Scala *-parameter

Let’s see how we can pass an array as argument smoothly.

We are going to see an example when this syntax shine the most: nested Node creation.

Do you experience the pain of writing Node(List(Node(List(.... Not pleasant huh ?!

But never again you will have to do this now that you know about * !

Let’s see how it works:

In a function, you can have TYPE* to take all the last parameters given to the function and turn it into an array. If you place the argument of the method with TYPE* not as last argument you will get an exception *-parameter must come last.

After doing this, instead of doing foo(List(a,b,c)) you are able to do foo(a,b,c). And inside the body of the method you can use the parameter the same as before.

But now, let’s say in some part of your code you do have a List already but you have changed the method, you want to convert the List into a *-parameter.

The weird syntax to do this is: foo(list:_*). It means you are casting (changing the type) , with : into _ which is a wildcard, so anything, in this case, the anything being the type A of the list which should be the same as the input type of the method A*, and finally *.

If we look at the type only we are doing this:

list      : _*
List[A] : A*

If I didn’t explain well, please feel free to reach out on the Discord server so we can improve the content of this article together.

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