
        let reader = new StringReader(pageResult.Source)
        let doc = XDocument.Load(reader)
        let ns = XNamespace.Get("http://www.w3.org/2005/Atom")

        let nextUrl = query { for link in doc.Descendants(ns + "link") do
                              where (link.Attribute(XName.Get("rel")).Value = "next")
                              select link.Value
                              headOrDefault }

        if nextUrl = null then
            { CurrentPageUri = pageResult.Uri; NextPageUri = null; NumberOfReviews = doc.Descendants(ns + "entry") |> Seq.length } 
        else
            { CurrentPageUri = pageResult.Uri; NextPageUri = ReviewUrl.getBaseUri(nextUrl); NumberOfReviews = doc.Descendants(ns + "entry") |> Seq.length } 

let downloadReviews(locale: string) =
    let appId = "4e08377c-1240-4f80-9c35-0bacde2c66b6"
    let country = locale.Substring(3)
    let pageResult = downloadReview country locale appId
    let parseResult = parse pageResult
    parseResult


[<EntryPoint>]
let main argv = 
    let locales = [| "en-US"; "en-GB"; |]
    let results = locales |> Array.map downloadReviews

    printfn "%A" results
    0

Answer 1

我正在更多地讨论这个问题并尝试使用XML类型提供程序和F＃Data的其他功能。它不是完整的代码，但它应该足以给你一个想法（并显示类型提供者非常好： - ））：

首先，我需要一些参考资料：

#r "System.Xml.Linq.dll"
#r "FSharp.Data.dll"
open FSharp.Data
open FSharp.Net

接下来，我编写了以下代码来下载一个示例页面。

let data =
  Http.Request
   ( "http://cdn.marketplaceedgeservice.windowsphone.com//v8/ratings/product/4e08377c-1240-4f80-9c35-0bacde2c66b6/reviews",
     query=["os", "8.0.0.0"; "cc", "US"; "lang", "en-US"; "hw", "520170499"; "dm", "Test"; "chunksize", "10" ],
     headers=["User-Agent", "F#"])

我将示例保存为D:\temp\appstore.xml，然后使用XML类型提供程序获取用于解析页面的好类型：

type PageDocument = XmlProvider< @"D:\temp\appstore.xml" >

然后你可以下载＆amp;像这样解析页面（这显示了如何获得评论的数量和有关下一个链接的信息）：

let parseAsync (locale:string) appId = async {
  let country = locale.Substring(3)

  // Make the request (asynchronously) using the parameters specified
  let! data =
    Http.AsyncRequest
     ( "http://cdn.marketplaceedgeservice.windowsphone.com//v8/ratings/product/" 
         + appId + "/reviews",
       query=[ "os", "8.0.0.0"; "cc", country; "lang", locale; 
               "hw", "520170499"; "dm", "Test"; "chunksize", "10" ],
       headers=["User-Agent", "F#"])

  // Parse the result using the type-provider generated type
  let page = PageDocument.Parse(data)

  // Now you can type 'page' followed by '.' and explore the results!
  // page.GetLinks() returns all links and page.GetEntries() returns
  // review entries. Each link also has 'Rel' and 'Href' properties:
  let nextLink = 
    page.GetLinks() 
    |> Seq.tryFind (fun link -> link.Rel = "next")
    |> Option.map (fun link -> link.Href)
  let reviewsCount = page.GetEntries().Length    
  return (reviewsCount, nextLink) }

Answer 2

使代码异步的一般模式是找到I / O昂贵的操作（在调用树的某个地方）然后从那里“向上”并使所有使用它的代码也异步，直到你达到一个点你需要阻止。

在您的示例中，原始操作正在下载，因此您首先要使downloadPage异步：

let downloadPage (uri: System.Uri) = async {
    try
        use webClient = new System.Net.WebClient()
        printfn "%s" (uri.ToString())
        // (Headers omitted)
        let! source = webClient.AsyncDownloadString(uri)
        return { Uri = uri; Source = source; ErrorOccured = false }
    with error -> 
        return { Uri = uri; Source = String.Empty; ErrorOccured = true } }

您需要在async { ... }中包装代码，使用DownloadString调用let!的异步版本，并使用return（在两个分支中）返回结果。

然后你需要创建像downloadReview和downloadReviews这样的函数（再次，将它们包装在异步块中，使用downloadPage或使用{{调用let!之类的其他异步操作1}}）。

最后，如果您正在编写控制台应用程序，则需要阻止，但您可以并行运行不同语言环境的下载。假设return!是异步的：

downloadReviews

要回答其他问题，我认为在上面的示例中使用let locales = [| "en-US"; "en-GB"; |] let results = locales |> Array.map downloadReviews // Build an array of asynchronous computations |> Async.Parallel // Compose them into a single, parallel computation |> Async.RunSynchronously // Run the computation and wait可能没问题（您正在调用返回它的LINQ，因此没有简单的方法可以避免这种情况）。实际上可以使用选项类型，但这有点棘手 - 请参阅this snippet if you're interested。

此外，您可以使用F# Data Library中的null方法，这样可以更简单地构建复杂的HTTP请求（但我是该库的贡献者之一，所以我是偏置！）

Answer 3

正如Tomas所说，创建一个基于异步的版本的DownloadString（或者只是使用他的FSharp.Data库来处理它）会更“功能”。

您还可以将FSharp.Data与ExtCore结合使用，以利用ExtCore中的asyncMaybe或asyncChoice工作流程。这些工作流程在普通async工作流程之上提供了非常易于使用的错误处理。

无论如何，我花了几分钟清理你的代码。它并不多，但它确实在几个方面简化了代码：

open System
open System.IO
open System.Xml
open System.Xml.Linq
open Printf

type DownloadPageResult = {
    Uri : System.Uri;
    ErrorOccured : bool;
    Source : string;
}

type ReviewData = {
    CurrentPageUri : System.Uri;
    NextPageUri : System.Uri option;
    NumberOfReviews : uint32;
}

module ReviewUrl =
    let baseUri = Uri ("http://cdn.marketplaceedgeservice.windowsphone.com/", UriKind.Absolute)

    let getUri country locale (appId : System.Guid) =
        let localUri =
            let appIdStr = appId.ToString "D"
            sprintf "/v8/ratings/product/%s/reviews?os=8.0.0.0&cc=%s&oc=&lang=%s&hw=520170499&dm=Test&chunksize=10" appIdStr country locale
        Uri (baseUri, localUri)

let downloadPage (uri : System.Uri) =
    try
        use webClient = new System.Net.WebClient()
        printfn "%s" (uri.ToString())
        webClient.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8")
        webClient.Headers.Add("Accept-Encoding", "zip,deflate,sdch")
        webClient.Headers.Add("Accept-Language", "en-US,en;q=0.8,fr;q=0.6")
        webClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1482.0 Safari/537.36")
        { Uri = uri; Source = webClient.DownloadString uri; ErrorOccured = false }
    with error ->
        { Uri = uri; Source = String.Empty; ErrorOccured = true }


let parse (pageResult : DownloadPageResult) =
    if pageResult.ErrorOccured then
        { CurrentPageUri = pageResult.Uri; NextPageUri = None; NumberOfReviews = 0u }
    else 
        use reader = new StringReader (pageResult.Source)
        let doc = XDocument.Load reader
        let ns = XNamespace.Get "http://www.w3.org/2005/Atom"

        let nextUrl =
            query {
            for link in doc.Descendants(ns + "link") do
            where (link.Attribute(XName.Get("rel")).Value = "next")
            select link.Value
            headOrDefault }

        {   CurrentPageUri = pageResult.Uri;
            NextPageUri =
                if System.String.IsNullOrEmpty nextUrl then None
                else Some <| Uri (ReviewUrl.baseUri, nextUrl);
            NumberOfReviews =
                doc.Descendants (ns + "entry") |> Seq.length |> uint32; }

let downloadReviews (locale : string) =
    System.Guid "4e08377c-1240-4f80-9c35-0bacde2c66b6"
    |> ReviewUrl.getUri (locale.Substring 3) locale
    |> downloadPage
    |> parse


[<EntryPoint>]
let main argv =
    let locales = [| "en-US"; "en-GB"; |]
    let results = locales |> Array.map downloadReviews

    printfn "%A" results
    0

使用F＃下载Windows Phone应用评论

3 个答案: