What does `var @ _*` signify in Scala

Question

I'm reviewing some Scala code trying to learn the language. Ran into a piece that looks like the following:

 case x if x startsWith "+" => 
          val s: Seq[Char] = x
          s match {
            case Seq('+', rest @ _*) => r.subscribe(rest.toString){ m => }
          }

In this case, what exactly is rest @ _* doing? I understand this is a pattern match for a Sequence, but I'm not exactly understanding what that second parameter in the Sequence is supposed to do.

Was asked for more context so I added the code block I found this in.

Sorry, I guess I was wrong - additional context was not necessary :) — Chris Martin, Oct 04 '13 at 04:02
See also [value binding](http://stackoverflow.com/a/2359365/298389). You case is actually composition of *value binding* with *varargs syntax sugar*. — om-nom-nom, Oct 04 '13 at 08:12

Shadowlands · Accepted Answer · 2013-10-04T02:44:31.393

5

If you have come across _* before in the form of applying a Seq as varargs to some method/constructor, eg:

val myList = List(args: _*)

then this is the "unapply" (more specifically, search for "unapplySeq") version of this: take the sequence and convert back to a "varargs", then assign the result to rest.

edited Oct 04 '13 at 02:44

answered Oct 04 '13 at 01:55

Shadowlands

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score 2 · Answer 2 · answered Oct 04 '13 at 01:45

2

x @ p matches the pattern p and binds the result of the whole match to x. This pattern matches a Seq containing '+' followed by any number (*) of unnamed elements (_) and binds rest to a Seq of those elements.

answered Oct 04 '13 at 01:45

Jon Purdy

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Is `*` the name of an extractor, or is it syntactic? – Owen Oct 04 '13 at 01:48
What would be great also is some terminology I can google to see more examples/explanations. I tried searching 'pattern matching', but it doesn't go into extractors or anything similar. – randombits Oct 04 '13 at 02:12
1

@randombits I tried googling for "scala pattern match unapplySeq" and came up with [this link](http://daily-scala.blogspot.com.au/2009/09/extract-sequences-unapplyseq.html) – Shadowlands Oct 04 '13 at 02:43
@Owen, it is syntactic feature related to varargs. – Vladimir Matveev Oct 04 '13 at 05:33

What does `var @ _*` signify in Scala

2 Answers2