Scala：是否可以使用宏注释来注释 class 的构造函数字段？（宏观天堂）

Question

我正在尝试使用宏注释来注释 class 的构造函数值。假设在 class 的 class 定义中实现并使用了一个名为 @identity 的宏注释 A:

class A(@identity val foo: String, // causes error
        val bar: String) {
@identity val foobar: String = "" // doesn't cause error
}

当只是注释 foobar 时，一切都编译得很好。但是，注释 foo 时出现以下编译时错误：

没有同伴的顶级class只能扩展到同名class或包含同名同伴的块

有人可以详细说明此错误及其发生的原因吗？

Answer 1

我怀疑你调用了宏

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

  @compileTimeOnly("enable macro paradise to expand macro annotations")
  class identity extends StaticAnnotation {
    def macroTransform(annottees: Any*): Any = macro identity.impl
  }

  object identity {
    def impl(c: whitebox.Context)(annottees: c.Tree*): c.Tree = {
      import c.universe._
      println(s"$annottees")
      q"..$annottees"
    }
  }

喜欢

  class A(@identity val foo: String,
          val bar: String) {
    @identity val foobar: String = ""
  }

  object A

那你有错误

Warning:scalac: List(<paramaccessor> val foo: String = _, class A extends scala.AnyRef {
  <paramaccessor> val foo: String = _;
  <paramaccessor> val bar: String = _;
  def <init>(foo: String, bar: String) = {
    super.<init>();
    ()
  };
  @new identity() val foobar: String = ""
}, object A extends scala.AnyRef {
  def <init>() = {
    super.<init>();
    ()
  }
})
Warning:scalac: 
Warning:scalac: List(<paramaccessor> val foo: String = _, def <init>(foo: String, bar: String) = {
  super.<init>();
  ()
})
Warning:scalac: List(val foobar: String = "")
Error:(8, 12) top-level class with companion can only expand into a block consisting in eponymous companions
  class A(@identity val foo: String,
Error:(8, 12) foo is already defined as value foo
  class A(@identity val foo: String,
Error:(8, 12) foo  is already defined as value foo
  class A(@identity val foo: String,

问题是你拿了一个 class（可能还有伴生对象）和 return 不仅是它们还有 val foo 所以你改变了 top- 的 number/flavor被禁止的级别定义 https://docs.scala-lang.org/overviews/macros/annotations.html

Top-level expansions must retain the number of annottees, their flavors and their names, with the only exception that a class might expand into a same-named class plus a same-named module, in which case they automatically become companions as per previous rule.

例如，如果我们更改宏

   def impl(c: whitebox.Context)(annottees: c.Tree*): c.Tree = {
      import c.universe._
      println(s"$annottees")
      q"..${annottees.tail}" // addded tail
    }

然后一切都会编译。

Scala：是否可以使用宏注释来注释 class 的构造函数字段？（宏观天堂）

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Scala：是否可以使用宏注释来注释 class 的构造函数字段？ （宏观天堂）

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Scala：是否可以使用宏注释来注释 class 的构造函数字段？（宏观天堂）