A simple generic Command/Query Agent in F# using MailboxProcessor

gistfile1.txt

type Message<'S> =
  | Query of ('S -> unit)
  | Command of ('S -> 'S)
  | Kill

type CQAgent<'S>(state: 'S) =
  let innerModel =
    MailboxProcessor<Message<'S>>.Start(fun inbox ->
      let rec messageLoop (state: 'S) =
        async {
          let! msg = inbox.Receive()
          match msg with
          | Query q ->
              q state
              return! messageLoop state
          | Command c ->
              let newState = c state
              return! messageLoop(newState)
          | Kill -> ()
        }
      messageLoop state)

  member this.Query<'T> (q: 'S -> 'T) =
    innerModel.PostAndReply(fun chan -> Query(fun state ->
      let res = q state
      chan.Reply(res)))

  member this.Command<'T> (c: 'S -> 'T * 'S) =
    innerModel.PostAndReply(fun chan -> Command(fun state ->
      let res, newState = c state
      chan.Reply(res)
      newState))