假设我想遍历here所述的案例类通用表示形式

我定义了一些类型类来描述字段:

trait Described[X] extends (X => String)
object Described{
  def apply[X](x: X)(implicit desc: Described[X]) = desc(x)
}

定义了一些实例:
implicit object DoubleDescribed extends Described[Double]{
  def apply(x: Double) =  x.formatted("%01.3f")
}

和一般用户:
import shapeless._
import shapeless.labelled.FieldType
import shapeless.ops.hlist.LeftFolder

object DescrFolder extends Poly2{
  implicit def field[X, S <: Symbol](implicit desc: Described[X],
                                              witness: Witness.Aux[S]):
  Case.Aux[Seq[String], FieldType[S, X], Seq[String]] =
  at[Seq[String], FieldType[S, X]](
    (descrs, value) => descrs :+ f"${witness.value.name}: ${desc(value)}")
}

def describe[T <: Product, Repr <: HList](struct: T)
      (implicit lgen: LabelledGeneric.Aux[T,Repr],
                folder: LeftFolder.Aux[Repr, Seq[String], DescrFolder.type, Seq[String]]
                             ): String = {
  val repr = lgen.to(struct)
  val descrs = folder(repr,Vector())
  descrs.mkString(struct.productPrefix + "{", ",", "}")
}

所以现在我可以写
case class Point(x: Double, y: Double, z: Double)
describe(Point(1,2,3.0))

并得到



现在,我想使用shapeless标记定义一些字段元数据:
import tag._
trait Invisible
val invisible = tag[Invisible]
implicit def invisibleDescribed[X](implicit desc: Described[X])
             : Described[X @@ Invisible] =
  new Described[X @@ Invisible]{
    def apply(x: X @@ Invisible) = desc(x: X) + "[invisible]"
  }

所以Described(invisible(0.5))现在可以成功产生



但随着重新定义
case class Point(x: Double, y: Double, z: Double @@ Invisible)

describe(Point(1,2,invisible(3.0)))

产生编译错误:



我认为X with Tag[Y] with KeyTag[K,X]类型不能识别为FieldType[S, X],但无法猜测如何解决。

如何为这种情况定义适当的LeftFolder

最佳答案

您的问题根本不涉及shapeless。实际上可以简化为:

trait Described[T]
trait Invisible

implicit val doubleDescribed: Described[Double] = ???

implicit def invisibleDescribed[T](
  implicit desc: Described[T]
): Described[T with Invisible] = ???

implicitly[Described[Double with Invisible]]
Double @@ Invisible可以“表示为” Double with Invisible。请注意Double with Invisible <: Double

当编译器尝试获取隐式Described[Double with Invisible]时,它会正确地提示发散的隐式扩展:doubleDescribedinvisibleDescribed

回到原始代码,一个简单的解决方法是将invisibleDescribed重写为:

implicit def invisibleDescribed[X, I <: X @@ Invisible](
  implicit desc: Described[X]
): Described[I] = new Described[I]{
  def apply(x: I) = desc(x: X) + "[invisible]"
}

关于scala - 在LabelledGenerics中使用无形标签,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/32250845/

10-11 05:23