Example of compiler discovering and warning missing cases in a simple calculator. sealed abstract class Expression is required for the warning message to appear. Sealed will require all direct subclasses (but not subclasses of subclasses) to be defined within the same file.

Calculator.scala

sealed abstract class Expression
case class Add(val exp1:Expression, val exp2:Expression) extends Expression
case class Subtract(val exp1:Expression, val exp2:Expression) extends Expression
case class Multiply(val exp1:Expression, val exp2:Expression) extends Expression
case class Divide(val exp1:Expression, val exp2:Expression) extends Expression
case class Value(value:Double) extends Expression


object Calculator {
  def calculate(expression:Expression):Double = {
    expression match {
      case Value(value) => value
      case Add(exp1, exp2) => calculate(exp1) + calculate(exp2)
      case Subtract(exp1, exp2) => calculate(exp1) - calculate(exp2)
      case Multiply(exp1, exp2) => calculate(exp1) * calculate(exp2)
      // case Divide(exp1, exp2) => calculate(exp1) / calculate(exp2)
    }
  }
}

object ScalaWithStyle {
  def main(args:Array[String]) {
    val result = Calculator.calculate(
        Multiply(
          Add(Value(1), Value(2)),
	        Add(Value(3), Value(4))))
    println(result) // result is 21.0
  }
}

Warning:
Description  Resource	Path	Location	Type
match may not be exhaustive. It would fail on the following input: Divide(_, _)	ScalaWithStyle.scala	/ScalaWithStyle/src	line 11	Scala Problem