Timers in F#

open System
open System.Threading
open System.Threading.Tasks

let main() = 
    // Timers represent a single event in the future. You
    // tell the timer how long you want to wait, and it
    // provides a Task that will complete at that time.
    // This timer will wait 2 seconds.
    let timer1 = Task.Delay(TimeSpan.FromSeconds(2.0))

    // The Await blocks until the task completes,
    // indicating that the timer fired.
    timer1.Wait()
    printfn "Timer 1 fired"

    // If you just wanted to wait, you could have used
    // Thread.Sleep. One reason a timer may be useful is
    // that you can cancel it before it fires.
    // Here's an example of that.
    let cts = new CancellationTokenSource()
    let timer2 = Task.Delay(TimeSpan.FromSeconds(1.0), cts.Token)

    // Start a task that will wait for the timer
    let timer2Task = Task.Run(fun () ->
        try
            timer2.Wait()
            printfn "Timer 2 fired"
        with
        | :? OperationCanceledException -> ()
    )

    // Cancel the timer
    cts.Cancel()
    if cts.IsCancellationRequested then
        printfn "Timer 2 stopped"

    // Give the timer2 enough time to fire, if it ever
    // was going to, to show it is in fact stopped.
    Thread.Sleep(TimeSpan.FromSeconds(2.0))

main()

This F# code demonstrates the use of timers, which are represented by Task.Delay in F#. Here’s a breakdown of the translation:

We use Task.Delay to create a timer that will complete after a specified duration.
The Wait() method is used to block until the timer completes.
For the second timer, we use a CancellationTokenSource to allow cancellation.
We start a separate task to wait for the second timer, wrapping it in a try-catch block to handle cancellation.
We use Thread.Sleep at the end to give enough time for the second timer to fire if it wasn’t cancelled.

The first timer will fire ~2s after we start the program, but the second should be stopped before it has a chance to fire.

To run the program, save it as Timers.fsx and use the F# interpreter:

$ dotnet fsi Timers.fsx
Timer 1 fired
Timer 2 stopped

This example demonstrates basic timer functionality in F#, including creating timers, waiting for them to complete, and cancelling them before they fire.

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