Can I have an asynchronous agent that listens and responds to 2 events?

Question

I'd like to have an agent that acts as an SMTP server that asynchronously waits for emails and also services requests to retrieve the list of emails received. The agent would look something like:

let agent = Agent.Start(fun inbox ->    
  let rec loop emails = async {
     let! email = ReceiveEmail()
     let emails = email::emails
     let! emailStatusRequest = inbox.Receive()
     match emailStatusRequest with
     | Get channel -> channel.Reply emails
     return! loop emails }
  loop [])

This would wait for an email and then wait for a status request. How would I decouple the ordering so that the agent responds to whichever event happens first - the email received event or the request to get the emails event? Is an agent even the right tool for modelling this?

Answer 1

I would move the ReceiveEmail part in it's own async-loop and have it post messages to your agent.

something like this (warning: not tested - treat as pseudo-code):

type AgentCommands =
   | Received of Email
   | StatusRequest of RequestParams
   | ...

let agent = Agent.Start(fun inbox ->    
  let rec loop emails = async {
     let! cmd = inbox.Receive
     match cmd with
     | Received email ->
        let emails = email::emails
        return! loop emails
     | StatusRequest emailStatusRequest ->
         match emailStatusRequest with
         | Get channel -> channel.Reply emails
     return! loop emails }
  loop [])

let receiveLoop = async {
      while true do
         let! email = ReceiveEmail()
         agent.Post (Received email)   
   } |> Async.Start

of course you probably need to at CancellationToken support ... you'll get it ;)