Port of Bryan Hunter's (@bryan_hunter) Cheap Process Erlang demo to F# (https://github.com/bryanhunter/IntroToErlangTalk/blob/nashjug-2013/cheap_process/cheap_process.erl)

CheapProcess.fsx

module CheapProcess =
    open System.Diagnostics
    
    type Agent<'T> = MailboxProcessor<'T>
    
    type Answer =
        Ok

    let rec start howMany (pid: Agent<Answer>) =
        Agent.Start(fun inbox ->
            async { 
                match howMany with
                | 0 -> 
                    pid.Post(Ok)
                    return ()
                | _ -> 
                    let child = start (howMany-1) pid
                    child.Post(Ok)
                    let! msg = inbox.Receive()
                    match msg with
                        | Ok -> return ()
            })

    let startAndTime howMany =
        Agent.Start(fun inbox ->
            async {
                let clock = Stopwatch.StartNew()
                let child=start howMany inbox
                let! msg=inbox.Receive()
                printfn "%O" clock.Elapsed
                return ()
            })