PartialFunction and MatchError

Question

There are two ways to define PF: 1) with literal case {} syntax and 2) as explicit class. I need the following function throw a MatchError, but in the second case that doesn't happen.

1) with case

val test: PartialFunction[Int, String] =  {
  case x if x > 100 => x.toString
}

2) as class

val test = new PartialFunction[Int, String] {
  def isDefinedAt(x: Int) = x > 100
  def apply(x: Int) = x.toString
}

Should i, in the seconds case, manually call isDefinedAt, shouldn't it be called implicitly by the compiler?

Answer 1

You will have to call isDefinedAt manually in your apply method:

val test = new PartialFunction[Int, String] {
  def isDefinedAt(x: Int) = x > 100
  def apply(x: Int) = if(isDefinedAt(x)) x.toString else throw new MatchError(x)
}

If you want to avoid this code, you can simply use the first way to define your partial function. It is syntactic sugar and will result in a valid definition of both isDefinedAt and apply. As described in the Scala language specification, your first definition will expand to the following:

val test = new scala.PartialFunction[Int, String] {
  def apply(x: Int): String = x match {
    case x if x > 100 => x.toString
  }
  def isDefinedAt(x: Int): Boolean = {
    case case x if x > 100 => true
    case _ => false
  }
}