How to overcome Scala 3 missing type projection?

In Scala 2 I used type projection in an simple ORM model which contains of IdEntity and some Repositories. Now I tried to migrate to Scala 3 but stuck since type projection with # is missing.

Simplified code looks like:

trait IdEntity:
  type ID_Type
  val id: ID_Type

trait IdLongEntity extends IdEntity:
  override type ID_Type = Long

trait IdStringEntity extends IdEntity:
  override type ID_Type = String

trait IdRepository[E <: IdEnity]:
  def findById(id: E#ID_Type): Option[E]:

Any ideas how to overcome the missing type projection in def findById(id: E#ID_Type)?

I already tried with type match like

object IdEntity:
  type Aux[K] = IdEntity {type ID_Type = K}

type IdType[T <: IdEntity] = T match
    case IdEntity.Aux[k] => k

but this does not work since the compile thinks that the types does not match.

Update:

Thanks to the answer I tried type classes as well as match type but both fail as soon as I try the following (https://scastie.scala-lang.org/UtW9u0mbS5ydiZmQNNWV6g):

trait IdEntity:
  type ID_Type
  val idDefault: ID_Type
  val id: Option[ID_Type]
  def idOrDefault: ID_Type = id.getOrElse(idDefault)

trait IdRepository[E <: IdEntity](using val e: E):
  type ID_Type = e.ID_Type  
  def delete(id: ID_Type): Either[String, Int] = deleteImpl(id)
  def delete(entity: E): Either[String, Int] = delete(entity.idOrDefault) // here the type error occures
  protected def deleteImpl(id: ID_Type): Either[String, Int]

Solution

Indeed, one of the ways is match types:

trait IdEntity:
  type ID_Type
  val id: ID_Type
object IdEntity:
  type Aux[K] = IdEntity {type ID_Type = K}

trait IdLongEntity extends IdEntity:
  override type ID_Type = Long

trait IdStringEntity extends IdEntity:
  override type ID_Type = String

// match type
type IdType[T <: IdEntity] = T match
  case IdEntity.Aux[k] => k

trait IdRepository[E <: IdEntity]:
  def findById(id: IdType[E]): Option[E] = ???

https://scastie.scala-lang.org/QoV6Z7xRTOa6hVhAYOb0uw

this does not work since the compile thinks that the types does not match

You should prepare reproduction and we'll see whether compilation can be fixed.

The other way is type classes:

trait IdEntity:
  type ID_Type
  val id: ID_Type
object IdEntity:
  type Aux[K] = IdEntity {type ID_Type = K}

trait IdLongEntity extends IdEntity:
  override type ID_Type = Long

trait IdStringEntity extends IdEntity:
  override type ID_Type = String

// type class
trait IdType[T <: IdEntity]:
  type Out
object IdType:
  given [K, T <: IdEntity.Aux[K]]: IdType[T] with
    type Out = K

trait IdRepository[E <: IdEntity]:
  def findById(using idType: IdType[E])(id: idType.Out): Option[E] = ???

https://scastie.scala-lang.org/166gIUf3Rg2DwSgLlheXvQ

Or you can even try to use type classes starting from IdEntity, for example making IdEntity a type class:

trait IdEntity:
  type ID_Type
  val id: ID_Type

trait IdLongEntity extends IdEntity:
  override type ID_Type = Long

trait IdStringEntity extends IdEntity:
  override type ID_Type = String

trait IdRepository[E <: IdEntity](using val e: E):
  def findById(id: e.ID_Type): Option[E]

https://scastie.scala-lang.org/ap4GJY9NQyOyt2eHwobXSg

In Scala 3, how to replace General Type Projection that has been dropped?

What does Dotty offer to replace type projections?

E is not a legal path since it is not a concrete type (Scala 3)

https://docs.scala-lang.org/scala3/reference/dropped-features/type-projection.html

Translation of

// scala 2.13, type projections

trait IdEntity {
  type ID_Type
  val idDefault: ID_Type
  val id: Option[ID_Type]
  def idOrDefault: ID_Type = id.getOrElse(idDefault)
}

trait IdRepository[E <: IdEntity] {
  type ID_Type = E#ID_Type
  def delete(id: ID_Type): Either[String, Int] = deleteImpl(id)
  def delete(entity: E): Either[String, Int] = delete(entity.idOrDefault)
  protected def deleteImpl(id: ID_Type): Either[String, Int]
}

can be

// scala 3, match types

trait IdEntity:
  type ID_Type
  val idDefault: ID_Type
  val id: Option[ID_Type]
  def idOrDefault: ID_Type = id.getOrElse(idDefault)
object IdEntity:
  type Aux[K] = IdEntity {type ID_Type = K}

type IdType[T <: IdEntity] = T match
  case IdEntity.Aux[k] => k

// unfortunately, E <: IdEntity.Aux[IdType[E]] doesn't work (match types are not lazy enough): Recursion limit exceeded. Maybe there is an illegal cyclic reference?
trait IdRepository[E <: IdEntity]:
  type ID_Type = IdType[E]
  def delete(id: ID_Type): Either[String, Int] = deleteImpl(id)
  def delete(entity: E {type ID_Type = IdType[E]}): Either[String, Int] = delete(entity.idOrDefault)
  protected def deleteImpl(id: ID_Type): Either[String, Int]

// scala 3, type classes (path-dependent types)

trait IdEntity:
  type ID_Type
  val idDefault: ID_Type
  val id: Option[ID_Type]
  def idOrDefault: ID_Type = id.getOrElse(idDefault)

trait IdRepository[E <: IdEntity](using val e: E):
  type ID_Type = e.ID_Type
  def delete(id: ID_Type): Either[String, Int] = deleteImpl(id)
  def delete(entity: E {type ID_Type = e.ID_Type}): Either[String, Int] = delete(entity.idOrDefault)
  protected def deleteImpl(id: ID_Type): Either[String, Int]