Pattern matching with more than one match

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?

possible duplicate of [Match "fallthrough": executing same piece of code for more than one case?](http://stackoverflow.com/questions/2325863/match-fallthrough-executing-same-piece-of-code-for-more-than-one-case) — nawfal, May 18 '13 at 11:02

score 25 · Accepted Answer · edited May 23 '17 at 12:25

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.)

This reminds me of http://stackoverflow.com/questions/6720205/idiomatic-way-to-convert-a-seqb/6720659#6720659. What's the benefit of the by name `=> S`? — huynhjl, Sep 27 '11 at 13:35
Wow. I had no idea this was there. Thanks for adding the link. I use the by name argument for the (unlikely) case someone would construct the MatchAll instance before using it, so that the potential effects resulting from the evaluation of the scrutinee appear in the right place. — Philippe, Sep 27 '11 at 13:46

Luigi Plinge · Answer 2 · 2011-09-27T13:55:05.620

At risk of being Captain Obvious, in a case like this it would be simplest just to forget pattern matching and use if.

if (a == "both" || a == "foo") println("foo")
if (a == "both" || a == "bar") println("bar")

If the repetition of a == worries you, you could instead write

if (Set("both", "foo")(a)) println("foo")
if (Set("both", "bar")(a)) println("bar")

using the fact that the apply method on Set does the same as contains, and is a bit shorter.

score 3 · Answer 3 · edited Nov 01 '13 at 20:04

3

match executes one, and only one, of the cases, so you can't do this as an or in the match. You can, however, use a list and map/foreach:

val a = "both"
(a match {
  case "both" => List("foo", "bar")
  case x => List(x)
}) foreach(_ match {
  case "foo" => println("foo")
  case "bar" => println("bar")
})

And you're not duplicating any of the important code (in this case the printlns).

edited Nov 01 '13 at 20:04

Peter Mortensen

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answered Sep 27 '11 at 08:08

Matthew Farwell

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score 1 · Answer 4 · edited Nov 01 '13 at 20:02

1

Just match twice:

val a = "both"

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

edited Nov 01 '13 at 20:02

Peter Mortensen

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answered Jul 30 '12 at 15:08

Nikita Volkov

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score 0 · Answer 5 · edited Nov 01 '13 at 20:04

0

One possible way could be:

val a = "both"

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

edited Nov 01 '13 at 20:04

Peter Mortensen

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answered Sep 27 '11 at 07:42

Jamil

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Pattern matching with more than one match

5 Answers5