scala.MatchError: <SomeStringvalue> (of class java.lang.String)

Question

I was getting the error:

Exception in thread "main" scala.MatchError: SomeStringValue (of class java.lang.String)

I found that it was causing because of "SomeStringValue" was not present in any of the cases:

val test = "SomeStringValue"
test match {
  case "Name" => println("Name")
  case "Age"  => println("Age")
  case "Sex"  => println("Sex")
}

When I added the else case: _ it ran correctly as below.

val test = "SomeStringValue"
test match {
  case "Name" => println("Name")
  case "Age"  => println("Age")
  case "Sex"  => println("Sex")
  case _      => println("Nothing Matched!!")
}

Question: What is the reason that there should always be a matching value in the case construct in Scala?

Answer 1

The match construct is an expression in itself.

Suppose, instead of having println statements, you had integers, then the whole block would be a value of type Integer:

val test = "SomeStringValue"
val count: Int = test match {
  case "Name" => 1
  case "Age"  => 2
  case "Sex"  => 3
}

Now, what value should count be ? That's why the match statement must handle all possible cases.

In some cases (such as when pattern matching against a sealed trait or sealed abstract class), the compiler will be able to give you a warning, but most of the time, the error will be thrown at runtime, so you really need to be careful about it.