CODEWITHSUNDEEP
scalascala-macros
Ford O.
Ford O.

Reputation: 1498

How to get a type of macro typeparameter

I am trying to implement a macro, which would generate new class, extending an existing class, given as an argument:

def impl[T: c.WeakTypeTag](c: Context)(p: c.Expr[T]): c.Expr[() => T] = {
  val t = weakTypeOf[T]
  q"class Too extends $t {..}; () => new Too()"
}

But trying to run such macro results in :

[error] Main.scala: exception during macro expansion:
[error] scala.ScalaReflectionException: object com.foo.MyClass in compiler mirror not found.
[error]         at scala.reflect.internal.Mirrors$RootsBase.staticClass(Mirrors.scala:129)
[error]         at scala.reflect.internal.Mirrors$RootsBase.staticClass(Mirrors.scala:29)
[error]         at impl(Macro.scala:54)

Upvotes: 2

Views: 89

Answers (1)

Dmytro Mitin
Dmytro Mitin

Reputation: 51648

You seem not to provide enough information to reproduce your error. The following code compiles without errors in 2.13

macros/src/main/scala/Macros.scala

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

object Macros {
  def too[T](p: T): () => T = macro impl[T]

  def impl[T: c.WeakTypeTag](c: blackbox.Context)(p: c.Expr[T]): c.Expr[() => T] = {
    import c.universe._
    val t = weakTypeOf[T]
    c.Expr[() => T](q"class Too extends $t {}; () => new Too()")
  }
}

core/src/main/scala/App.scala

import Macros._

object App {
  class MyClass

  too(new MyClass) //App$$$Lambda$3/1967205423@77f03bb1

  too(new MyClass)() //App$Too$1@77f03bb1
}

//Warning:scalac: {
//  class Too extends App.MyClass {
//    def <init>() = {
//      super.<init>();
//      ()
//    };
//    <empty>
//  };
//  (() => new Too())
//}

Upvotes: 1

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