MailboxProcessor.PostAndReply<'Msg,'Reply> Method (F#)

Article
04/28/2015

Posts a message to an agent and await a reply on the channel, synchronously.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
member this.PostAndReply : (AsyncReplyChannel<'Reply> -> 'Msg) * ?int -> 'Reply

// Usage:
mailboxProcessor.PostAndReply (buildMessage)
mailboxProcessor.PostAndReply (buildMessage, timeout = timeout)

Parameters

buildMessage
Type: AsyncReplyChannel<'Reply> -> 'Msg

The function to incorporate the AsyncReplyChannel into the message to be sent.
timeout
Type: int

An optional timeout parameter (in milliseconds) to wait for a reply message. The default is -1 which corresponds to Infinite.

Return Value

The reply from the agent.

Remarks

The message is generated by applying buildMessage to a new reply channel to be incorporated into the message. The receiving agent must process this message and invoke the Reply method on this reply channel precisely once.

Example

The following code example shows how to start a mailbox processor agent. In this example, each line of input from the console is posted to a message queue. The program reads each message and replies by using a reply channel. When the special message "Stop" is received, the Stop reply is sent and the program exits.

open System

type Message = string * AsyncReplyChannel<string>

let formatString = "Message number {0} was received. Message contents: {1}" 

let printThreadId note =

    // Append the thread ID.
    printfn "%d : %s" System.Threading.Thread.CurrentThread.ManagedThreadId note


let agent = MailboxProcessor<Message>.Start(fun inbox ->
    let rec loop n =
        async {
                let! (message, replyChannel) = inbox.Receive();
                printThreadId "MailboxProcessor" 
                if (message = "Stop") then
                    replyChannel.Reply("Stopping.")
                else
                    replyChannel.Reply(String.Format(formatString, n, message))
                do! loop (n + 1)
        }
    loop 0)

printfn "Mailbox Processor Test"
printfn "Type some text and press Enter to submit a message."
printfn "Type 'Stop' to close the program." 

let rec loop() =
    printf "> " 
    let input = Console.ReadLine()
    printThreadId("Console loop")
    let reply = agent.PostAndReply(fun replyChannel -> input, replyChannel)
    if (reply <> "Stopping.") then
        printfn "Reply: %s" reply
        loop()
    else
        ()
loop()

printfn "Press Enter to continue."
Console.ReadLine() |> ignore

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.

Platforms

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

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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