impressive! I need to learn more about scala.
What I mean stripping away conditional check in Java is this.
static final boolean isLogInfoEnabled = false;
public void logMessage(String message) {
if(isLogInfoEnabled) {
log.info(message)
}
}
If you look at the byte code the dead if check will be removed.
On Sun, Jun 25, 2017 at 12:46 PM, Sean Owen <so...@cloudera.com> wrote:
> I think it's more precise to say args like any expression are evaluated
> when their value is required. It's just that this special syntax causes
> extra code to be generated that makes it effectively a function passed, not
> value, and one that's lazily evaluated. Look at the bytecode if you're
> curious.
>
> An if conditional is pretty trivial to evaluate here. I don't think that
> guidance is sound. The point is that it's not worth complicating the caller
> code in almost all cases by checking the guard condition manually.
>
> I'm not sure what you're referring to, but no, no compiler can elide these
> conditions. They're based on runtime values that can change at runtime.
>
> scala has an @elidable annotation which you can use to indicate to the
> compiler that the declaration should be entirely omitted when configured to
> elide above a certain detail level. This is how scalac elides assertions if
> you ask it to and you can do it to your own code. But this is something
> different, not what's happening here, and a fairly niche/advanced feature.
>
>
> On Sun, Jun 25, 2017 at 8:25 PM kant kodali <kanth...@gmail.com> wrote:
>
>> @Sean Got it! I come from Java world so I guess I was wrong in assuming
>> that arguments are evaluated during the method invocation time. How about
>> the conditional checks to see if the log is InfoEnabled or DebugEnabled?
>> For Example,
>>
>> if (log.isInfoEnabled) log.info(msg)
>>
>> I hear we should use guard condition only when the string "msg"
>> construction is expensive otherwise we will be taking a performance hit
>> because of the additional "if" check unless the "log" itself is declared
>> static final and scala compiler will strip away the "if" check and produce
>> efficient byte code. Also log.info does the log.isInfoEnabled check
>> inside the body prior to logging the messages.
>>
>> https://github.com/qos-ch/slf4j/blob/master/slf4j-
>> simple/src/main/java/org/slf4j/simple/SimpleLogger.java#L509
>> https://github.com/qos-ch/slf4j/blob/master/slf4j-
>> simple/src/main/java/org/slf4j/simple/SimpleLogger.java#L599
>>
>> Please correct me if I am wrong.
>>
>>
>>
>>
>> On Sun, Jun 25, 2017 at 3:04 AM, Sean Owen <so...@cloudera.com> wrote:
>>
>>> Maybe you are looking for declarations like this. "=> String" means the
>>> arg isn't evaluated until it's used, which is just what you want with log
>>> statements. The message isn't constructed unless it will be logged.
>>>
>>> protected def logInfo(msg: => String) {
>>>
>>>
>>> On Sun, Jun 25, 2017 at 10:28 AM kant kodali <kanth...@gmail.com> wrote:
>>>
>>>> Hi All,
>>>>
>>>> I came across this file https://github.com/
>>>> apache/spark/blob/master/core/src/main/scala/org/apache/
>>>> spark/internal/Logging.scala and I am wondering what is the purpose of
>>>> this? Especially it doesn't prevent any string concatenation and also the
>>>> if checks are already done by the library itself right?
>>>>
>>>> Thanks!
>>>>
>>>>
>>
