When to use call-by-name and call-by-value?

Question

I understand the basic concept of call-by-name and call-by-value, and I have also looked into handful number of examples. However, I am not very clear about when to use call-by-

Answer 1

There are plenty of places were call-by-name may gain performance or even correctness.

Simple performance example: logging. Imagine an interface like this:

trait Logger {
  def info(msg: => String)
  def warn(msg: => String)
  def error(msg: => String)
}

And then used like this:

logger.info("Time spent on X: " + computeTimeSpent)

If the info method doesn't do anything (because, say, the logging level was configured for higher than that), then computeTimeSpent never gets called, saving time. This happens a lot with loggers, where one often sees string manipulation which can be expensive relative to the tasks being logged.

Correctness example: logic operators.

You have probably seen code like this:

if (ref != null && ref.isSomething)

Say you declared && method like this:

trait Boolean {
  def &&(other: Boolean): Boolean
}

then, whenever ref is null, you'll get an error because isSomething will be called on a null reference before being passed to &&. For this reason, the actual declaration is:

trait Boolean {
  def &&(other: => Boolean): Boolean =
    if (this) other else this
}

So one may actually wonder is when to use call-by-value. In fact, in the Haskell programming language everything works similar to how call-by-name works (similar, but not the same).

There are good reasons not to use call-by-name: it is slower, it creates more classes (meaning the program takes longer to load), it consumes more memory, and it is different enough that many have difficult reasoning about it.