How does the Scala toSeq:_* syntax work?

Question

In the Scala code

val s = Set(List(1,2,3).toSeq:_*)

how does the toSeq:_* syntax work? I know what the code does, I know what toSeq does, I understand List(1,2,3).toSeq:Seq[Int]. Is toSeq:_* a special case?

Answer 1

The toSeq isn't accomplishing anything here, and should be omitted. The following is equivalent:

Set(List(1, 2, 3): _*)

(Or even better, just write List(1, 2, 3).toSet.)

Since you say you know what the code does, I'll assume you understand the _* type annotation that marks the list as a sequence argument (if not, see section 4.6.2 of the language specification). This will work on any Seq, including List, so converting the list explicitly with toSeq is just extra clutter.