Question

Consider the following Scala code.

val a = "both"

a match {
    case "both" | "foo" => println ("foo")   // case 1
    case "both" | "bar" => println ("bar")   // case 2
}

I would like match to work so that if a == "both", Scala will execute both cases. Is this possible or are there any alternatives to achieve what I want?

Answer 1

Standard pattern-matching will always match on only exactly one case. You can get close to what you want by using the fact that patterns can be treated as partial functions (see the Language Specification, Section 8.5, Pattern Matching Anonymous Functions) and by defining your own matching operator, though:

class MatchAll[S](scrutinee : =>S) {
  def matchAll[R](patterns : PartialFunction[S,R]*) : Seq[R] = {
    val evald : S = scrutinee
    patterns.flatMap(_.lift(evald))
  }
}

implicit def anyToMatchAll[S](scrut : =>S) : MatchAll[S] = new MatchAll[S](scrut)

def testAll(x : Int) : Seq[String] = x matchAll (
  { case 2 => "two" },
  { case x if x % 2 == 0 => "even" },
  { case x if x % 2 == 1 => "neither" }
)

println(testAll(42).mkString(",")) // prints 'even'
println(testAll(2).mkString(","))  // prints 'two,even'
println(testAll(1).mkString(","))  // prints 'neither'

The syntax is slightly off the usual, but to me such a construction is still a witness to the power of Scala.

Your example is now written as:

// prints both 'foo' and 'bar'
"both" matchAll (
  { case "both" | "foo" => println("foo") },
  { case "both" | "bar" => println("bar") }
)

(Edit huynhjl pointed out that he gave a frighteningly similar answer to this question.)