| layout | title | partof | num | next-page | previous-page | redirect_from |
|---|---|---|---|---|---|---|
tour | By-name Parameters | scala-tour | 33 | annotations | operators | /tutorials/tour/automatic-closures.html |
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.
{% tabs by-name-parameters_1 %} {% tab 'Scala 2 and 3' for=by-name-parameters_1 %}
defcalculate(input: =>Int) = input *37{% endtab %} {% endtabs %}
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:
{% tabs by-name-parameters_2 class=tabs-scala-version %} {% tab 'Scala 2' for=by-name-parameters_2 %}
defwhileLoop(condition: =>Boolean)(body: =>Unit):Unit=if (condition) { body whileLoop(condition)(body) } vari=2 whileLoop (i >0) { println(i) i -=1 } // prints 2 1{% endtab %} {% tab 'Scala 3' for=by-name-parameters_2 %}
defwhileLoop(condition: =>Boolean)(body: =>Unit):Unit=if condition then body whileLoop(condition)(body) vari=2 whileLoop (i >0) { println(i) i -=1 } // prints 2 1{% endtab %} {% endtabs %}
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.
