在Scala中引发异常是否被视为副作用？

Question

Having the code below:

val prices = cars map (car => {
  val price = car.getPrice
  Logger.info(s"Car price is $price")
  price
})

Is using parentheses fine in the above case or there is a strict imperial to use curly braces like below:

val prices = cars map { car => {
  val price = car.getPrice
  Logger.info(s"Car price is $price")
  price
}}

I do not like two curly braces and prefer ({ ... }) style, but was warned by another developer that the inside function is side-effecting (logging message) and is 3 lines of code so it must be used with two curly braces and that the whole Scala community uses double curly braces in these cases.

I googled scala code and found this one :

def failed: Future[Throwable] =
  transform({
    case Failure(t) => Success(t)
    case Success(v) => Failure(new NoSuchElementException("Future.failed not completed with a throwable."))
  })(internalExecutor)

It has multiple lines but no side effects and my nice notation ({...}) is used here. Should it be changed when the code contains side-effects? Personally I do not like that even if the whole community says "Yes, use double curly braces!" :)

Answer 1

In your particular case I would actually only use curly braces and no parens:

val prices = cars map { car =>
  val price = car.getPrice
  Logger.info(s"Car price is $price")
  price
}

The example you posted from the scala code is only using parentheses out of necessity, since transform is actually a function with multiple argument lists (ie transform(args1)(arg2) ).

I have not heard of any convention relating side-effecting code to a particular choice of block delimiter ( () vs {} ), but I do find I prefer the look of {} when there are multiple lines in that block.

Answer 2

I have never heard about that you should curly braces when there is a side effect in a block of code. Use () for a single statement, and {} for multiple statements. What you call nice notation ({...}) is perfectly fine.