什么会导致宏扩展中的“genLoad中的意外树”？

Question

我试图生成一个小宏来隔离我遇到的另一个问题，并开始遇到这个编译时错误。

Error:scalac: 
  Unexpected tree in genLoad: test.MacroTest$Baz.type/class scala.reflect.internal.Trees$TypeTree at: source-/Users/jpatterson/test/src/test/scala/test/MacroTest.scala,line-5,offset=114
     while compiling: /Users/jpatterson/test/src/test/scala/test/MacroTest.scala
        during phase: jvm
     library version: version 2.13.0-RC1
    compiler version: version 2.13.0-RC1
  reconstructed args: -deprecation -Vimplicits -language:higherKinds -language:implicitConversions -language:postfixOps -classpath ... (cut)
last tree to typer: Literal(Constant(test.MacroTest.MacroTest$Baz.type))
       tree position: line 4 of /Users/jpatterson/test/src/test/scala/test/MacroTest.scala
            tree tpe: Class(classOf[test.MacroTest$Baz$])
              symbol: null
           call site: constructor MacroTest$$anon$1 in package test
== Source file context for tree position ==
     1 package test
     2 
     3 object MacroTest {
     4   case class Baz(x: Int, y: Int)
     5   implicit def bazRead: Read[Baz] = Read.readFor[Baz]
     6 
     7   def main(args: Array[String]): Unit = {

我开始使用Scala2.12.8。我试着切换到2.13.0-RC1，只是想看看它是否已经修复了。这两个版本的scala都失败了。

宏代码：

package test

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context

trait Read[A] {
  def read(in: String): A
}

object Read {
  implicit def intRead = new Read[Int] {
    override def read(in: String): Int = in.toInt
  }

  def CaseClassReadImpl[A: c.WeakTypeTag](c: Context): c.Expr[Read[A]] = {
    import c.universe._
    val aType = weakTypeOf[A]

    val params = aType.decls.collect {
      case m: MethodSymbol if m.isCaseAccessor => m
    }.toList

    val paramList = params.map(param => q"Read.read[${param.typeSignature}](in)")

    val src = q"""
      new Read[$aType] {
        def read(in: String) = ${aType.companion}.apply(..$paramList)
      }
    """

    println(src)
    c.Expr[Read[A]](src)
  }

  def readFor[A]: Read[A] = macro CaseClassReadImpl[A]

  def read[A](in: String)(implicit A: Read[A]): A = A.read(in)
}

练习它的守则：

package test

object MacroTest {
  case class Baz(x: Int, y: Int)
  implicit def bazRead: Read[Baz] = Read.readFor[Baz]

  def main(args: Array[String]): Unit = {
    println(Read.read[Baz]("4"))
  }
}

编译第二个块会导致上述错误。

我以为这能正确编译。我将这个println放入宏定义中，这样我就可以获取代码并尝试编译它。当我将它添加到第二个块时，它会编译得很好。我甚至可以用它替换bazRead的值，一切都如预期的那样:它打印出Baz(4,4)。

Answer 1

对于宏，您正在尝试将一个类型(aType.companion)拼接到一个需要一个术语( tpe: Type转换为TypeTree(tpe))的位置。

尝试将${aType.companion}替换为${aType.typeSymbol.companion}。

对于派生类型类，最好使用无形、木兰或Scalaz-派生，而不是原始宏。

例如，在无形状的Read中，可以如下所示

import shapeless.{Generic, HList, HNil, ::}

trait Read[A] {
  def read(in: String): A
}

object Read {
  implicit def intRead: Read[Int] = _.toInt

  implicit def hNilRead: Read[HNil] = _ => HNil

  implicit def hConsRead[H, T <: HList](implicit r: Read[H], r1: Read[T]): Read[H :: T] = 
    in => r.read(in) :: r1.read(in)

  implicit def caseClassRead[A, L <: HList](implicit gen: Generic.Aux[A, L], r: Read[L]): Read[A] = 
    in => gen.from(r.read(in))

  def read[A](in: String)(implicit A: Read[A]): A = A.read(in)
}

case class Baz(x: Int, y: Int)

Read.read[Baz]("123") // Baz(123,123)