Compile time check on some property

Question

Given the following scala code:

sealed trait Color
case object Red extends Color
case object Blue extends Color

sealed trait Car {
  def isBroken: Boolean
          


        

Answer 1

I am assuming that you can change your class structure.

One way to achieve what you want is using Type Level Programming. There is a pretty good stack overflow post on this here: Scala type programming resources

Here is some sample code based on your original code which demonstrates how this can be achieved using the type system within Scala.

/* 
 * Color traits I've left the same except converted the objects 
 * to classes. This would work just as well with traits. It might 
 * even be better if the Colors such as Red and Blue are traits
 * themselves that extend Color, I'm still just learning this 
 * method myself.
 */
sealed trait Color
class Red extends Color
class Blue extends Color

/* New trait to represent whether something is broken or not */
sealed trait IsBroken    
class Broken extends IsBroken
class NotBroken extends IsBroken

/* Change Car trait to have two type parameters, broken and color */   
trait Car[T <: Color, S <: IsBroken]

/* fixBrokenCar signature includes whether a car is broken and it's color */
def fixBrokenRedCar(c: Car[Red, Broken]): Car[Red, NotBroken]
    = new Car[Red, NotBroken]{}

val brokenRedCar = new Car[Red, Broken]{}
val fixedRedCar = new Car[Red, NotBroken]{}
val brokenBlueCar = new Car[Blue, Broken]{}

/* Compiles */
fixBrokenRedCar(brokenRedCar)

/* Doesn't compile */
fixBrokenRedCar(fixedRedCar)

/* Doesn't compile */
fixBrokenRedCar(brokenBlueCar)