Scala宏:"无法从具有未解析类型参数的类型T创建TypeTag"

Question

我正在玩Scalas的新宏,并从akshaal中找到了这个要点.因为它接缝我不太明白.给出以下特征(fieldsMacro或多或少与akshaal示例相同)

case class Field[I <: AnyRef](name: String, get: I => Any)

type Fields[I <: AnyRef] = List[Field[I]]

trait FieldAccess {
import FieldMacors._
    import Field._
    import language.experimental.macros

    def fields[T <: AnyRef]: Fields[T] = macro fieldsMacro[T]

    def field[T <: AnyRef](name: String): Fields[T] = fields[T].headOption <-- does not work!
                                                             ^
}
 object FieldMacors {

import language.experimental.macros
import Field._

def fields[T <: AnyRef]: Fields[T] = macro fieldsMacro[T]

/**
 * Get a list of fiels
 */
def fieldsMacro[T <: AnyRef: c.TypeTag](c: Context): c.Expr[Fields[T]] = {
  import c.universe._
  val instanceT = c.typeOf[T]

  val fields = instanceT.members.filter(member => member.isTerm && !member.isMethod)

  // transform an iterable of expr in a expr of list.
  def foldIntoListExpr[T: c.TypeTag](exprs: Iterable[c.Expr[T]]): c.Expr[List[T]] =
    exprs.foldLeft(reify { Nil: List[T] }) {
      (accumExpr, expr) =>
        reify { expr.splice :: accumExpr.splice }
    }

  val fieldAccessores = for (field <- fields) yield {
    val name = field.name.toString.trim // Why is there a space at the end of field name?!
    val nameExpr = c literal name

    // Construct expression (x : $I) => x.$name
    val getFunArgTree = ValDef(Modifiers(), newTermName("x"), TypeTree(instanceT), EmptyTree)
    val getFunBodyTree = Select(Ident(newTermName("x")), newTermName(name))
    val getFunExpr = c.Expr[T => Any](Function(List(getFunArgTree), getFunBodyTree))
    reify {
      Field[T](name = nameExpr.splice, get = getFunExpr.splice)
    }
  }
  foldIntoListExpr(fieldAccessores)
}
}

编译器抱怨'无法从具有未解析类型参数的类型T创建TypeTag'

如何设置将T传递给宏,或者我是否必须实现另一个使用fieldsMacro的宏

Answer 1

T: TypeTag类型参数的上下文绑定T意味着您需要提供代替此参数的类型参数是具体的(即不包含对未标记的类型参数或抽象类型成员的引用).否则会发生错误.

例子:

scala> val ru = scala.reflect.runtime.universe
ru @ 6d657803: scala.reflect.api.JavaUniverse = scala.reflect.runtime.JavaUniverse@6d657803

scala> def foo[T: ru.TypeTag] = implicitly[ru.TypeTag[T]]
foo: [T](implicit evidence$1: reflect.runtime.universe.TypeTag[T])reflect.runtime.universe.TypeTag[T]

scala> foo[Int]
res0 @ 7eeb8007: reflect.runtime.universe.TypeTag[Int] = TypeTag[Int]

scala> foo[List[Int]]
res1 @ 7d53ccbe: reflect.runtime.universe.TypeTag[List[Int]] = TypeTag[scala.List[Int]]

scala> def bar[T] = foo[T] // T is not a concrete type here, hence the error
<console>:26: error: No TypeTag available for T
       def bar[T] = foo[T]
                       ^

scala> def bar[T] = foo[List[T]] // T being not concrete renders 
                                 // the entire compound type not concrete
<console>:26: error: No TypeTag available for List[T]
       def bar[T] = foo[List[T]]
                       ^

scala> def bar[T: TypeTag] = foo[T] // to the contrast T is concrete here
                                    // because it's bound by a concrete tag bound
bar: [T](implicit evidence$1: reflect.runtime.universe.TypeTag[T])reflect.runtime.universe.TypeTag[T]

scala> bar[Int]
res2 @ 7eeb8007: reflect.runtime.universe.TypeTag[Int] = TypeTag[Int]

scala> def bar[T: TypeTag] = foo[List[T]]
bar: [T](implicit evidence$1: reflect.runtime.universe.TypeTag[T])reflect.runtime.universe.TypeTag[List[T]]

scala> bar[Int]
res3 @ 1a108c98: reflect.runtime.universe.TypeTag[List[Int]] = TypeTag[scala.List[Int]]

scala> bar[List[Int]]
res4 @ 76d5989c: reflect.runtime.universe.TypeTag[List[List[Int]]] = TypeTag[scala.List[scala.List[Int]]]

在编译时具有可强制执行的具体类型的概念是有用的.默认情况下启用具体类型标记非常有用,如https://issues.scala-lang.org/browse/SI-5884中所述.

然而,正如您所见,宏中的具体类型标记可能会引起混淆,因为宏通常应该适用于具体类型和非具体类型.因此,应该总是使用c.AbsTypeTag.由于这个原因,我们不再允许c.TypeTag2.10.0-M7中的上下文界限:https://github.com/scala/scala/commit/788478d3ab.

编辑.在2.10.0-RC1中,有些AbsTypeTag已重命名为WeakTypeTag.关于类型标签的其他所有内容都保持不变.