How does | (pipe) in pattern matching work?

Question

You can write:

str match { case "foo" | "bar" => ... } 

At first glance it looks like | could be an extractor object, however:

str match { case |("foo", "bar") => ... } 

does not work. (And I can't see how that could be implemented anyway.)

So it is a magic built-in operator?

(I believe I have seen this question on SO before, but it's impossible to search for...)

Answer 1

| is not implemented in the library, it is interpreted by the Scala compiler. It builds a new pattern that is defined as the disjunction between two subpatterns that don't bind any variable (although the newly formed pattern can itself be bound; i.e., you can write stuff like

try { /*...*/ } catch {   case e @ (_: IOException | _: IllegalArgumentException) => /*...*/ } 

and e gets as type the most specific supertype of the listed alternatives).