从光滑的HLIST中获取元素(或将光滑的HLIst转换为无形状的HList)

Question

我有自动生成的scala代码使用灵活的编码。我看到一些表行被实现为HLists。(但这些都是光滑的HList，而不是普通的无型HList)

现在，我需要一个来自HList的特定元素，它以行的形式返回给灵活的查询。

我在谷歌上找到了这条线

Getting elements from an HList

但这并不适用于光滑的HList。它适用于无形状的HList。

我也尝试了应用的方法。

val : Long = slickHList(2)

但这不能编译，因为类型Any不符合exected类型的Long。我不想做.asInstanceOf

是否有一种类型错误的方法可以访问光滑的HList元素？

编辑:基于下面的输入，我编写了下面的代码

package com.abhi

object SlickAndShapeless {
   import slick.collection.heterogeneous.{HCons, HList, HNil}
   import slick.collection.heterogeneous.syntax.HNil

   type MyRow = HCons[Long, HCons[String, HNil]]
   val row : MyRow = 1L :: "foo" :: HNil
   import HListExtensions._
   val hlist = row.asShapeless
   val container = new Container(hlist)
   val item = container.get(1)
}

class Container[L <: shapeless.HList](list: L) {
   import shapeless._
   import nat._
   import ops.hlist._
   def get(n: Nat)(implicit at: At[L, n.N]): at.Out = list[n.N]
}

object HListExtensions {
   import slick.collection.heterogeneous.{HNil => SHNil, HList => SHList, HCons}
   import shapeless.{::, HList, HNil}

   implicit class HListShapelessSlick(val list: HList) extends AnyVal {
      def asSlick : SHList = list match {
         case HNil => SHNil
         case head :: tail => head :: tail.asSlick
      }
   }

   implicit class HListSlickShapeless(val list: SHList) extends AnyVal {
      def asShapeless : HList = list match {
         case SHNil => HNil
         case HCons(head, tail) => head :: tail.asShapeless
      }
   }
}

上面的代码的问题是，从val item = container.get(1)获得的val item = container.get(1)类型是at.Out，而不是我所期望的Long。

build.sbt

libraryDependencies ++= Seq(
   "com.typesafe.slick" % "slick_2.12" % "3.2.1",
   "com.chuusai" % "shapeless_2.12" % "2.3.2"
)

我还看到两个编译器错误。

Error:(19, 35) Implicit not found: shapeless.Ops.At[shapeless.HList, shapeless.Succ[shapeless._0]]. You requested to access an element at the position shapeless.Succ[shapeless._0], but the HList shapeless.HList is too short.
   val item : Long = container.get(1)
Error:(19, 35) not enough arguments for method get: (implicit at: shapeless.ops.hlist.At[shapeless.HList,shapeless.Succ[shapeless._0]])at.Out.
Unspecified value parameter at.
   val item : Long = container.get(1)

Answer 1

可以创建扩展方法：

object HListExtensions {

  import slick.collection.heterogeneous.{HNil => SHNil, HList => SHList, HCons}
  import shapeless.{ ::, HList, HNil }

  implicit class HListShapelessSlick(val list:HList) extends AnyVal {
    def asSlick:SHList = list match {
      case HNil => SHNil
      case head :: tail => head :: tail.asSlick
    }
  }

  implicit class HListSlickShapeless(val list:SHList) extends AnyVal {
    def asShapeless:HList = list match {
      case SHNil => HNil
      case HCons(head, tail) => head :: tail.asShapeless
    }
  }
}

示例：

scala>import HListExtensions._
import HListExtensions._

scala> val x1:HList = 1 :: 2 ::  HNil
x1: slick.collection.heterogeneous.HList = 1 :: 2 :: HNil

scala> x1.asShapeless
res1: shapeless.HList = 1 :: 2 :: HNil

scala> x1.asShapeless.asSlick
res2: slick.collection.heterogeneous.HList = 1 :: 2 :: HNil

我希望这能帮到你。

编辑:这是类型级别的解决方案。

object HListsConvertersTypeLevel {

  import shapeless.{::}

  sealed trait HConv[From <: heterogeneous.HList, To <: shapeless.HList] {
    def convert(list: From): To
  }

  implicit def buildHConvNil: HConv[heterogeneous.HNil.type, shapeless.HNil] =
    new HConv[heterogeneous.HNil.type, shapeless.HNil] {
      override def convert(list: heterogeneous.HNil.type): shapeless.HNil = shapeless.HNil
    }

  implicit def buildHConv[H, T <: heterogeneous.HList, T2 <: shapeless.HList](
      implicit conv: HConv[T, T2]): HConv[HCons[H, T], ::[H, T2]] = new HConv[HCons[H, T], ::[H, T2]] {

    override def convert(list: HCons[H, T]): ::[H, T2] = {
      list.head :: conv.convert(list.tail)
    }
  }

  def toShapeless[A <: heterogeneous.HList, B <: shapeless.HList](list: A)(implicit conv: HConv[A, B]): B = conv.convert(list)

}

示例：

object SlickAndShapeless {
  import slick.collection.heterogeneous.{HCons, HNil}
  import slick.collection.heterogeneous.syntax.HNil

  type MyRow = HCons[Long, HCons[String, HNil]]
  val row: MyRow = 1L :: "foo" :: HNil

  import HListsConvertersTypeLevel._
  val hlist         = toShapeless(row)
  val item: Long    = hlist.head
  val item2: String = hlist.tail.head
}