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! >>> >>> >
