使用无形状的类型级筛选

Question

有没有人知道如何使用Shapeless使这个测试工作。

package net.jtownson.swakka.jsonschema
import org.scalatest.FlatSpec
import org.scalatest.Matchers._
class OptionalFieldSpec extends FlatSpec {
  case class A(i: Int, j: Option[Int])
  "an extractor of some kind" should "get the (non)optional fields from a case class" in {
    extractNonOptionalFieldNames[A] shouldBe List("i")
    extractOptionalFieldNames[A] shouldBe List("j")
  }
  def extractNonOptionalFieldNames[T <: Product](/* implicit typeclass instances? */): List[String] = ???
  def extractOptionalFieldNames[T <: Product]: List[String] = ???
}

我没有 A 或其泛型等效项的运行时实例，因为我正在努力为案例类 A 创建独立于任何给定实例的 JsonSchema。架构具有必填字段，该字段是非可选字段的列表。例如

{
  "type" -> "object",
  "required" -> ["i"],
  "properties" -> {
    "i" -> {
      "type" -> "integer",
      "format" -> "int32"
     }
   }
}

Answer 1

像这样：

trait FieldNameExtractor[T] extends Serializable {
  import shapeless.ops.hlist.{RightFolder, ToTraversable}
  import shapeless.ops.record.Keys
  import shapeless.{HList, HNil, LabelledGeneric, Poly2}
  /**
    * Extracts filtered field names for type [[T]],
    * given a polymorphic function that acts as the type filter
    */
  def extract[L <: HList, R <: HList, O <: HList](op: Poly2)(
      implicit lgen: LabelledGeneric.Aux[T, L],
      folder: RightFolder.Aux[L, HNil.type, op.type, R],
      keys: Keys.Aux[R, O],
      traversable: ToTraversable.Aux[O, List, Symbol]
  ): List[String] = {
    val result = keys().to[List]
    result.map(_.name)
  }
}
object FieldNameExtractor {
  def apply[T] = new FieldNameExtractor[T] {}
}

用法：

import org.scalatest.FlatSpec
import org.scalatest.Matchers._
class Test extends FlatSpec {
  /* type filters */
  import shapeless.{HList, Poly2}
  import shapeless.labelled.KeyTag, shapeless.tag.Tagged
  type FilterO[A, T] = Option[A] with KeyTag[Symbol with Tagged[T], Option[A]]
  trait Ignore extends Poly2 {
    implicit def default[A, L <: HList] = at[A, L]((_, l) => l)
  }
  trait Accept extends Poly2 {
    implicit def default[A, L <: HList] = at[A, L](_ :: _)
  }
  object allOptions extends Ignore {
    implicit def option[A, T, L <: HList] = at[FilterO[A, T], L](_ :: _)
  }
  object noOptions extends Accept {
    implicit def option[A, T, L <: HList] = at[FilterO[A, T], L]((_, l) => l)
  }
  "an extractor of some kind" should "get the (non)optional fields from a case class" in {
    case class A(i: Int, j: Option[Int], k: String)
    val fne = FieldNameExtractor[A]
    fne.extract(noOptions) shouldBe List("i", "k") // extractNonOptionalFieldNames
    fne.extract(allOptions) shouldBe List("j")     // extractOptionalFieldNames
  }
}

Answer 2

以下是使用类型类的一种方法：

import shapeless._
import shapeless.labelled.FieldType

trait OptionExtractor[A] {
  type B <: HList
}
trait LowPriorityOptionExtractor {
  implicit def hconsExtractor[K, V, T <: HList](implicit
                                                       extractor: OptionExtractor[T]):
  OptionExtractor.Aux[FieldType[K, V] :: T, extractor.B] = new OptionExtractor[FieldType[K, V] :: T] {
    type B = extractor.B
  }
}
object OptionExtractor extends LowPriorityOptionExtractor {
  type Aux[A, B0 <: HList] = OptionExtractor[A] {type B = B0}
  def apply[A](implicit extractor: OptionExtractor[A]): OptionExtractor.Aux[A, extractor.B] = extractor
  implicit val hnilOptionExtractor: OptionExtractor.Aux[HNil, HNil] = new OptionExtractor[HNil] {
    type B = HNil
  }
  implicit def hconsOptionExtractor[K, V, T <: HList](implicit extractor: OptionExtractor[T]):
  OptionExtractor.Aux[FieldType[K, Option[V]] :: T, K :: extractor.B] = new OptionExtractor[FieldType[K, Option[V]] :: T] {
    type B = K :: extractor.B
  }
}

有几件事可能需要解释：

• 既然你提到你没有A的运行时实例.您希望返回的类型级别表示形式是什么？在此解决方案中，我只是为可选证人返回了HList见证人。我认为List[String]表示是不够的，因为过滤掉非可选值与什么都不做具有相同的类型。
• 类型类具有优先级，因此筛选选项与反向优先级相同。

它可以像这样使用：

case class A(i: Int, j: Option[Int], k: Option[Long])
val x = LabelledGeneric[A]
type filteredType = OptionExtractor[x.Repr] 
//type B = Symbol with shapeless.tag.Tagged[String("j")] :: Symbol with shapeless.tag.Tagged[String("k")] :: shapeless.HNil