Question

我现在正在Erlang做一个实验室，这是我第一次写Erlang。我有一个initial_state函数，它应该在聊天程序中为客户端设置初始状态。但是如果你没有像Java或C那样存储它的东西，那么设置这个初始状态有什么意义呢？我的意思是感觉我只是在创建初始状态然后把它扔掉。那有什么意义呢？我想在某处存放它以便以后可以使用它。

initial_state（Nick，GUIName） - ＆gt; #cl_st {gui = GUIName}。

Answer 1

你的问题缺乏一些背景，让整个模块给出一个好的答案可能是有用的。

无论如何，你展示的功能非常简单，它是一个返回客户端状态记录的函数，字段gui等于GUIName。

这个函数看起来很奇怪，因为它有2个参数，参数Nick未使用。

在erlang中只有局部变量，它们属于一个进程，不能与另一个进程共享。这意味着两件事：

如果你想跟踪变量值，它所属的过程一定不会死，所以它必须是递归的。
如果您想在流程之间进行交换，则必须使用消息

通常将服务器拆分为init函数，一些回调和接口函数以及无限循环。我想这是对OTP的gen_server行为的一个很好的介绍。我可以想象以这种方式补充你的代码：

%% the record state definition, it will include
%% the client nickname,
%% the gui name (or pid ?)
%% the client cart with a default value equals to the empty list
-record(cl_st,{nick,gui,cart=[]}).

initial_state(Nick, GUIName) -> 
%% the role of this function could be to start processes such as the gui
%% and return the initial state
    %% add some code to start the gui
    #cl_st { gui = GUIName, nick = Nick}.

%% an example of an interface function to add some item to the client cart
%% it simply pack the parameters into a tuple and send it to the server
%% Note that the server is identified by its pid, so somwhere there must be a unique
%% server that keep the list of all clients and their server pid
add_to_cart(Pid,Item,Price,Quantity) ->
    Pid ! {add_to_cart,Item,Price,Quantity}.

%% this function calls the init function, starts the server in a new process (usage of spawn) with the
%% initial state and returns the server pid 
start(Nick,GUIName) ->
    State = initial_state(Nick, GUIName),
    spawn(?MODULE,cl_loop,[State]).

stop(Pid) ->
    Pid ! stop.

%% the server loop
%% this example manages 2 kind of messages
cl_loop(State) ->
    receive
        %% add to cart simply builds a tuple made of item, price and quantity and add it to a list
        %% of tuple representing the cart content.
        %% it calls itself recursively with a new state as parameter where the old cart
        %% is replaced by the new one
        {add_to_cart,Item,Price,Quantity} ->
            NewCart = [{Item,Price,Quantity}|State#cl_st.cart],
            cl_loop(State#cl_st{cart=NewCart});
        %% to stop the server, it calls the terminate callback function and does not call itself recursively 
        stop ->
            terminate(State);
        %% other messages are ignored, the server simply calls itself with an unchanged state
        Ignored ->
            cl_loop(State)
    end. 

%% a callback function to terminate the server properly
terminate(State#cl_st{gui = GUIName, nick = Nick}) ->
    %% some code to stop the gui
    {stopped_client, Nick}.

（它必须在代码中出错，我甚至没有编译它）

Erlang，为什么它没有全局变量和记录？

1 个答案: