by-name-parameters.md

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By-name parameters are only evaluated when used. They are in contrast to by-value parameters. To make a parameter called by-name, simply prepend => to its type.

def calculate(input: => Int) = input * 37

By-name parameters have the advantage that they are not evaluated if they aren't used in the function body. On the other hand, by-value parameters have the advantage that they are evaluated only once.

Here's an example of how we could implement a while loop:

def whileLoop(condition: => Boolean)(body: => Unit): Unit =
  if (condition) {
    body
    whileLoop(condition)(body)
  }

var i = 2

whileLoop (i > 0) {
  println(i)
  i -= 1
}  // prints 2 1

The method whileLoop uses multiple parameter lists to take a condition and a body of the loop. If the condition is true, the body is executed and then a recursive call to whileLoop is made. If the condition is false, the body is never evaluated because we prepended => to the type of body.

Now when we pass i > 0 as our condition and println(i); i-= 1 as the body, it behaves like the standard while loop in many languages.

This ability to delay evaluation of a parameter until it is used can help performance if the parameter is computationally intensive to evaluate or a longer-running block of code such as fetching a URL.