如何调试宏注释？

Question

背景

我尝试向Monocle添加@Prisms注释，其工作原理如下。给出：

@Prisms sealed trait Foo
case object A extends Foo
case object B extends Foo

它将为Foo生成一个配套对象：

object Foo {
  val a = monocle.macros.GenPrism.apply[Foo, A]
  val b = monocle.macros.GenPrism.apply[Foo, B]
}

（或者如果伴侣对象已经存在，它会将这些方法添加到它中。）

宏依赖于directKnownSubclasses来查找A和B，因此Monocle自述文件中会有一条注释，建议人们使用Typelevel Scala。我已将Monocle配置为在我的分支上使用TLS。

首次尝试

我尝试使用TypeName来表示父类型Foo和子类型（A或B）：

def findSubclasses(typeName: TypeName): Set[TypeName] = {
  // Note: disable macros to prevent stack overflow caused by infinite typing loop!
  val tpe = c.typecheck(Ident(typeName), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
  tpe.symbol.asClass.knownDirectSubclasses.map(_.asType.name)
}

def prisms(parentTypename: TypeName, childNames: Set[TypeName]): Set[Tree] = childNames.map { childName =>
  val prismName = TermName(prefix + childName.decodedName.toString.toLowerCase)
  q"""val $prismName = monocle.macros.GenPrism.apply[$parentTypename, $childName]"""
}

这会生成看似合理的代码：

{
  sealed trait Foo;
  object Foo {
    val a = monocle.macros.GenPrism.apply[Foo, A];
    val b = monocle.macros.GenPrism.apply[Foo, B]
  };
  ()
}

但它似乎不起作用：

[error] /Users/chris/code/Monocle/test/shared/src/test/scala/other/PrismsAnnotationSpec.scala:5: not found: type A
[error] @monocle.macros.Prisms sealed trait Foo
[error]  ^
[error] one error found

第二次尝试

我还尝试使用TypeSymbol代替TypeName s：

def prisms(parentTypename: TypeName, childSymbols: Set[TypeSymbol]): Set[Tree] = {
  val parentSymbol: TypeSymbol = 
    c.typecheck(Ident(parentTypename), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
      .symbol
      .asType
  childSymbols.map { childSymbol =>
    val prismName = TermName(prefix + childSymbol.name.decodedName.toString.toLowerCase)
    q"""val $prismName = monocle.macros.GenPrism.apply[$parentSymbol, $childSymbol]"""
  }
}

但是这给了我以下错误，所以我猜测生成的树是无效的：

[error] /Users/chris/code/Monocle/test/shared/src/test/scala/other/PrismsAnnotationSpec.scala:5: macro annotation could not be expanded (the most common reason for that is that you need to enable the macro paradise plugin; another possibility is that you try to use macro annotation in the same compilation run that defines it)
[error] @monocle.macros.Prisms sealed trait Foo
[error]  ^
[error] one error found

第三次尝试

我也尝试将TypeSymbol转换为Type s：

def prisms(parentTypename: TypeName, childSymbols: Set[TypeSymbol]): Set[Tree] = {
  val parentSymbol: TypeSymbol = 
    c.typecheck(Ident(parentTypename), mode = c.TYPEmode, silent = true, withMacrosDisabled = true)
      .symbol
      .asType
  childSymbols.map { childSymbol =>
    val prismName = TermName(prefix + childSymbol.name.decodedName.toString.toLowerCase)
    q"""val $prismName = monocle.macros.GenPrism.apply[${parentSymbol.toType}, ${childSymbol.toType}]"""
  }
}

但我得到同样的错误，说注释无法扩展。

现在我没有想法。

如何重现

此分支上提供了完整的代码：https://github.com/cb372/Monocle/tree/sealed-trait-macro-annotation。

• First attempt
• Second attempt
• Third attempt

宏用于test/shared/src/test/scala/other/PrismsAnnotationSpec.scala。要编译它，请运行sbt test:compile。