It's the other way around.

'async'-annotated methods in C# enable 'await'ing on task-shaped types. It is bespoke and async-specific. There is nothing wrong with it but it's necessary to acknowledge this limitation.

let!, and!, return!, etc. keywords in F# are generic - you can build your own state machines/coroutines with resumable code, you can author completely custom logic with CEs. I'm not sure what led you to believe the opposite. `await Task.Delay` in C# is `do! Task.Delay` in F#. `let! response = http.SendAsync` is for asynchronous calls that return a value rather than unit.

In the same vein, seq is another CE that is more capable than iterator methods with yield return:

    let values = seq {
        // yield individual values
        for i in 1..10 -> i
        // yield a range, merged into the sequence
        yield! [11..20] // note the exclamation mark
    }

Adding support for this in C# would require explicit compiler changes. CEs are generic and very powerful at building execution blocks with fine control over the behavior, DSLs and more.

Reference: https://learn.microsoft.com/en-us/dotnet/fsharp/language-ref...