Erlang gen_server 投射错误的返回值
我尝试将消息投射到 gen_server：
gen_server:cast({global, ID}, {watchers}).
处理程序是：
handle_cast({watchers}, State) ->
case State#table_state.watchers of
[] ->
{reply, no_watchers, State};
_ ->
{reply, State#table_state.watchers, State}
end;
但是当我执行 gen_server:cast 时，gen_server 终止并出现错误：
=ERROR REPORT==== 29-Apr-2011::18:26:07 ===
** Generic server 1 terminating
** Last message in was {'$gen_cast',{watchers}}
** When Server state == {table_state,1,"1",11,[]}
** Reason for termination ==
** {bad_return_value,{reply, no_watchers, {table_state,3,"3",11,[]}}}
为什么我会得到 bad_return_value？
I try to cast message to a gen_server:
gen_server:cast({global, ID}, {watchers}).
The handler is:
handle_cast({watchers}, State) ->
case State#table_state.watchers of
[] ->
{reply, no_watchers, State};
_ ->
{reply, State#table_state.watchers, State}
end;
But when I execute gen_server:cast the gen_server terminates with error:
=ERROR REPORT==== 29-Apr-2011::18:26:07 ===
** Generic server 1 terminating
** Last message in was {'$gen_cast',{watchers}}
** When Server state == {table_state,1,"1",11,[]}
** Reason for termination ==
** {bad_return_value,{reply, no_watchers, {table_state,3,"3",11,[]}}}
Why do I get bad_return_value?
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您无法使用强制类型转换进行回复（请参阅
gen_server文档）。这就是转换异步消息而不是使用调用的全部要点。在您的情况下，您想返回回复，因此请使用
gen_server:call/2。You cannot reply using cast (see
gen_serverdocumentation). That is the whole point of casting an asynchronous message instead of using call.In your case you want to return a reply, so use
gen_server:call/2instead.