scala - Syntax sugar: _* for treating Seq as method parameters

Question

I just noticed this construct somewhere on web:

val list = List(someCollection: _*)

What does _* mean? Is this a syntax sugar for some method call? What constraints should my custom class satisfy so that it can take advantage of this syntax sugar?

Answer 1

Generally, the : notation is used for type ascription, forcing the compiler to see a value as some particular type. This is not quite the same as casting.

val b = 1 : Byte
val f = 1 : Float
val d = 1 : Double

In this case, you're ascribing the special varargs type. This mirrors the asterisk notation used for declaring a varargs parameter and can be used on a variable of any type that subclasses Seq[T]:

def f(args: String*) = ... //varargs parameter, use as an Array[String]
val list = List("a", "b", "c")
f(list : _*)