gistfile1.scala

// scala @ syntax in patternmatching implemented as a library 

object `@` {
  def unapply[A](a:A) = Some(a, a)
}

// usage

val Nums = """(\d),(\d)""".r

"1,2" match {
  case a `@` Nums(first, second `@` "2") => println(a + "-"+first + "-" + second)
}

// prints

1,2-1-2


// it's actually more general

"1,2" match {
  case Nums(a, b) `@` Nums(c, d) => println(List(a,b,c,d))
}

//prints

List(1, 2, 1, 2)

// -----

"1,2" match {
  case Nums(a, b) @ Nums(c, d) => println(List(a,b,c,d))
}

error: '=>' expected but '@' found.

@retronym thanks for the links.. Love that hand rolled matcher!

Does anyone know why @ is part of the patternmatching syntax, when it can be implemented as a library so easily ?

Actually case x => is syntactic sugar for case x @ _ =>. I showed Martin Odersky my &&[A, B] a couple weeks ago and he was a little surprised to see that this could be implemented as a library! So Scala can really suprise all of us, usually in good ways :)