CODEWITHSUNDEEP
scalascala-macros
Some Name
Some Name

Reputation: 9521

Pass type identifier to static macro annotation

Is there a way to pass a type identifier to a macro annotation? Here is what I mean:

@compileTimeOnly("Compile-time only annotation")
class mymacro(val typeName: universe.TypeName) extends StaticAnnotation { // <-- does not work
  def macroTransform(annottees: Any*): Any = macro impl
}

object mymacro {
  def impl(c: whitebox.Context)(annottees: c.Expr[Any]*) = //...
}

use-case:

trait SomeTrait

@mymacro(SomeTrait)
class Test {
    //...
}

Or maybe there is any other way to pass Type identifier of an arbitrary non-generic type to a macro annotation implementation?

Motivation behind this: I need to generate some class Test member function def foo depending on the type passed as an argument to the macro annotation (SomeTrait).

Upvotes: 0

Views: 83

Answers (1)

Dmytro Mitin
Dmytro Mitin

Reputation: 51683

For example you can pass typeName as a string

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("Compile-time only annotation")
class mymacro(typeName: String) extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro mymacro.impl
}

object mymacro {
  def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._
    val typeName = TypeName(c.prefix.tree match {
      case q"new mymacro(${str: String})" => str
    })

    println(typeName)

    q"..$annottees"
  }
}

Usage:

trait SomeTrait

@mymacro("SomeTrait")
class Test 

// scalac: SomeTrait

Getting Parameters from Scala Macro Annotation

If you prefer the companion-object approach proposed by @user in comments then you can do

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("Compile-time only annotation")
class mymacro(companionObject: Any) extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro mymacro.impl
}

object mymacro {
  def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._

    val companionTrait = c.typecheck(c.prefix.tree) match {
      case q"new mymacro($arg)" => arg.symbol.companion
    }

    println(companionTrait)

    q"..$annottees"
  }
}

Usage:

trait SomeTrait
object SomeTrait

@mymacro(SomeTrait)
class Test

//scalac: trait SomeTrait

Upvotes: 3

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