How to Use SAM in Scala for Discriminator Criteria Types

The Issue at Hand

When working with mirrors and type reflections in Scala, particularly in macros or complex type derivation, it is common to encounter type mismatches. In your case, the issue arises when specifying headType as a type argument for the getFor method. This happens due to the strict upper bound A of the getFor method not being met by the inferred type from headType.

Step-by-Step Solution

To successfully call the getFor method on an instance of DiscriminatorCriteria, follow these steps:

Step 1: Define Your Derive Serializer Implementation

First, ensure you have your derive serializer implementation set up correctly. This function will extract the MirroredElemTypes necessary to provide the correct encapsulated types for calling the getFor method.

import scala.quoted.*
import scala.deriving.Mirror

def deriveSerializerImpl[A: Type](using quotes: Quotes): Expr[Serializer[A]] = {
    import quotes.reflect.*
    Expr.summon[Mirror.Of[A]] match {
        case Some('{ $m: Mirror.SumOf[A] {type MirroredElemTypes = elemTypes} }) =>
            Type.of[elemTypes] match {
                case '[headType *: tailTypes] =>
                    // Further implementation needed
                case _ =>
                    report.error("Unsupported type structure")
            }
    }
}

Step 2: Calling getFor Method

In the section where we pattern match on elemTypes, we will generate code to invoke getFor. Be sure to correctly pass headType while conforming to the type bounds.

case '[headType *: tailTypes] =>
    val discriminatorCriteriaExpr = Expr.summon[DiscriminatorCriteria[A]].getOrElse(
        report.error("DiscriminatorCriteria is not defined")
    )
    val discriminator: Expr[Int] = '{ $discriminatorCriteriaExpr.getFor[headType] }

Here, discriminatorCriteriaExpr is an expression that summons the appropriate DiscriminatorCriteria. The important note here is that headType must satisfy the required upper bound, which is A.

Handling Type Mismatches

To prevent the compiler from throwing upper bound errors:

• Ensure that headType derives correctly from A. You might need to introduce a type constraint or use a different approach to ensure conformity within your type definitions.
• Review the type structure to ensure that each type you pass maintains the expected bounds and relationships.

Frequently Asked Questions

What is SAM in Scala?

SAM stands for Single Abstract Method, a pattern that allows you to define functional interfaces for concise lambda expressions in Scala.

How do I handle reflections in Scala?

Use Scala's reflection capabilities through scala.reflect.runtime.universe or scala.quoted for macro annotations. Ensure that your types are well-defined and compliant.

What to do if I encounter type inferring issues?

Check your type bounds and ensure that the types you are working with inherit correctly from the parents. You can also use explicit type annotations for better clarity.

Conclusion

This post outlined how to work with the DiscriminatorCriteria trait and call its getFor method using Scala's type system and quotations. By following the structured approach above, you can navigate common pitfalls related to type bounds effectively and derive robust serializers with ease. Feel free to explore more about Scala's reflection and type systems to enhance your macros and functional programming skills.