如何在 Scala 的编译时验证类型存在

Question

I have the following traits and classes:

sealed trait Signal

sealed trait Description[T]

final case class S1(name: String) extends Signal

final case class D1(name: String) extends Description[S1]

What I try to achieve is that anyone who wants to add Signal will have (at compile time) to create a description.

I don't want to change the signature of Description but for sure not of Signal

I set my compiler to fail on warning, so I can leverage the fact that my ADT is sealed.

My idea was to have such a "compilation guard":

def compilationGuard[S <: Signal](s: S): Description[S] = s match { case S1(name) => D1(name) }

but I get the following error:

<console>:17: error: type mismatch;
 found   : D1
 required: Description[S]
       def compilationGuard[S <: Signal](s: S): Description[S] = s match { case S1(name) => D1(name) }
                                                                                              ^

Answer 1

def compilationGuard[S <: Signal](s: S): Description[S] = s match { case S1(name) => D1(name) }

can't compile for the same reason as

def returnItself[S <: Signal](s: S): S = s match { case S1(name) => S1(name) }

Reasons are explained here in details:

Why can't I return a concrete subtype of A if a generic subtype of A is declared as return parameter?

Type mismatch on abstract type used in pattern matching

If you don't want to mix Description logic to ADT or define instances of a type class like SignalMapper manually you can use Shapeless

import shapeless.ops.coproduct.Mapper
import shapeless.{:+:, CNil, Coproduct, Generic, Poly1}

def compilationGuard[C <: Coproduct]()(implicit
  gen: Generic.Aux[Signal, C],
  mapper: Mapper[uniqueDescriptionPoly.type, C]
) = null

object uniqueDescriptionPoly extends Poly1 {
  implicit def cse[S <: Signal, C1 <: Coproduct](implicit
    gen1: Generic.Aux[Description[S], C1],
    ev: C1 <:< (_ :+: CNil)
  ): Case.Aux[S, Null] = null
}

compilationGuard()

Testing:

final case class S1(name: String) extends Signal
final case class S2(name: String) extends Signal
final case class D1(name: String) extends Description[S1] 
// doesn't compile

final case class S1(name: String) extends Signal
final case class S2(name: String) extends Signal
final case class D1(name: String) extends Description[S1]
final case class D2(name: String) extends Description[S1]
// doesn't compile

final case class S1(name: String) extends Signal
final case class S2(name: String) extends Signal
final case class D1(name: String) extends Description[S1]
final case class D2(name: String) extends Description[S2]
// compiles

Answer 2

What I try to achieve is that anyone who wants to add Signal will have (at compile time to create a description.

The easy way to do so is to make it part of Signal :

sealed trait Signal[S <: Signal[S, D], D <: Description[S]] {
  // optionally
  def description: D
}

final case class S1(name: String) extends Signal[S1, D1] {
  def description = D1(name)
}

or

sealed trait Signal[S <: Signal[S]] {
  type Descr <: Description[S]
  // optionally
  def description: Descr
}

final case class S1(name: String) extends Signal[S1] {
  type Descr = D1
  def description = D1(name)
}

Of course it isn't far from the simpler

sealed trait Signal[S <: Signal] {
  def description: Description[S]
}

depending on your requirements.

Answer 3

Your program is failing, because it can't prove, that type S is S1 .

Instead of doing pattern matching, you could introduce typeclass which would do a mapping on compile time:

trait SignalMapper[S] { //typeclass handling of mapping S to D
  type D <: Description[S]

  def map(signal: S): D
}

//instance of typeclass SignalMapper for S1
//if you'd put it in a companion object of S1, it would be always in scope
object S1 { 

  implicit val mapperS1: SignalMapper[S1] = new SignalMapper[S1] {
    type D = D1

    def map(signal: S1) = D1(signal.name)
  }

}

Then you can rewrite compilationGuard as:

def compilationGuard[S <: Signal](s: S)(implicit mapper: SignalMapper[S]): Description[S] = mapper.map(s)

Scastie