如何使用 Scala macro/quasiquote 作为代码模板？

Question

我想在编译时生成一组遵循简单模式的对象，所以我编写了以下宏：

object MyMacro {

  def readWrite[T](taName: String, readParse: String => T, label: String, format: T => String): Any = macro readWriteImpl[T]

  def readWriteImpl[T: c.WeakTypeTag](c: Context)(taName: c.Expr[String], readParse: c.Expr[String => T], label: c.Expr[String], format: c.Expr[T => String]): c.Expr[Any] = {
    import c.universe._

    def termName(s: c.Expr[String]): TermName = s.tree match {
      case Literal(Constant(s: String)) => TermName(s)
      case _ => c.abort(c.enclosingPosition, "Not a string literal")
    }

    c.Expr[Any](q"""
    object ${termName(taName)} extends TypeAdapter.=:=[${implicitly[c.WeakTypeTag[T]].tpe}] {
      def read[WIRE](path: Path, reader: Transceiver[WIRE], isMapKey: Boolean = false): ${implicitly[c.WeakTypeTag[T]].tpe} =
        reader.readString(path) match {
          case null => null.asInstanceOf[${implicitly[c.WeakTypeTag[T]].tpe}]
          case s => Try( $readParse(s) ) match {
            case Success(d) => d
            case Failure(u) => throw new ReadMalformedError(path, "Failed to parse "+${termName(label)}+" from input '"+s+"'", List.empty[String], u)
          }
        }

      def write[WIRE](t: ${implicitly[c.WeakTypeTag[T]].tpe}, writer: Transceiver[WIRE], out: Builder[Any, WIRE]): Unit =
        t match {
          case null => writer.writeNull(out)
          case _    => writer.writeString($format(t), out)
        }
    }
    """)
  }
}

我不确定 readWrite 和 readWriteImpl 的 return 值是否正确——编译器强烈抱怨某些断言失败！

我也不确定如何实际使用这个宏。首先我尝试了（在一个单独的编译单元中）：

object TimeFactories {
    MyMacro.readWrite[Duration](
      "DurationTypeAdapterFactory",
      (s: String) => Duration.parse(s),
      "Duration",
      (t: Duration) => t.toString)
}

没有成功。如果我尝试引用 TimeFactories.DurationTypeAdapterFactory，我会收到一条错误消息，提示未找到。接下来我想我会尝试将它分配给一个 val...也没有用：

object Foo {
  val duration = MyMacro.readWrite[Duration](
    "DurationTypeAdapterFactory",
    (s: String) => Duration.parse(s),
    "Duration",
    (t: Duration) => t.toString).asInstanceOf[TypeAdapterFactory]
}

我如何将其连接起来，以便生成如下编译的代码：

object TimeFactories{
    object DurationTypeAdapterFactory extends TypeAdapter.=:=[Duration] {
      def read[WIRE](path: Path, reader: Transceiver[WIRE], isMapKey: Boolean = false): Duration =
        reader.readString(path) match {
          case null => null.asInstanceOf[Duration]
          case s => Try( Duration.parse(s) ) match {
            case Success(d) => d
            case Failure(u) => throw new ReadMalformedError(path, "Failed to parse Duration from input 'Duration'", List.empty[String], u)
          }
        }

      def write[WIRE](t: Duration, writer: Transceiver[WIRE], out: Builder[Any, WIRE]): Unit =
        t match {
          case null => writer.writeNull(out)
          case _    => writer.writeString(t.toString, out)
        }
    }

// ... More invocations of the readWrite macro with other types for T
}

Answer 1

我认为，您不能使用宏生成新的标识符并公开使用它们。

相反，尝试将 object ${termName(taName)} extends TypeAdapter 简单地替换为 new TypeAdapter 并将宏的调用分配给 val（如第二个示例所示）。然后，您将引用存储在 val 中的匿名（和生成的）class。参数 taName 变得多余。

如何使用 Scala macro/quasiquote 作为代码模板？

How can I consume a Scala macro/quasiquote for code templates?

scala

scala-macros

scala-quasiquotes