警告：使用运算符“<”无效？

Question

我是 erlang 的新手，我正在尝试实现一个简单的功能，如下所示：

% * ChatServers is a dictionary of usernames with tuples of the form:
%   {server, Pid, Reference,LoggedInUsers}
get_chat_server([], _) ->        
    undefined;

get_chat_server([Key|_], ChatServers) ->
    {server, Pid, Reference,LoggedInUsers} = dict:fetch(Key,ChatServers),
    LoggedInUsers < 100,
    {server, Pid, Reference,LoggedInUsers};

get_chat_server([_|T], ChatServers) ->        
    get_chat_server(T, ChatServers).

基本上我要做的是找到我的字典中第一个 LoggedInUsers 数小于 100 的元组。

但是，一旦我编译了我的代码，我就收到了以下 2 个警告：

main_server_distributed.erl:63: Warning: use of operator '<' has no effect main_server_distributed.erl:66: Warning: this clause cannot match because a previous clause at line 61 always matches

我对 prolog 有一些经验，据我所知，这是对模式匹配和递归的有效使用。你能指出我在这里做错了什么吗？提前致谢。

Answer 1

子句的主体（-> 右侧的所有内容）不是要满足的条件列表，而只是要计算的以逗号分隔的表达式列表。除最后一个表达式之外的所有结果值都将被丢弃。因此，您的 < 比较的布尔值未在任何地方使用。

Answer 2

main_server_distributed.erl:66: Warning: this clause cannot match because a previous clause at line 61 always matches

你基本上写了：

get_chat_server(NonEmptyList, ChatServers) ->
    {server, Pid, Reference,LoggedInUsers} = dict:fetch(Key,ChatServers),
    LoggedInUsers < 100,
    {server, Pid, Reference,LoggedInUsers};

get_chat_server(NonEmptyList, ChatServers) ->        
    get_chat_server(T, ChatServers).

因此，第一个子句将始终匹配第二个子句可以匹配的所有内容。更具体地说，在模式中：

[Key|_]

Key 将匹配任何内容，_ 将匹配任何内容。同样，在模式中：

[_|T]

_ 将匹配任何内容，T 将匹配任何内容。

模仿@dsmith 的回答：

-module(my).
-export([get_chat_server/3, get_chat_server_test/0]).

get_chat_server(_MaxLoggedIn, []=_Keys, _ChatServers) ->        
    none;
get_chat_server(MaxLoggedIn, [Key|Keys], ChatServers) ->
    get_chat_server(MaxLoggedIn, Keys, ChatServers, dict:fetch(Key, ChatServers) ).

get_chat_server(MaxLoggedIn, _, _, {_,_,_,LoggedInUsers}=ChatServer) when LoggedInUsers < MaxLoggedIn ->
    ChatServer;
get_chat_server(MaxLoggedIn, [Key|Keys], ChatServers, _ChatServer) ->
    get_chat_server(MaxLoggedIn, Keys, ChatServers, dict:fetch(Key, ChatServers) ).

%---------

get_chat_server_test() ->
    Keys = [a, c],
    ChatServers = [
         {a, {server, a, a_, 200}},
         {b, {server, b, b_, 100}},
         {c, {server, c, c_, 30}}
    ],  
    ChatServerDict = dict:from_list(ChatServers),

    none = get_chat_server(10, [], ChatServerDict),
    {server, c, c_, 30} = get_chat_server(50, Keys, ChatServerDict),
    {server, c, c_, 30} = get_chat_server(150, Keys, ChatServerDict),
    PossibleResults = sets:from_list([{server,a,a_, 200},{server,c,c_,30}]),
    true = sets:is_element(
             get_chat_server(250, Keys, ChatServerDict),
             PossibleResults
           ),
    all_tests_passed.

您还可以使用高阶函数，即dict:fold()，来获取满足您要求的所有 ChatServers 的列表：

max_fun(Max, Keys) ->
    fun(Key, {_,_,_,LoggedInUsers}=Server, Acc) ->
            case lists:member(Key, Keys) andalso LoggedInUsers<Max of
                true -> [Server | Acc];
                false -> Acc
            end
    end.

在shell:

44>     ChatServers = [
44>          {a, {server, a, a_, 200}},
44>          {b, {server, b, b_, 100}},
44>          {c, {server, c, c_, 30}}
44>     ].
[{a,{server,a,a_,200}},
 {b,{server,b,b_,100}},
 {c,{server,c,c_,30}}]

45> ChatServerDict = dict:from_list(ChatServers).
{dict,3,16,16,8,80,48,
      {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],[]},
      {{[],
        [[a|{server,a,a_,200}]],
        [[b|{server,b,b_,100}]],
        [[c|{server,c,c_,30}]],
        [],[],[],[],[],[],[],[],[],[],[],[]}}}

46> Keys = [a,c].
[a,c]

47> MaxLoggedIn = 150.
150

50> F = my:max_fun(MaxLoggedIn, Keys).
#Fun<fl.0.128553666>

51> dict:fold(F, [], ChatServerDict).
[{server,c,c_,30}]

Answer 3

你可以这样做...

   get_chat_server([], _) ->        
       undefined;

   get_chat_server([Key|T], ChatServers) ->
       {server, Pid, Reference,LoggedInUsers} = dict:fetch(Key,ChatServers),
       if
          LoggedInUsers < 100 -> 
              {server, Pid, Reference,LoggedInUsers};
          true -> 
              get_chat_server(T, ChatServers)
       end.

或者这个

   get_chat_server([], _) ->        
       undefined;

   get_chat_server([Key|T], ChatServers) ->
       Result = dict:fetch(Key,ChatServers),
       case Result of
          {_, _, _, LoggedInUsers} when LoggedInUsers < 100 ->
             Result;
          _ -> 
             get_chat_server(T, ChatServers)
       end.

警告：使用运算符“<”无效？

Warning: use of operator '<' has no effect?

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