Why do I have to wrap an Async<T> into another async workflow and let! it?

Question

I'm trying to understand async workflows in F# but I found one part that I really don't understand.

The following code works fine:

let asynWorkflow = async{
    let! result = Stream.TryOpenAsync(partition) |> Async.AwaitTask 
    return result
    } 

let stream = Async.RunSynchronously asynWorkflow
             |> fun openResult -> if openResult.Found then openResult.Stream else Stream(partition)

I define a async workflow where TryOpenAsync returns a Task<StreamOpenResult> type. I convert it to Async<StreamOpenResult> with Async.AwaitTask. (Side quest: "Await"Task? It doesn't await it just convert it, does it? I think it has nothing to do with Task.Wait or the await keyword). I "await" it with let! and return it. To start the workflow I use RunSynchronously which should start the workflow and return the result (bind it). On the result I check if the Stream is Found or not.

But now to my first question. Why do I have to wrap the TryOpenAsync call in another async computation and let! ("await") it? E.g. the following code does not work:

let asynWorkflow =  Stream.TryOpenAsync(partition) |> Async.AwaitTask  

let stream = Async.RunSynchronously asynWorkflow
             |> fun openResult -> if openResult.Found then openResult.Stream else Stream(partition)

I thought the AwaitTask makes it an Async<T> and RunSynchronously should start it. Then use the result. What do I miss?

My second question is why is there any "Async.Let!" function available? Maybe because it does not work or better why doesn't it work with the following code?

let ``let!`` task = async{
    let! result = task |> Async.AwaitTask 
   return result
   } 

let stream = Async.RunSynchronously ( ``let!`` (Stream.TryOpenAsync(partition))  )
         |> fun openResult -> if openResult.Found then openResult.Stream else Stream(partition)

I just insert the TryOpenAsync as a parameter but it does not work. By saying does not work I mean the whole FSI will hang. So it has something to do with my async/"await".

--- Update:

Result of working code in FSI:

>

Real: 00:00:00.051, CPU: 00:00:00.031, GC gen0: 0, gen1: 0, gen2: 0
val asynWorkflow : Async<StreamOpenResult>
val stream : Stream

Result of not working code in FSI:

>

And you cannot execute anything in the FSI anymore

--- Update 2

I'm using Streamstone. Here the C# example: https://github.com/yevhen/Streamstone/blob/master/Source/Example/Scenarios/S04_Write_to_stream.cs

and here the Stream.TryOpenAsync: https://github.com/yevhen/Streamstone/blob/master/Source/Streamstone/Stream.Api.cs#L192

Answer 1

The second code block looks like it should work to me. It does run it if I provide dummy implementations for Stream and StreamOpenResult.

You should avoid using Async.RunSynchronously wherever possible because it defeats the purpose of async. Put all of this code within a larger async block and then you will have access to the StreamOpenResult:

async {
    let! openResult = Stream.TryOpenAsync(partition) |> Async.AwaitTask  
    let stream = if openResult.Found then openResult.Stream else Stream(partition)
    return () // now do something with the stream
    }

You may need to put a Async.Start or Async.RunSynchronously at the very outer edge of your program to actually run it, but it's better if you have the async (or convert it to a Task) and pass it to some other code (e.g. a web framework) that can call it in a non-blocking manner.

Answer 2

Not that I want to answer your question with another question, but: why are you doing code like this anyway? That might help to understand it. Why not just:

let asyncWorkflow = async {
    let! result = Stream.TryOpenAsync(partition) |> Async.AwaitTask 
    if result.Found then return openResult.Stream else return Stream(partition) }

There's little point in creating an async workflow only to immediately call RunSynchronously on it - it's similar to calling .Result on a Task - it just blocks the current thread until the workflow returns.

Answer 3

I can't tell you why the second example doesn't work without knowing what Stream and partition are and how they work.

However, I want to take this opportunity to point out that the two examples are not strictly equivalent.

F# async is kind of like a "recipe" for what to do. When you write async { ... }, the resulting computation is just sitting there, not actually doing anything. It's more like declaring a function than like issuing a command. Only when you "start" it by calling something like Async.RunSynchronously or Async.Start does it actually run. A corollary is that you can start the same async workflow multiple times, and it's going to be a new workflow every time. Very similar to how IEnumerable works.

C# Task, on the other hand, is more like a "reference" to an async computation that is already running. The computation starts as soon as you call Stream.TryOpenAsync(partition), and it's impossible to obtain a Task instance before the task actually starts. You can await the resulting Task multiple times, but each await will not result in a fresh attempt to open a stream. Only the first await will actually wait for the task's completion, and every subsequent one will just return you the same remembered result.

In the async/reactive lingo, F# async is what you call "cold", while C# Task is referred to as "hot".