等待在F＃的IAsyncOperation [英] Awaiting an IAsyncOperation in F#

问题描述

我在F＃以下code：

I have the following code in F#:

   let CreateSampleDataFromJson<'T>(path) = 
      let uri = new Uri(path)
      async {
         let file = StorageFile.GetFileFromApplicationUriAsync(uri)
         let jsonText = FileIO.ReadTextAsync(file)
         return JsonObject<'T>.Parse(jsonText)
      }

我遇到的问题是，文件是 IAsyncOperation＆LT; StorageFile＆GT; ，而不是 StorageFile 为 ReadTextAsync 预计。

在C＃中，你可以做一些类似的：

In C# you can do something similar to this:

var file = await StorageFile.GetFileFromApplicationUriAsync(uri) 

即

public async Task<T> CreateSampleDataFromUrl<T>(string path)
{
  var uri = new Uri(path);
  var file = await StorageFile.GetFileFromApplicationUriAsync(uri);
  var jsonText = await FileIO.ReadTextAsync(file);
  return JsonObject<T>.Parse(jsonText);
}

问题是，我不知道如何等待一个 IAsyncOperation F＃中。通常让！不起作用。即以下编译失败：

The problem is that I don't know how to await an IAsyncOperation in F#. The usual let! doesn't work. i.e. the following fails to compile:

async {
   let! file = StorageFile.GetFileFromApplicationUriAsync(uri)

通过编译器错误：

error FS0001: This expression was expected to have type    Async<'a>    but here has type    IAsyncOperation<StorageFile>

我发现了一份文件，称有一个在 System.WindowsRuntimeSystemExtensions 类中定义的 AsTask（）扩展方法，我可以使用如下：

I found a document that said there's an AsTask() extension method defined in the System.WindowsRuntimeSystemExtensions class which I can use as follows:

 let! file = StorageFile.GetFileFromApplicationUriAsync(uri).AsTask() |> Async.AwaitTask

有没有使这是一个更好一点的地方这样做，或可用的东西在F＃库的标准方式？

Is there a standard way of doing this or something available in an F# library somewhere that makes this a bit nicer?

推荐答案

您的解决方案由我似乎罚款。如果你正在寻找一个更好的语法，如何滚动它变成这样的功能（没有可能无偿类型注释）：

Your solution seems fine by me. If you're looking for a nicer syntax, how about rolling it into a function like this (without the possibly gratuitous type annotations):

let await<'a> (op: IAsyncOperation<'a>) : Async<'a> =
   op.AsTask() |> Async.AwaitTask

这会给你你会在C＃中看到几乎相同的语法：

This will give you the almost exact same syntax you'd see in c#:

async {
   let! file = await <| StorageFile.GetFileFromApplicationUriAsync(uri)
   ...
}

您与您的previous方法被越来越编译器错误是可以预期的。所有异步工作流程在乎的是F＃异步特异型。这种类型给你一个的方式与.NET世界中通过任务其余的互操作，但仅此而已。 IAsyncOperation是从'世界不同部分'，我不希望F＃核心库很快支持。

The compiler errors you were getting with your previous approaches are to be expected. All async workflow cares about is the F#-specific Async type. This type gives you a way to interop with the rest of .NET world through Tasks, but that's it. IAsyncOperation is from a 'different part of the world', I wouldn't expect F# core libraries to support it anytime soon.

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