Dotty Documentation

0.9.0-bin-SNAPSHOT

Vararg Patterns

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The syntax of vararg patterns has changed. In the new syntax one writes varargs in patterns exactly like one writes them in expressions, using a : _* type annotation:

xs match {
  case List(1, 2, xs: _*) => println(xs)    // binds xs
  case List(1, _ : _*) =>                   // wildcard pattern
}

The old syntax, which is shorter but less regular, is no longer supported:

/*!*/ case List(1, 2, xs @ _*)       // syntax error
/*!*/ case List(1, 2, _*) => ...     // syntax error

Another change relates to extractors producing values with T* types, which can then be matched by vararg patterns. Previously, such extractors used an unapplySeq whereas now they use an unapply (in the long term, we plan to get rid of unapplySeq altogether, replacing all its usages with unapply).

Example: Previously, this was correct:

class Person(val name: String, val children: Person *)
object Person {
  def unapplySeq(p: Person) = Some((p.name, p.children))
}

def childCount(p: Person) = p match {
  case Person(_, ns @ _*) => ns.length
}

Now, the equivalent program is written as follows:

class Person(val name: String, val children: Person *)
object Person {
  def unapply(p: Person) = Some((p.name, p.children))
}

def childCount(p: Person) = p match {
  case Person(_, ns : _*) => ns.length
}