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MailboxProcessor.TryScan<'Msg,'T> Method (F#)

Den Delimarsky|Last Updated: 8/18/2017
1 Contributor

Scans for a message by looking through messages in arrival order until a provided function returns a Some value. Other messages remain in the queue.

Namespace/Module Path: Microsoft.FSharp.Control

Assembly: FSharp.Core (in FSharp.Core.dll)


// Signature:
member this.TryScan : ('Msg -> Async<'T> option) * ?int -> Async<'T option>

// Usage:
mailboxProcessor.TryScan (scanner)
mailboxProcessor.TryScan (scanner, timeout = timeout)


scanner Type: 'Msg ->Async<'T>option

A function that returns None if the message is to be skipped, or Some if the message is to be processed and removed from the queue.

timeout Type: int

An optional timeout in milliseconds. Defaults to -1 which corresponds to System.Threading.Timeout.Infinite.

Return Value

An asynchronous computation (Async object) that scanner built off the read message.


If a timeout period is exceeded, None is returned. This method is for use within the body of the agent. For each agent, at most one concurrent reader can be active, so no more than one concurrent call to Receive, TryReceive, Scan or TryScan can be active. The body of the scanner function is locked during its execution, but the lock is released before the execution of the asynchronous workflow.


The following code example shows how to use the TryScan method. This example is a job submission agent. There are three agents: one called runAgent that starts each job, another called inprogressAgent that represents all running jobs, and one called completeAgent that represents notification that a job is completed. TryScan is used in the cancelJob function to find a job to cancel, or fail if there is no matching job.

open System
open System.Threading

let random = System.Random()

// Generates mock jobs by using Async.Sleep.
let createJob(id:int, source:CancellationTokenSource) =
    let job = async {
        // Let the time be a random number between 1 and 10000.
        // The mock computed result is a floating point value.
        let time = random.Next(10000)
        let result = random.NextDouble()
        let count = ref 1
        while (!count <= 100 && not source.IsCancellationRequested) do
            do! Async.Sleep(time / 100)
            count := !count + 1
        return result
    id, job, source

type Result = double

// A Job consists of a job ID, a computation that produces a single result,
// and a cancellation token source object that can be used to cancel the job.
type Job = int * Async<Result> * CancellationTokenSource

type Message = int * Result

let context = System.Threading.SynchronizationContext.Current

// This agent processes when jobs are completed.
let completeAgent = MailboxProcessor<Message>.Start(fun inbox ->
    let rec loop n =
        async {
            let! (id, result) = inbox.Receive()
            printfn "The result of job #%d is %f" id result
            do! loop (n + 1)
    loop (0))

// inprogressAgent maintains a queue of in-progress jobs that can be
// scanned to remove canceled jobs. It never runs its processor function,
// so we set it to do nothing.
let inprogressAgent = new MailboxProcessor<Job>(fun _ -> async { () })

// This agent starts each job in the order in which it is received.
let runAgent = MailboxProcessor<Job>.Start(fun inbox ->
    let rec loop n =
        async {          
            let! (id, job, source) = inbox.Receive()
            printfn "Starting job #%d" id
            // Post to the in-progress queue.
            inprogressAgent.Post(id, job, source)
            // Start the job.
                (fun result -> completeAgent.Post(id, result)),
                (fun _ -> ()),
                (fun cancelException -> printfn "Canceled job #%d" id),
            do! loop (n + 1)
    loop (0))

for id in 1 .. 10 do
    let source = new CancellationTokenSource()
    runAgent.Post(createJob(id, source))

let cancelJob(cancelId) =
         inprogressAgent.TryScan((fun (jobId, result, source) ->
                let action =
                    async {
                        printfn "Canceling job #%d" cancelId
                        return cancelId
                // Return Some(async) if the job ID matches.
                if (jobId = cancelId) then
                    None), 1000))

printfn "Specify a job by number to cancel it, then press Enter."

let mutable finished = false
while not finished do
    let input = System.Console.ReadLine()
    let a = ref 0
    if (Int32.TryParse(input, a) = true) then
        match cancelJob(!a) with
        | Some id -> printfn "A job was canceled: job #%d" id
        | None -> printfn "Job not found."
        printfn "Terminating."
        finished <- true

Following is an example session.

Mailbox Processor Test
Type some text and press Enter to submit a message.
Type 'Stop' to close the program.
> hello
1 : Console loop
4 : mailboxProcessor
Reply: Message number 0 was received. Message contents: hello
> testing
1 : Console loop
3 : mailboxProcessor
Reply: Message number 1 was received. Message contents: testing
> hello?
1 : Console loop
4 : mailboxProcessor
Reply: Message number 2 was received. Message contents: hello?
> testing 1 2 3
1 : Console loop
3 : mailboxProcessor
Reply: Message number 3 was received. Message contents: testing 1 2 3
> Stop
1 : Console loop
4 : mailboxProcessor
Press Enter to continue.


Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

See Also

Control.MailboxProcessor<'Msg> Class (F#)

Microsoft.FSharp.Control Namespace (F#)

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