什么会导致宏扩展中出现“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此编译时错误。

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 scala 2.12.8。我尝试切换到 2.13.0-RC1 只是为了看看它是否已经修复。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)
}

执行它的代码：

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面的错误。

我期待它能够正确编译。我将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，它编译得很好。我什至可以用它替换bazRead 的值，一切都按预期工作：它打印出Baz(4,4)。

Answer 1

关于您的宏，尝试将${aType.companion} 替换为${aType.typeSymbol.companion}。

---

对于派生类型类，最好使用Shapeless、Magnolia 或Scalaz-deriving，而不是原始宏。

例如在Shapeless中Read可以导出如下

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)