Simple F# agents (MailboxProcessor) example. Try it here: http://www.tryfsharp.org/create/vkobel/Agent.fsx

AgentsSimple.fs

type CounterMessage = 
  | Update of float
  | Reset

let inbox = MailboxProcessor.Start(fun agent -> 
    // Function that implements the body of the agent
    let rec loop sum count = async {
      // Asynchronously wait for the next message
      let! msg = agent.Receive()
      match msg with
      | Reset -> 
          // Restart loop with initial values
          return! loop 0.0 0.0
     
      | Update value -> 
          // Update the state and print the statistics
          let sum, count = sum + value, count + 1.0
          printfn "Average: %f" (sum / count)
     
          // Wait before handling the next message
          do! Async.Sleep(1000)
          return! loop sum count
    }     
    // Start the body with initial values
    loop 0.0 0.0
)

inbox.Post(Update 10.0)
inbox.Post(Update 5.0)
inbox.Post(Reset)
inbox.Post(Update 2.0)