5 记录和宏

较大的程序通常被编写为包含多个文件，这些文件之间具有明确定义的接口。

5.1 将较大示例划分为多个文件

为了说明这一点，上一节中的信使示例被划分为以下五个文件

mess_config.hrl

配置数据的头文件
mess_interface.hrl

客户端和信使之间的接口定义
user_interface.erl

用户界面的函数
mess_client.erl

信使客户端的函数
mess_server.erl

信使服务器端的函数

在执行此操作时，Shell、客户端和服务器之间消息传递接口已清理，并使用**记录**进行定义。此外，还引入了**宏**

%%%----FILE mess_config.hrl----

%%% Configure the location of the server node,
-define(server_node, messenger@super).

%%%----END FILE----

%%%----FILE mess_interface.hrl----

%%% Message interface between client and server and client shell for
%%% messenger program 

%%%Messages from Client to server received in server/1 function.
-record(logon,{client_pid, username}).
-record(message,{client_pid, to_name, message}).
%%% {'EXIT', ClientPid, Reason}  (client terminated or unreachable.

%%% Messages from Server to Client, received in await_result/0 function 
-record(abort_client,{message}).
%%% Messages are: user_exists_at_other_node, 
%%%               you_are_not_logged_on
-record(server_reply,{message}).
%%% Messages are: logged_on
%%%               receiver_not_found
%%%               sent  (Message has been sent (no guarantee)
%%% Messages from Server to Client received in client/1 function
-record(message_from,{from_name, message}).

%%% Messages from shell to Client received in client/1 function
%%% spawn(mess_client, client, [server_node(), Name])
-record(message_to,{to_name, message}).
%%% logoff

%%%----END FILE----

%%%----FILE user_interface.erl----

%%% User interface to the messenger program
%%% login(Name)
%%%     One user at a time can log in from each Erlang node in the
%%%     system messenger: and choose a suitable Name. If the Name
%%%     is already logged in at another node or if someone else is
%%%     already logged in at the same node, login will be rejected
%%%     with a suitable error message.

%%% logoff()
%%%     Logs off anybody at that node

%%% message(ToName, Message)
%%%     sends Message to ToName. Error messages if the user of this 
%%%     function is not logged on or if ToName is not logged on at
%%%     any node.

-module(user_interface).
-export([logon/1, logoff/0, message/2]).
-include("mess_interface.hrl").
-include("mess_config.hrl").

logon(Name) ->
    case whereis(mess_client) of 
        undefined ->
            register(mess_client, 
                     spawn(mess_client, client, [?server_node, Name]));
        _ -> already_logged_on
    end.

logoff() ->
    mess_client ! logoff.

message(ToName, Message) ->
    case whereis(mess_client) of % Test if the client is running
        undefined ->
            not_logged_on;
        _ -> mess_client ! #message_to{to_name=ToName, message=Message},
             ok
end.

%%%----END FILE----

%%%----FILE mess_client.erl----

%%% The client process which runs on each user node

-module(mess_client).
-export([client/2]).
-include("mess_interface.hrl").

client(Server_Node, Name) ->
    {messenger, Server_Node} ! #logon{client_pid=self(), username=Name},
    await_result(),
    client(Server_Node).

client(Server_Node) ->
    receive
        logoff ->
            exit(normal);
        #message_to{to_name=ToName, message=Message} ->
            {messenger, Server_Node} ! 
                #message{client_pid=self(), to_name=ToName, message=Message},
            await_result();
        {message_from, FromName, Message} ->
            io:format("Message from ~p: ~p~n", [FromName, Message])
    end,
    client(Server_Node).

%%% wait for a response from the server
await_result() ->
    receive
        #abort_client{message=Why} ->
            io:format("~p~n", [Why]),
            exit(normal);
        #server_reply{message=What} ->
            io:format("~p~n", [What])
    after 5000 ->
            io:format("No response from server~n", []),
            exit(timeout)
    end.

%%%----END FILE---

%%%----FILE mess_server.erl----

%%% This is the server process of the messenger service

-module(mess_server).
-export([start_server/0, server/0]).
-include("mess_interface.hrl").

server() ->
    process_flag(trap_exit, true),
    server([]).

%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server(User_List) ->
    io:format("User list = ~p~n", [User_List]),
    receive
        #logon{client_pid=From, username=Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
        {'EXIT', From, _} ->
            New_User_List = server_logoff(From, User_List),
            server(New_User_List);
        #message{client_pid=From, to_name=To, message=Message} ->
            server_transfer(From, To, Message, User_List),
            server(User_List)
    end.

%%% Start the server
start_server() ->
    register(messenger, spawn(?MODULE, server, [])).

%%% Server adds a new user to the user list
server_logon(From, Name, User_List) ->
    %% check if logged on anywhere else
    case lists:keymember(Name, 2, User_List) of
        true ->
            From ! #abort_client{message=user_exists_at_other_node},
            User_List;
        false ->
            From ! #server_reply{message=logged_on},
            link(From),
            [{From, Name} | User_List]        %add user to the list
    end.

%%% Server deletes a user from the user list
server_logoff(From, User_List) ->
    lists:keydelete(From, 1, User_List).

%%% Server transfers a message between user
server_transfer(From, To, Message, User_List) ->
    %% check that the user is logged on and who he is
    case lists:keysearch(From, 1, User_List) of
        false ->
            From ! #abort_client{message=you_are_not_logged_on};
        {value, {_, Name}} ->
            server_transfer(From, Name, To, Message, User_List)
    end.
%%% If the user exists, send the message
server_transfer(From, Name, To, Message, User_List) ->
    %% Find the receiver and send the message
    case lists:keysearch(To, 2, User_List) of
        false ->
            From ! #server_reply{message=receiver_not_found};
        {value, {ToPid, To}} ->
            ToPid ! #message_from{from_name=Name, message=Message}, 
            From !  #server_reply{message=sent} 
    end.

%%%----END FILE---

5.2 头文件

如上所示，一些文件扩展名为 .hrl。这些是头文件，通过以下方式包含在 .erl 文件中：

-include("File_Name").

例如

-include("mess_interface.hrl").

在上面的情况下，该文件是从与信使示例中所有其他文件相同的目录中获取的。（*手册*）。

.hrl 文件可以包含任何有效的 Erlang 代码，但最常用于记录和宏定义。

5.3 记录

记录被定义为

-record(name_of_record,{field_name1, field_name2, field_name3, ......}).

例如

-record(message_to,{to_name, message}).

这等效于

{message_to, To_Name, Message}

创建记录最好通过示例说明

#message_to{message="hello", to_name=fred)

这将创建

{message_to, fred, "hello"}

请注意，您不必担心在创建记录时为记录的各个部分分配值的顺序。使用记录的优势在于，通过将它们的定义放在头文件中，您可以方便地定义易于更改的接口。例如，如果您想在记录中添加一个新字段，您只需要更改使用新字段的代码，而不需要在引用记录的每个地方都更改。如果您在创建记录时省略了一个字段，它将获得原子 undefined 的值。（*手册*）

使用记录进行模式匹配与创建记录非常相似。例如，在 case 或 receive 内部

#message_to{to_name=ToName, message=Message} ->

这与以下相同

{message_to, ToName, Message}

5.4 宏

添加到信使中的另一项是宏。文件 mess_config.hrl 包含以下定义：

%%% Configure the location of the server node,
-define(server_node, messenger@super).

此文件包含在 mess_server.erl 中

-include("mess_config.hrl").

现在，mess_server.erl 中的每个 ?server_node 都会被替换为 messenger@super。

在生成服务器进程时也使用宏

spawn(?MODULE, server, [])

这是一个标准宏（即，由系统定义，而不是由用户定义）。?MODULE 始终被替换为当前模块的名称（即，文件开头附近的 -module 定义）。还有一些更高级的使用宏的方法，例如使用参数（*手册*）。

信使示例中的三个 Erlang (.erl) 文件分别编译成目标代码文件 (.beam)。Erlang 系统在执行代码时引用这些文件时，会将这些文件加载并链接到系统中。在这种情况下，它们只是被放在我们的当前工作目录中（即，您执行了 "cd" 的位置）。还有一些方法可以将 .beam 文件放在其他目录中。

在信使示例中，没有关于正在发送的消息的任何假设。它可以是任何有效的 Erlang 项。