erlang - 如果它不为零，则打印一个值

Question

如果它高于零，我需要打印出一个值：

info({mail, BoxPid, Messages, MessageCount, MessageDropCount}, Req, S) ->
    if MessageDropCount > 0 ->
            io:format("dropped=~p~n", [MessageDropCount]);
       true -> true
    end,
    ...,
    {loop, Req, S};

我不喜欢表达的true -> true部分if。有没有办法避免它？有成语吗？

Answer 1

The If Then page at the ErlangCentral wiki suggests these patterns:

• [io:format("dropped=~p~n", [MessageDropCount]) || MessageDropCount > 0]
• MessageDropCount > 0 andalso io:format("dropped=~p~n", [MessageDropCount])
• MessageDropCount =< 0 orelse io:format("dropped=~p~n", [MessageDropCount])

Answer 2

您可以使用具有模式匹配的函数：

info({mail, BoxPid, Messages, MessageCount, MessageDropCount}, Req, S) ->
    printIfNotZero(MessageDropCount,"dropped=~p~n"),
    ...,
    {loop, Req, S};

...
printIfNotZero(0,_) -> ok;
printIfNotZero(C,M) -> io:format(M, [C]).

编辑：更通用的功能可能如下：

info({mail, BoxPid, Messages, MessageCount, MessageDropCount}, Req, S) ->
    printCond(MessageDropCount =/= 0,"dropped=~p~n",[MessageDropCount]),
    ...,
    {loop, Req, S};

...
printCond(false,_,_) -> ok;
printCond(true,Format,Arglist) -> io:format(Format,Arglist).