Unable to keep state with mailbox processor (F#)

Question

I am attempting to create a list of strings which gets elements gradually inserted into asynchronously with the help of a mailbox processor. However I am not getting the desired output.

I have pretty much followed the code from https://fsharpforfunandprofit.com/posts/concurrency-actor-model/ however it does not seem to work as intended in my case. The code I have is as follows:

type TransactionQueue ={

queue : string list

} with

static member UpdateState (msg : string) (tq : TransactionQueue) =
    {tq with queue = (msg :: tq.queue)}

static member Agent = MailboxProcessor.Start(fun inbox ->
                            let rec msgLoop (t : TransactionQueue) =
                                async{
                                   let! msg = inbox.Receive()
                                   let newT = TransactionQueue.UpdateState msg t
                                   printfn "%A" newT
                                   return! msgLoop newT
                                }
                            msgLoop {queue = []}
                        )

static member Add i = TransactionQueue.Agent.Post i



[<EntryPoint>]
let main argv =

// test in isolation
printfn "welcome to test"
let rec loop () =
    let str = Console.ReadLine()
    TransactionQueue.Add str
    loop ()

loop ()



0 

The result i keep getting is a list of the latest input only, the state is not kept. So if I enter "a" then "b" then "c" the queue will only have the value "c" instead of "a";"b";"c"

Any help or pointers would be most appreciated!

Answer 1

Just like C# Properties, your Agent is really a Property and thus behaves like a method with void parameter. That’s why you will get a new agent everytime Agent property is accessed.

In idiomatic F# there are two styles when implementing agents. If you don’t need to have many agent instances, just write a module and encapsule the agent-related stuff inside. Otherwise, OOP style should be used.

Code for style #1

module TransactionQueue =
    type private Queue = Queue of string list
    let private empty = Queue []
    let private update item (Queue items) = Queue (item :: items)
    let private agent = MailboxProcessor.Start <| fun inbox ->
        let rec msgLoop queue = async {
            let! msg = inbox.Receive ()
            return! queue |> update msg |> msgLoop
        }
        msgLoop empty
    let add item = agent.Post item

[<EntryPoint>]
let main argv =
    // test in isolation
    printfn "welcome to test"
    let rec loop () =
        let str = Console.ReadLine()
        TransactionQueue.add str
        loop ()
    loop ()

Code for style #2

type Queue = Queue of string list with
    static member Empty = Queue []
    static member Update item (Queue items) =
        Queue (item :: items)

type Agent () =
    let agent = MailboxProcessor.Start <| fun inbox ->
        let rec msgLoop queue = async {
            let! msg = inbox.Receive ()
            return! queue |> Queue.Update msg |> msgLoop
        }
        msgLoop Queue.Empty
    member this.Add item = agent.Post item

[<EntryPoint>]
let main argv =
    // test in isolation
    printfn "welcome to test"
    let agent = new Agent ()
    let rec loop () =
        let str = Console.ReadLine()
        agent.Add str
        loop ()
    loop ()

Notice the use of Single-case union types for the Queue type.