Aggregate.fsx

type State =
    | Ping
    | Pong

type Command = | Send

type Event = | Received

let apply item =
    function
    | Received ->
        printfn "Received %A" item

        match item with
        | Ping -> Pong
        | Pong -> Ping

let exec state =
    function
    | Send -> Received

type Aggregate<'TState, 'TCommand, 'TEvent> =
    { zero: 'TState
      apply: 'TState -> 'TEvent -> 'TState
      exec: 'TState -> 'TCommand -> 'TEvent }

let makeHandler (aggregate: Aggregate<'TState, 'TCommand, 'TEvent>) events command =
    let state =
        Seq.fold aggregate.apply aggregate.zero events

    aggregate.exec state command

let aggregate =
    { zero = Ping
      apply = apply
      exec = exec }

let events =
    seq {
        while true do
            yield Received
            Async.Sleep 1000 |> Async.RunSynchronously
    }

let handler = makeHandler aggregate events

let x = handler Send