Channel.fs

open System
open System.Collections.Generic
open System.Threading
open System.Threading.Tasks

[<Sealed>]
type Channel<'T>() =
    let root = obj ()
    let qF = Queue<'T->unit>()
    let qX = Queue<'T>()
    let mutable n = 0
    let enq x =
        lock root (fun () ->
            let k = n
            n <- n + 1
            if k < 0 then
                let f = qF.Dequeue()
                fun () -> f x
            else
                qX.Enqueue(x)
                ignore) ()
    let deq f =
        lock root (fun () ->
            let k = n
            n <- n - 1
            if k > 0 then
                let x = qX.Dequeue()
                fun () -> f x
            else
                qF.Enqueue(f)
                ignore) ()
    let receiveAsync =
        Async.FromContinuations(fun (ok, _, _) -> deq ok)

    member this.Receive() = receiveAsync
    member this.Receive(f) = deq f
    member this.Send(x) = enq x

[<Sealed>]
type Agent<'T,'R>(f: Channel<'T> -> Async<'R>, opts) =
    let chan = Channel<'T>()
    let task = Async.StartAsTask(f chan, opts)
    member this.Send(m) = chan.Send(m)
    member this.Task = task

[<Sealed>]
type Agent =
    static member Start(f) = Agent(f, TaskCreationOptions.None)
    static member Start(f, opts) = Agent(f, opts)