F# MailboxProcessor limit parallelism

I'm new to F# and trying to experiment with the MailboxProcessor to ensure that state changes are done in isolation.

In short, I am posting actions (immutable objects describing state chanage) to the MailboxProcessor, in the recursive function I read the message and generate a new state (i.e. add an item to a collection in the example below) and send that state to the next recursion.

open System

type AppliationState =
    {
        Store : string list
    }
    static member Default = 
        {
            Store = List.empty
        }
    member this.HandleAction (action:obj) =
        match action with
        | :? string as a -> { this with Store = a :: this.Store }
        | _ -> this

type Agent<'T> = MailboxProcessor<'T>     

[<AbstractClass; Sealed>]
type AppHolder private () =
    static member private Processor = Agent.Start(fun inbox ->
        let rec loop (s : AppliationState) =
            async {
                let! action = inbox.Receive()
                let s' = s.HandleAction action
                Console.WriteLine("{s: " + s.Store.Length.ToString() + " s': " + s'.Store.Length.ToString())
                return! loop s'
                }
        loop AppliationState.Default)

    static member HandleAction (action:obj) =
        AppHolder.Processor.Post action

[<EntryPoint>]
let main argv =
    AppHolder.HandleAction "a"
    AppHolder.HandleAction "b"
    AppHolder.HandleAction "c"
    AppHolder.HandleAction "d"

    Console.ReadLine()
    0 // return an integer exit code

Expected output is:

s: 0 s': 1
s: 1 s': 2
s: 2 s': 3
s: 3 s': 4

What I get is:

s: 0 s': 1
s: 0 s': 1
s: 0 s': 1
s: 0 s': 1

Reading the documentation for the MailboxProcessor and googling about it my conclusion is that it is a Queue of messages, processed by a 'single-thread', instead it looks like they are all processed in parallel.

Am I totally off the field here?

Solution

I think the issue must be in your implementation of HandleAction. I implemented the following, and it produces the expected output.

open System

type ApplicationState =
    {
        Items: int list
    }
    static member Default = {Items = []}
    member this.HandleAction x = {this with Items = x::this.Items}

type Message = Add of int

let Processor = MailboxProcessor<Message>.Start(fun inbox ->
    let rec loop (s : ApplicationState) =
        async {
            let! (Add action) = inbox.Receive()
            let s' = s.HandleAction action
            Console.WriteLine("s: " + s.Items.Length.ToString() + " s': " + s'.Items.Length.ToString())
            return! loop s'
        }
    loop ApplicationState.Default)

Processor.Post (Add 1)
Processor.Post (Add 2)
Processor.Post (Add 3)
Processor.Post (Add 4)


// OUTPUT
// s: 0 s': 1
// s: 1 s': 2
// s: 2 s': 3
// s: 3 s': 4

EDIT

After seeing the updated code sample, I believe the correct F# solution would just be to switch the AppHolder type from being a class to a module. The updated code would like this:

open System

type AppliationState =
    {
        Store : string list
    }
    static member Default = 
        {
            Store = List.empty
        }
    member this.HandleAction (action:obj) =
        match action with
        | :? string as a -> { this with Store = a :: this.Store }
        | _ -> this

type Agent<'T> = MailboxProcessor<'T>     

module AppHolder =
    let private processor = Agent.Start(fun inbox ->
        let rec loop (s : AppliationState) =
            async {
                let! action = inbox.Receive()
                let s' = s.HandleAction action
                Console.WriteLine("{s: " + s.Store.Length.ToString() + " s': " + s'.Store.Length.ToString())
                return! loop s'
            }
        loop AppliationState.Default)

    let handleAction (action:obj) =
        processor.Post action


AppHolder.handleAction "a"
AppHolder.handleAction "b"
AppHolder.handleAction "c"
AppHolder.handleAction "d"

This outputs the same result as before:

{s: 0 s': 1
{s: 1 s': 2
{s: 2 s': 3
{s: 3 s': 4