scala - 如何使用宏注释将无参数构造函数添加到 Scala 案例类？

Question

我试图回答这个问题。

而不是写：

case class Person(name: String, age: Int) {
  def this() = this("",1)
}

我想我会使用宏注释来扩展它：

@Annotation
case class Person(name: String, age: Int)

DefDef因此，我尝试在宏注释的 impl 中使用准引号将新构造函数添加为普通旧构造函数，例如：

val newCtor = q"""def this() = this("", 1)"""
val newBody = body :+ newCtor
q"$mods class $name[..$tparams](..$first)(...$rest) extends ..$parents { $self => ..$newBody }"

但这会返回一个错误：called constructor's definition must precede calling constructor's definition

有没有办法解决这个问题？我错过了什么？

感谢您的观看，-朱利安

Answer 1

事实证明，在宏注释中生成辅助构造函数的非常自然的意图暴露了两个不同的问题。

1）第一个问题（https://issues.scala-lang.org/browse/SI-8451）是关于准引号为二级构造函数发出错误的树形。这已在 2.11.0-RC4（尚未发布，目前以 2.11.0-SNAPSHOT 提供）和天堂 2.0.0-M6 的 2.10.x（昨天发布）中修复。

2）第二个问题是关于未分配的位置在类型检查期间造成严重破坏。奇怪的是，在对构造函数的调用进行类型检查时，typer 使用位置来确定这些调用是否合法。这不能那么容易地修补，我们必须解决：

         val newCtor = q"""def this() = this(List(Some("")))"""
-        val newBody = body :+ newCtor
+
+        // It looks like typer sometimes uses positions to decide whether stuff
+        // (secondary constructors in this case) typechecks or not (?!!):
+        // https://github.com/xeno-by/scala/blob/c74e1325ff1514b1042c959b0b268b3c6bf8d349/src/compiler/scala/tools/nsc/typechecker/Typers.scala#L2932
+        //
+        // In general, positions are important in getting error messages and debug
+        // information right, but maintaining positions is too hard, so macro writers typically don't care.
+        //
+        // This has never been a problem up until now, but here we're forced to work around
+        // by manually setting an artificial position for the secondary constructor to be greater
+        // than the position that the default constructor is going to get after macro expansion.
+        //
+        // We have a few ideas how to fix positions in a principled way in Palladium,
+        // but we'll have to see how it goes.
+        val defaultCtorPos = c.enclosingPosition
+        val newCtorPos = defaultCtorPos.withEnd(defaultCtorPos.endOrPoint + 1).withStart(defaultCtorPos.startOrPoint + 1).withPoint(defaultCtorPos.    point + 1)
+        val newBody = body :+ atPos(newCtorPos)(newCtor)