模式匹配灵丹妙药中Map的键值

Question

我想对地图中某个键的值进行模式匹配

  def handle_in("new_message", payload, socket) do
    case payload.message do
      "hello" -> broadcast! socket, "new_message", payload
    end

    {:noreply, socket}
  end

[error] GenServer #PID<0.378.0> terminating ** (KeyError) key :message not found in: %{"message" => "hello", "name" => "x"} (chatroom) web/channels/lobby_channel.ex:9: Chatroom.LobbyChannel.handle_in/3 (phoenix) lib/phoenix/channel/server.ex:226: anonymous fn/4 in Phoenix.Channel.Server.handle_info/2 (stdlib) gen_server.erl:601: :gen_server.try_dispatch/4 (stdlib) gen_server.erl:667: :gen_server.handle_msg/5

当我发送%{"message" => "hello", "name" => "x"}的有效负载时，会发生该错误，或​​者发送任何其他消息。

Answer 1

此处的密钥为"message"而不是:message。使用：payload["message"]

  def handle_in("new_message", payload, socket) do
    case payload["message"] do
      "hello" -> broadcast! socket, "new_message", payload
    end

    {:noreply, socket}
  end

你也可以这样做：

  def handle_in("new_message", %{"message" => message}, socket) do
    case message do
      # ...
  end