``` fun eval(expr: Expr): Double = when(expr) { ...

# language-proposals

[orangy]

fun eval(expr: Expr): Double = when(expr) {
  is Expr.Const -> expr.number
  is Expr.Sum -> eval(expr.e1) + eval(expr.e2)
  is NotANumber -> Double.NaN
}

This will work, and if you add Product that will cause compile-time error