HList 中的类型类包含元素？

Question

我正在尝试定义一个类型类，以便 HList L 包含特定类型 A。

trait Contains[A] {
  type Out
}
object Contains {

  type Aux[A, O] = Contains[A] { type Out = O }

  def contains[L <: HList, A]
       (implicit ev: Contains.Aux[L, A]) = ev

  implicit def containsInt[H, L <: HList, A]
       (implicit ev: H == A): Contains.Aux[H :: L, A] =
    new Contains[H :: L] {
      type Out = A
    }
}

当然因为ev: H == A.

编译不了

这样的 "equivalent type" 类型类存在吗？

一旦成功，我希望编译以下内容：

implicitly[Contains.Aux[Int :: HNil, Int]] 因为 HList 中有一个 Int。

然而，我预计 implicitly[Contains.Aux[String :: HNil, Boolean] 无法编译，因为输入 HList 中没有 Boolean。

Answer 1

您正在寻找 shapeless.ops.hlist.Selector。

使用 Selector 你的例子是这样的，

scala> import shapeless._, ops.hlist._
import shapeless._
import ops.hlist._

scala> Selector[Char :: HNil, Char]
res0: shapeless.ops.hlist.Selector[shapeless.::[Char,shapeless.HNil],Char] = shapeless.ops.hlist$Selector$$anon@20eebe9c

scala> Selector[Int :: Char :: HNil, Char]
res1: shapeless.ops.hlist.Selector[shapeless.::[Int,shapeless.::[Char,shapeless.HNil]],Char] = shapeless.ops.hlist$Selector$$anon@10cf3c87

scala> Selector[Int :: HNil, Int]
res2: shapeless.ops.hlist.Selector[shapeless.::[Int,shapeless.HNil],Int] = shapeless.ops.hlist$Selector$$anon@79d8c266

scala> Selector[Boolean :: Char :: HNil, Int]
<console>:18: error: Implicit not found: shapeless.Ops.Selector[shapeless.::[Boolean,shapeless.::[Char,shapeless.HNil]], Int]. You requested an element of type Int, but there is none in the HList shapeless.::[Boolean,shapeless.::[Char,shapeless.HNil]].
       Selector[Boolean :: Char :: HNil, Int]
               ^

scala> Selector[Boolean :: Char :: HNil, Char]
res4: shapeless.ops.hlist.Selector[shapeless.::[Boolean,shapeless.::[Char,shapeless.HNil]],Char] = shapeless.ops.hlist$Selector$$anon@bf88c94

scala> Selector[Boolean :: Int :: Char :: HNil, Char]
res5: shapeless.ops.hlist.Selector[shapeless.::[Boolean,shapeless.::[Int,shapeless.::[Char,shapeless.HNil]]],Char] = shapeless.ops.hlist$Selector$$anon@653f47f

scala> Selector[Int :: HNil, HNil]
<console>:18: error: Implicit not found: shapeless.Ops.Selector[shapeless.::[Int,shapeless.HNil], shapeless.HNil]. You requested an element of type shapeless.HNil, but there is none in the HList shapeless.::[Int,shapeless.HNil].
       Selector[Int :: HNil, HNil]
               ^

HList 中的类型类包含元素？

Typeclass Containing Element in HList?

scala

shapeless