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

Yes the pipe (|) is a built-in for pattern matching (see the scala language reference). The Pattern matching section (section 8) defines in section 8.1.11 what is called Pattern Alternatives. The definition says:

A pattern alternative p1 | … | pn
consists of a number of alternative
patterns pi . All alternative patterns
are type checked with the expected
type of the pattern. They may no bind
variables other than wildcards. The
alternative pattern matches a value v
if at least one its alternatives
matches v.

So yes, the pipe is a built-in that is context sensitive to pattern matching.