Scala 删除不成形的HList文本类型？

我试图按文本类型筛选HList，但无法使其正常工作，它只是筛选所有字符串：

import shapeless._
import shapeless.ops.hlist._

def filterLabel[Labels <: HList, Label, Out <: HList](labels: Labels, label: Label)(
implicit sel: Filter.Aux[Labels, label.type, Out]): Out = sel(labels)
val a: "a" = "a"
val b: "b" = "b"
val c1: 1 = 1
val h1 = a :: b :: a :: c1 :: HNil
//result: hl: String :: String :: String :: Int :: HNil = "a" :: "b" :: "a" :: 1 :: HNil 
//looks like literal-types are no longer present?
filterLabel(h1, "a")

更新： 对于Lightbend Scala，我现在有以下内容：

import shapeless._
import shapeless.ops.hlist._
import syntax.singleton._

implicit def stringToF(label: String) = () => label.narrow

def filterLabel[Labels <: HList, T, Label <: String, Out <: HList](labels: Labels, label: () => Label)(
implicit
sel: Filter.Aux[Labels, Label, Out]): Out = sel(labels)

val h1 = "a".narrow :: "b".narrow :: "a".narrow :: "c".narrow :: HNil

filterLabel(h1, "a")
//res92: a :: a :: HNil = "a" :: "a" :: HNil

import shapeless.ops.hlist.Filter
import shapeless.{::, HList, HNil, Witness}
import shapeless.syntax.singleton._

def filterLabel[Labels <: HList, Label, Out <: HList](labels: Labels, label: Label)(
  implicit sel: Filter.Aux[Labels, Label, Out]): Out = sel(labels)

val h1: Witness.`"a"`.T :: Witness.`"b"`.T :: Witness.`"a"`.T :: Witness.`1`.T :: HNil = 
  "a".narrow :: "b".narrow :: "a".narrow :: 1.narrow :: HNil

filterLabel(h1, "a".narrow) // a :: a :: HNil

导入无形状_
导入shapeless.ops.hlist_
导入syntax.singleton_
隐式def stringToF（label:String）=（）=>label.窄
def filterLabel[标签我想这适用于Lightbend Scala：

import shapeless._
import shapeless.ops.hlist._
import syntax.singleton._

implicit def stringToF(label: String) = () => label.narrow

def filterLabel[Labels <: HList, T, Label <: String, Out <: HList](labels: Labels, label: () => Label)(
implicit
sel: Filter.Aux[Labels, Label, Out]): Out = sel(labels)

val h1 = "a".narrow :: "b".narrow :: "a".narrow :: "c".narrow :: HNil

filterLabel(h1, "a")
//res92: a :: a :: HNil = "a" :: "a" :: HNil

import shapeless.ops.hlist.Filter
import shapeless.{::, HList, HNil, Witness}
import shapeless.syntax.singleton._

def filterLabel[Labels <: HList, Label, Out <: HList](labels: Labels, label: Label)(
  implicit sel: Filter.Aux[Labels, Label, Out]): Out = sel(labels)

val h1: Witness.`"a"`.T :: Witness.`"b"`.T :: Witness.`"a"`.T :: Witness.`1`.T :: HNil = 
  "a".narrow :: "b".narrow :: "a".narrow :: 1.narrow :: HNil

filterLabel(h1, "a".narrow) // a :: a :: HNil

导入shapeless.ops.hlist.Filter
导入无形状。{：：，HList，HNil，Witness}
导入shapless.syntax.singleton_
def filterLabel[Labels这是我在类型级别scala 2.12.4上执行的方式

trait Narrow[T] {
  def value: T
}

object Narrow {
  def apply[T](implicit n: Narrow[T]): n.type = n

  implicit def hnil = new Narrow[HNil] { def value = HNil }

  implicit def cons[T, L <: HList: Narrow](implicit t: ValueOf[T]) = new Narrow[T::L] {
   def value = t.value :: Narrow[L].value
  }
}

我怀疑这个问题与不成形有关。我猜如果你选中
：键入a
，你会得到
字符串
。即使使用stock编译器，尤其是在REPL和2.12上，这个东西也有惊人的缺陷。你可以尝试使用
val a:Witness。
a“
.T=”a“
或类型别名—我知道可以通过多种方法让它记住singleton类型，至少在vanilla 2.12中是这样。
scala> val h1 = Narrow["a"::"b"::"a"::1::HNil].value
h1: "a" :: "b" :: "a" :: 1 :: shapeless.HNil = a :: b :: a :: 1 :: HNil