1-programsync.fs

open System.Threading.Tasks

let add a b = a + b
let generateAString() = "foo"

let oldMain argv =
    let anInt = add 1 2
    let aString = generateAString()
    printfn "Number is %d and string is '%s'" anInt aString
    0

2-programasync.fs

open System.Threading.Tasks
open FSharp.Control.Tasks

let addAsync a b = Task.FromResult (a + b)
let generateAStringAsync() = Task.FromResult "foo"

[<EntryPoint>]
let main argv =
    async {
        let! anInt = addAsync 1 2 |> Async.AwaitTask
        let! aString = generateAStringAsync() |> Async.AwaitTask
        printfn "Number is %d and string is '%s'" anInt aString
        return 0
    } |> Async.RunSynchronously

3-programtask.fs

// Need to add     <PackageReference Include="TaskBuilder.fs" Version="2.1.0" />

open System.Threading.Tasks
open FSharp.Control.Tasks

let addAsync a b = Task.FromResult (a + b)
let generateAStringAsync() = Task.FromResult "foo"

[<EntryPoint>]
let main argv =
    task {
        let! anInt = addAsync 1 2
        let! aString = generateAStringAsync()
        printfn "Number is %d and string is '%s'" anInt aString
        return 0
    } |> fun t -> t.Result