Scala Language Pattern Matching on a Seq


Example

To check for a precise number of elements in the collection

def f(ints: Seq[Int]): String = ints match {
  case Seq() =>
      "The Seq is empty !"
  case Seq(first) =>
      s"The seq has exactly one element : $first"
  case Seq(first, second) =>
      s"The seq has exactly two elements : $first, $second"
  case  s @ Seq(_, _, _) => 
      s"s is a Seq of length three and looks like ${s}"  // Note individual elements are not bound to their own names.
  case s: Seq[Int] if s.length == 4 =>
      s"s is a Seq of Ints of exactly length 4"  // Again, individual elements are not bound to their own names.
  case _ =>
      "No match was found!"
}

Live demo

To extract the first(s) element(s) and keeping the rest as a collection:

def f(ints: Seq[Int]): String = ints match {
  case Seq(first, second, tail @ _*) =>
      s"The seq has at least two elements : $first, $second. The rest of the Seq is $tail"
  case Seq(first, tail @ _*) =>
      s"The seq has at least one element : $first. The rest of the Seq is $tail"
  // alternative syntax
  // here of course this one will never match since it checks
  // for the same thing as the one above
  case first +: tail =>
      s"The seq has at least one element : $first. The rest of the Seq is $tail"
  case _ =>
      "The seq didn't match any of the above, so it must be empty"
}

In general, any form that can be used to construct a sequence can be used to pattern match against an existing sequence.

Note that while using Nil and :: will work when pattern matching a Sequence, it does convert it to a List, and can have unexpected results. Constrain yourself to Seq( ...) and +: to avoid this.

Note that while using :: will not work for WrappedArray, Vector etc, see:

scala> def f(ints:Seq[Int]) = ints match {
     | case h :: t => h
     | case _ => "No match"
     | }
f: (ints: Seq[Int])Any

scala> f(Array(1,2))
res0: Any = No match

And with +:

scala> def g(ints:Seq[Int]) = ints match {
     | case h+:t => h
     | case _ => "No match"
     | }
g: (ints: Seq[Int])Any

scala> g(Array(1,2).toSeq)
res4: Any = 1