Ignoring Operation Results while using F# Async Computation Expressions

This post explains a simple pitfall where C# developers trying out F# may fail when writing async code

Introduction 

 

This post explains a simple pitfall where C# developers trying out F# may fail when writing async code. 

 

Consider this simple code in which we download page contents using Puppeteer-sharp:

let renderHtml = async {
    BrowserFetcher().DownloadAsync(BrowserFetcher.DefaultRevision) |> Async.AwaitTask |> ignore
    let options = LaunchOptions()
    options.Headless <- true
    let! browser = Puppeteer.LaunchAsync(options) |> Async.AwaitTask
    let! page = browser.NewPageAsync() |> Async.AwaitTask
    page.GoToAsync("https://i.ua") |> Async.AwaitTask |> ignore
    return! page.GetContentAsync() |> Async.AwaitTask
}

Since we actually don't care about download browser result, we naturally would expect our line to look like:

BrowserFetcher().DownloadAsync(BrowserFetcher.DefaultRevision) |> Async.AwaitTask |> ignore

This would be equivalent to the following C# code:

await new BrowserFetcher().DownloadAsync(BrowserFetcher.DefaultRevision);

However, when we execute the following method, we get AggregateException which in turn contains the following inner exception: "Chromium revision is not downloaded. Run BrowserFetcher.DownloadAsync or download Chromium manually". It seems like we've called the following... 

let! browser = Puppeteer.LaunchAsync(options) |> Async.AwaitTask

...without waiting for the BrowserFetcher result.

And indeed, in order to await async call, we have to use let! as the construct. The code below works as expected:

let renderHtml = async {
    let! _ = BrowserFetcher().DownloadAsync(BrowserFetcher.DefaultRevision) |> Async.AwaitTask
    let options = LaunchOptions()
    options.Headless <- true
    let! browser = Puppeteer.LaunchAsync(options) |> Async.AwaitTask
    let! page = browser.NewPageAsync() |> Async.AwaitTask
    let! _ = page.GoToAsync("https://i.ua") |> Async.AwaitTask
    return! page.GetContentAsync() |> Async.AwaitTask
}

Note how we use an underscore to show that the variable value is ignored.