Method Override with Scala 3 Macros

by

With new method Symbol.newClass (Scala 3.1.3) this becomes quite easy:

import scala.annotation.experimental
import scala.quoted.*

object NewClass {
  inline def newClass[A]: A = ${newClassImpl[A]}

  @experimental
  def newClassImpl[A: Type](using Quotes): Expr[A] = {
    import quotes.reflect.*

    val name: String = TypeRepr.of[A].typeSymbol.name + "Impl"
    val parents = List(TypeTree.of[A])

    def decls(cls: Symbol): List[Symbol] =
      List(Symbol.newMethod(cls, "func", MethodType(List("s"))(_ => List(TypeRepr.of[String]), _ => TypeRepr.of[String]), Flags.Override, Symbol.noSymbol))

    val cls = Symbol.newClass(Symbol.spliceOwner, name, parents = parents.map(_.tpe), decls, selfType = None)
    val funcSym = cls.declaredMethod("func").head

    val funcDef = DefDef(funcSym, argss => Some('{"override"}.asTerm))
    val clsDef = ClassDef(cls, parents, body = List(funcDef))
    val newCls = Typed(Apply(Select(New(TypeIdent(cls)), cls.primaryConstructor), Nil), TypeTree.of[A])

    Block(List(clsDef), newCls).asExprOf[A]
  }
}

Usage:

class TestClass {
  def func(s: String) = "base"
}

val res: TestClass = NewClass.newClass[TestClass]

//{
//  class TestClassImpl extends TestClass {
//    override def func(s: java.lang.String): java.lang.String = "override"
//  }
//
//  (new TestClassImpl(): TestClass)
//}

res.func("xxx") // override

Blog post: Possibility to generate arbitrary class implementations in macros

Scaladoc: Symbol.newClass

Issue: Support creating a new instance of a given Type[A] with Scala 3 macro

How to access constructor:

Scala 2 Append A Method To Class Body (Metaprogramming) (Scala 2, compiler plugin)

More Related Contents:

  1. Scala3: Crafting Types Through Metaprogramming?
  2. Iteration over a sealed trait in Scala?
  3. Weird behavior trying to convert case classes to heterogeneous lists recursively with Shapeless
  4. Scala Macros: Making a Map out of fields of a class in Scala
  5. How to access parameter list of case class in a dotty macro
  6. Scala macros: What is the difference between typed (aka typechecked) and untyped Trees
  7. Static return type of Scala macros
  8. Creating a method definition tree from a method symbol and a body
  9. Howto model named parameters in method invocations with Scala macros?
  10. “dynamically” creating case classes with macros
  11. Getting subclasses of a sealed trait
  12. How to compile and execute scala code at run-time in Scala3?
  13. What are Scala context and view bounds?
  14. (Why) do we need to call cache or persist on a RDD
  15. Abort early in a fold
  16. How to pass -D parameter or environment variable to Spark job?
  17. Why is “Unable to find encoder for type stored in a Dataset” when creating a dataset of custom case class?
  18. Append a column to Data Frame in Apache Spark 1.3
  19. MatchError while accessing vector column in Spark 2.0
  20. How do I list all files in a subdirectory in scala?
  21. How do you do dependency injection with the Cake pattern without hardcoding?
  22. Why is it possible to declare variable with same name in the REPL?
  23. Get TypeTag[A] from Class[A]
  24. Spark: long delay between jobs
  25. Addition of two RDD[mllib.linalg.Vector]’s
  26. Treating an SQL ResultSet like a Scala Stream
  27. Scala asInstanceOf with parameterized types
  28. Queries with streaming sources must be executed with writeStream.start();
  29. Singletons as Synthetic classes in Scala?
  30. Akka-Stream implementation slower than single threaded implementation

Leave a Comment