返回 handle_call 值时在 case_clause 中抛出错误

Question

我想要 handle_call \ 3 到 return 我一个以下形式的元组 {message, timestamp, integer} 然后在 case 子句中，它在我应该的时候给我一个错误通过与该响应对应的分支进入。

附上例子

execute(Method, Url, Headers, Body, PoolId) ->
    BeforeGetConn = erlang:now(),
    TimeStartBusy = undefined,
    PosBusy = undefined,
    TimeStartBusyAfterWait = undefined,
    PosBusyAfterWait = undefined,
    TimeInWaiting = undefined,
    PosWaiting = undefined,
    ShouldContinue =
    case gen_server:call(?MODULE, {get_conn, PoolId, self(), BeforeGetConn}) of
        proceed -> ok;
        {wait, Ref, PosWaiting} ->
            ...
        %Throw error in the following line
        {proceed, TimeStartBusy, PosBusy} -> {ok, TimeStartBusy, PosBusy};
        unknown_pool -> ...
    end,
    case ShouldContinue of
        {ok, TimeStartBusy, PosBusy}  ->
           execute_process(...);
        ...
    end.

handle_call({get_conn, PoolId, Caller, Time}, _From, #state{pools = Pools} = State) ->
  case proplists:get_value(PoolId, Pools) of
    undefined ->
      ...
    Pool ->
      case is_busy_full(Pool) of
        false ->
          Pool2 = add_to_busy({Caller}, Pool),
          TimeStartBusy = os:timestamp(),
          PosBusy = queue:len(Pool2#pool.busy),
          {reply,
          {proceed, TimeStartBusy, PosBusy},
          State#state{pools = [{PoolId, Pool2} |
                                proplists:delete(PoolId, Pools)]}};
        true ->
          ...
      end
  end;

我在控制台上看到的错误如下：

exit with reason: {[{reason,{case_clause,{proceed,{1614,588434,663546},1}}}

谢谢

已编辑代码：

ShouldContinue =
    case gen_server:call(?MODULE, {get_conn, PoolId, self(), BeforeGetConn}) of
        proceed -> ok;
        {wait, Ref, PosWaiting} ->
            receive
                {proceed, Ref, TimeStartBusyAfterWait, PosBusyAfterWait, TimeInWaiting} ->
                TimeStartBusy = undefined, 
                PosBusy = undefined,
                {ok, TimeStartBusy, PosBusy, TimeInWaiting, PosWaiting, TimeStartBusyAfterWait, PosBusyAfterWait}
            after ?CONF_BACKEND_HTTP_TIMEOUT ->
                ok
            end;
        {proceed, TimeStartBusy, PosBusy} -> 
            PosWaiting = undefined,
            TimeStartBusyAfterWait = undefined, 
            PosBusyAfterWait = undefined, 
            TimeInWaiting = undefined,
            {ok, TimeStartBusy, PosBusy, TimeInWaiting, PosWaiting, TimeStartBusyAfterWait, PosBusyAfterWait};
        unknown_pool -> unknown_pool
end,
case ShouldContinue of
    {ok, TimeStartBusy, PosBusy, TimeInWaiting, PosWaiting, TimeStartBusyAfterWait, PosBusyAfterWait}  ->
         execute_process(BeforeGetConn, Url, Headers, Body, Method, PoolId, TimeStartBusy, PosBusy, TimeStartBusyAfterWait, PosBusyAfterWait, PosWaiting, TimeInWaiting);
    ok ->
        execute_process(BeforeGetConn, Url, Headers, Body, Method, PoolId, undefined, undefined, undefined, undefined, undefined, undefined);
    unknown_pool ->
         ...
end.

Answer 1

出现这种情况是因为你在函数的开头将TimeStartBusy和PosBusy设置为undefined，所以case子句{proceed, TimeStartBusy, PosBusy}只能匹配元组{proceed, undefined, undefined}。实际值为{proceed,{1614,588434,663546},1}，所以不匹配

与大多数其他语言不同，Erlang 变量是单一赋值的：一旦设置了变量的值，就无法更改它，即使在 case 子句中的模式匹配中也是如此。要执行您想要的操作，只需删除 case 表达式之前将变量设置为 undefined 的行。这样，变量就会在比赛期间分配。

返回 handle_call 值时在 case_clause 中抛出错误

Throw error in case_clause when returning handle_call value

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