scala how to declare a type define which must have implicit conversion?

Question

I need call a function def fromJsonString[Result <: B : C](jsonString: String): Result = ???.

And I define a trait MyContext in order to matching a various instance of type Result.

MyContext define like this:

trait MyContext {
  type Result <: B : C // cannot compile accutally 
  def getResult(jsonString): Result = fromJsonString(jsonString)
}

How can I declare the common type of type Result <: B : C? Thanks

Answer 1

Foo : Bar is not a type, nor an implicit conversion.
Is just syntax for an implicit / using parameter.

// This is sugar syntax for:
def fromJsonString[A <: GeneratedMessage : GeneratedMessageCompanion](str: String): A

// In Scala 2:
def fromJsonString[A <: GeneratedMessage](str: String)(implicit ev: GeneratedMessageCompanion[A]): A

// In Scala 3:
def fromJsonString[A <: GeneratedMessage](str: String)(using GeneratedMessageCompanion[A]): A

As such, the best you can do is to add that value to your interface.

// Scala 2:
trait MyContext {
  type Result <: GeneratedMessage
  implicit val companion: GeneratedMessageCompanion[Result]

  final def getResult(jsonString): Result = fromJsonString(jsonString)
}

// Scala 3:
trait MyContext {
  type Result <: GeneratedMessage
  given GeneratedMessageCompanion[Result]

  final def getResult(jsonString): Result = fromJsonString(jsonString)
}