By-name Parameters

By-name parameters are evaluated every time they are used. They won’t be evaluated at all if they are unused. This is similar to replacing the by-name parameters with the passed expressions. They are in contrast to by-value parameters. To make a parameter called by-name, simply prepend => to its type.

defcalculate(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:

defwhileLoop(condition:=>Boolean)(body:=>Unit):Unit=if(condition){bodywhileLoop(condition)(body)}vari=2whileLoop(i>0){println(i)i-=1}// prints 2 1

defwhileLoop(condition:=>Boolean)(body:=>Unit):Unit=ifconditionthenbodywhileLoop(condition)(body)vari=2whileLoop(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.