echo_server.sml

module EchoServer
  let start () = listen 1234

  let listen port =
    let (:ok, socket) = gen_tcp.listen port [(:packet, 0), (:active, false)]
    in accept socket

  let accept listen_socket =
    let (:ok, socket) = gen_tcp.accept listen_socket
    in do
      spawn fn () -> echo socket
      accept listen_socket
    end

  let echo socket =
    match gen_tcp.recv socket 0 with
      (:ok, data) ->
        gen_tcp.send socket data
        echo socket
      (:error, :closed) ->
        :ok
    end
end

Brilliant code.