pattern matching - What does `:_*` (colon underscore star) do in Scala?

Question

I have the following piece of code from this question:

def addChild(n: Node, newChild: Node) = n match {
  case Elem(prefix, label, attribs, scope, child @ _*) => Elem(prefix, label, attribs, scope, child ++ newChild : _*)
  case _ => error("Can only add children to elements!")
}

Everything in it is pretty clear, except this piece: child ++ newChild : _*

What does it do?

I understand there is Seq[Node] concatenated with another Node, and then? What does : _* do?

Answer 1

It "splats"¹ the sequence.

Look at the constructor signature

new Elem(prefix: String, label: String, attributes: MetaData, scope: NamespaceBinding,
         child: Node*)

which is called as

new Elem(prefix, label, attributes, scope,
         child1, child2, ... childN)

but here there is only a sequence, not child1, child2, etc. so this allows the result sequence to be used as the input to the constructor.

Happy coding.

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¹ This doesn't have a cutesy-name in the SLS, but here are the details. The important thing to get is that it changes how Scala binds the arguments to the method with repeated parameters (as denoted with Node* above).

The _* type annotation is covered in "4.6.2 Repeated Parameters" of the SLS.

The last value parameter of a parameter section may be suf?xed by “*”, e.g. (..., x:T *). The type of such a repeated parameter inside the method is then the sequence type scala.Seq[T]. Methods with repeated parameters T * take a variable number of arguments of type T . That is, if a method m with type (p1 : T1, . . . , pn : Tn,ps : S*)U is applied to arguments (e1, . . . , ek) where k >= n, then m is taken in that application to have type (p1 : T1, . . . , pn : Tn,ps : S, . . . , ps0S)U, with k ? n occurrences of type S where any parameter names beyond ps are fresh. The only exception to this rule is if the last argument is marked to be a sequence argument via a _* type annotation. If m above is applied to arguments (e1, . . . , en,e0 : _*), then the type of m in that application is taken to be (p1 : T1, . . . , pn : Tn,ps :scala.Seq[S])