如何将泛型用于 Scala (2.12) 宏

Question

我定义了一个简单的通用宏：

object MyMacro {
  def readWrite[T](readParse: String => T, label: String, format: T => String): Unit = macro readWriteImpl[T]
  def readWriteImpl[T: c.WeakTypeTag](c: Context)(readParse: c.Expr[String => T], label: c.Expr[String], format: c.Expr[T => String]): c.Tree = {
    import c.universe._
    q"""
    def read[WIRE](path: Path, reader: Transceiver[WIRE], isMapKey: Boolean = false): T =
      reader.readString(path) match {
        case null => null.asInstanceOf[T]
        case s => Try( $readParse(s) ) match {
          case Success(d) => d
          case Failure(u) => throw new ReadMalformedError(path, "Failed to parse "+$label+" from input '"+s+"'", List.empty[String], u)
        }
      }
    def write[WIRE](t: T, writer: Transceiver[WIRE], out: Builder[Any, WIRE]): Unit =
      t match {
        case null => writer.writeNull(out)
        case _    => writer.writeString($format(t), out)
      }
      """
  }
}

在一个单独的编译单元中，我像这样使用它：

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

构建时，编译器抱怨它不知道 T：

[error] /Users/me/git/ScalaJack/core/src/main/scala/co.blocke.scalajack/typeadapter/TimePrimitives.scala:13:30: not found: type T
[error]   MyMacro.readWrite[Duration]((s: String) => Duration.parse(s), "Duration", (t: Duration) => t.toString)
[error] 

它不喜欢我的准引用中的"T"引用，我有点理解。如何在准引用中表示传递给 readWriteImpl 的 T 参数，以便它正确解包？

Answer 1

使用标记检查类型或使用 =:= 将其与其他类型进行比较，或在扩展中使用它。

例如

scala 2.13.0-M5> def fImpl[A: c.WeakTypeTag](c: Context)(a: c.Expr[A]) = { import c._, universe._
               | q"null.asInstanceOf[${implicitly[c.WeakTypeTag[A]].tpe}]" }
fImpl: [A](c: scala.reflect.macros.blackbox.Context)(a: c.Expr[A])(implicit evidence$1: c.WeakTypeTag[A])c.universe.Tree
scala 2.13.0-M5> import language.experimental.macros ; def f[A](a: A): A = macro fImpl[A]
import language.experimental.macros
defined term macro f: [A](a: A)A
scala 2.13.0-M5> f(42)
res2: Int = 0
scala 2.13.0-M5> f("")
res3: String = null