Enforcing one Async Observable at a time in F# - f#

Question

I have an observable sequence of things that need to be mapped to
a C# task. These C# tasks should not run concurrently, but one after
another. Basically, I need to achieve the F# equivalent of this C#
question:
Enforcing one async observable at a time
Naively translating this C# code gives something like the following:
let run (idx:int) (delay:int) =
async {
sprintf "start: %i (%i)" idx delay |> System.Diagnostics.Trace.WriteLine
let! t = System.Threading.Tasks.Task.Delay(delay) |> Async.AwaitTask
sprintf "finish: %i" idx |> System.Diagnostics.Trace.WriteLine
t
}
Observable.generate (new Random()) (fun _ -> true) id (fun s -> s.Next(250, 500))
|> Observable.take 20
|> Observable.mapi(fun idx delay -> idx, delay)
|> Observable.bind(fun (idx, delay) -> Observable.ofAsync (run idx delay))
|> Observable.subscribe ignore
|> ignore
Which does not work as expected because I don't wait for a result anywhere. Is there even
a way to do this in F# without blocking a thread, just like C#'s await would?

Answer 1

There's a handy library that exists in F# called AsyncSeq:
https://www.nuget.org/packages/FSharp.Control.AsyncSeq/
It's very similar to IAsyncEnumerable<T> that was added to C# 8.0, this gives you a nice solution to what you are after.
Solution:
open System
open FSharp.Control
open FSharp.Control.Reactive
[<EntryPoint>]
let main _ =
let run (idx:int) (delay:int) =
async {
sprintf "start: %i (%i)" idx delay |> Console.WriteLine
do! Async.Sleep delay
sprintf "finish: %i" idx |> Console.WriteLine
}
Observable.generate (new Random()) (fun _ -> true) id (fun s -> s.Next(250, 500))
|> Observable.take 20
|> Observable.mapi(fun idx delay -> idx, delay)
|> AsyncSeq.ofObservableBuffered
|> AsyncSeq.iterAsync (fun (idx,delay) -> run idx delay)
|> Async.RunSynchronously
0
AsyncSeq.ofObservableBuffered does the work of subscribing to your Observable and acts as AsyncSeq source which you can pipeline on top of. Finally we call Async.RunSynchronously to kick it off and wait on the entrypoint thread.
Note: I also update run as it was returning Async<Async<unit>> and I assume that wasn't intended.