Jam Jam - 4 months ago 10
Scala Question

Call by name vs call by value in Scala, clarification needed

As i understand it, in Scala, a function may be called either


  • by-value or

  • by-name



For example, given the following declarations, do we know how the function will be called?

Declaration:

def f (x:Int, y:Int) = x;


Call

f (1,2)
f (23+55,5)
f (12+3, 44*11)


What are the rules please?

dhg dhg
Answer

The example you have given only uses call-by-value, so I will give a new, simpler, example that shows the difference.

First, let's assume we have a function with a side-effect. This function prints something out and then returns an Int.

def something() = {
  println("calling something")
  1 // return value
}

Now we are going to define two function that accept Int arguments that are exactly the same except that one takes the argument in a call-by-value style (x: Int) and the other in a call-by-name style (x: => Int).

def callByValue(x: Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

def callByName(x: => Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

Now what happens when we call them with our side-effecting function?

scala> callByValue(something())
calling something
x1=1
x2=1

scala> callByName(something())
calling something
x1=1
calling something
x2=1

So you can see that in the call-by-value version, the side-effect of the passed-in function call (something()) only happened once. However, in the call-by-name version, the side-effect happened twice.

This is because call-by-value functions compute the passed-in expression's value before calling the function, thus the same value is accessed every time. However, call-by-name functions recompute the passed-in expression's value every time it is accessed.

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