Scala pattern matching: How to match on an element inside a list?

Question

Is it possible to rewrite the following code using Scala pattern matching?

val ls: List[String] = ??? // some list of strings

val res = if (ls.contains("foo")) FOO
     else if (ls.contains("bar")) BAR
     else SOMETHING_ELSE

Answer 1

You can add if conditions to matches like this:

ls match {
  case x if x.contains("foo") => // FOO
  case x if x.contains("bar") => // BAR
  case _ => // ELSE
}

However, it is not the nicest way, as each if check needs to traverse the list, so this doesn't scale well. There are various different ways to deal with this problem, but we would need to know more about your intensions, as normally the runtime semantics would differ from your code (for example, you could recursively traverse the list looking for either "foo" or "bar", but that would assume you only have either one in the list).

Answer 2

You could implement this using a function like

def onContains[T](xs: Seq[String], actionMappings: (String, T)*): Option[T] = {
  actionMappings collectFirst {
    case (str, v) if xs contains str => v
  }
}

And use it like this:

val x = onContains(items,
  "foo" -> FOO,
  "bar" -> BAR
)