This post is not to say that Scala is a bad programming language it is just my opinion about how sometimes the pattern matching syntax can be ambiguous. I promise to keep it short.

Let’s take an example:

val data = List("First", "Second", "Third", "Fourth")

  def check(data: List[String]) = data match {
    case List(_, second, _)=> println(s"$second in a list of 3")
    case List(_, second, _*) => println(s"$second in a list of 4 or more")
    case _ => println("Unknown")
  }

  check(data)         //Point: 1
  check(data.take(3)) //Point: 2
  check(data.take(2)) //Point: 3
  check(data.take(1)) //Point: 4

/*
 * Output:
 * Second in a list of 4 or more
 * Second in a list of 3
 * Second in a list of 4 or more
 * Unknown
*/

Based on whatever scarce documentation is around the language you would assume that the statement tagged above with Point: 3 would match against the default case i.e it would output “unknown” but actually it matches against the second case i.e its output is “Second in a list of 4 or more”.

Why would you think that it should match against the default case?

Because as human beings we (or at least I) assume that the pattern matching is saying that:

1. If it is list of 3 (see the deconstruction syntax) take the second element and interpolate it with a string and print the resulting string.
2. If it is a list of 3 or more (see the star in the deconstruction) take the second element and interpolate it with another string and print the resulting string.
3. If it is anything else print “unknown”

Whereas semantically in Scala terms it translates to:

1. If it is list of exactly 3 (see the deconstruction syntax) take the second element and interpolate it with a string and print the resulting string.
2. If it is list of with at least 2 items and it is not a list with exactly 3 items (see the deconstruction syntax) take the second element and interpolate it with a string and print the resulting string.
3. If it is anything else print “unknown”

Definition of ambiguity is (from the almighty Google):