无形与注释 [英] Shapeless and annotations

问题描述

我想将一些函数应用于案例类中的字段，这些字段使用 MyAnnotation 进行注释.这个想法是将类型 T 转换为其通用表示，提取注释，压缩，向右(或向左)折叠以重建通用表示，最后回到类型 T.我按照这里和这个要点.

I would like to have some function applied to fields in a case class, that are annotated with MyAnnotation. The idea is to transform type T into its generic representation, extract annotations, zip, fold right (or left) to reconstruct a generic representation and finally get back to type T. I followed the answer provided here and this gist.

我使用的是 Scala 2.11.12 和 shapeless 2.3.3.

I'm using scala 2.11.12 and shapeless 2.3.3.

以下是我的代码:

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

case class MyAnnotation(func: String) extends scala.annotation.StaticAnnotation

trait Modifier[T] {
  def modify(t: T): T
}

object Modifier {

  def apply[A: Modifier]: Modifier[A] = implicitly[Modifier[A]]

  def create[T](func: T => T): Modifier[T] = new Modifier[T] { override def modify(t: T): T = func(t) }

  private def id[T](t: T) = t

  implicit val stringModifier: Modifier[String] = create(id)

  implicit val booleanModifier: Modifier[Boolean] = create(id)

  implicit val byteModifier: Modifier[Byte] = create(id)

  implicit val charModifier: Modifier[Char] = create(id)

  implicit val doubleModifier: Modifier[Double] = create(id)

  implicit val floatModifier: Modifier[Float] = create(id)

  implicit val intModifier: Modifier[Int] = create(id)

  implicit val longModifier: Modifier[Long] = create(id)

  implicit val shortModifier: Modifier[Short] = create(id)

  implicit val hnilModifier: Modifier[HNil] = create(id)

  implicit def hlistModifier[H, T <: HList, AL <: HList](
    implicit
    hser: Lazy[Modifier[H]],
    tser: Modifier[T]
  ): Modifier[H :: T] = new Modifier[H :: T] {
    override def modify(ht: H :: T): H :: T = {
      ht match {
        case h :: t =>
          hser.value.modify(h) :: tser.modify(t)
      }
    }
  }

  implicit val cnilModifier: Modifier[CNil] = create(id)

  implicit def coproductModifier[L, R <: Coproduct](
    implicit
    lser: Lazy[Modifier[L]],
    rser: Modifier[R]
  ): Modifier[L :+: R] = new Modifier[L :+: R] {
    override def modify(t: L :+: R): L :+: R = t match {
      case Inl(l) => Inl(lser.value.modify(l))
      case Inr(r) => Inr(rser.modify(r))
    }
  }

  object Collector extends Poly2 {
    implicit def myCase[ACC <: HList, E] = at[(E, Option[MyAnnotation]), ACC] {
      case ((e, None), acc) => e :: acc
      case ((e, Some(MyAnnotation(func))), acc) => {
        println(func)
        e :: acc
      }
    }
  }

  implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
    implicit
    gen: Generic.Aux[T, HL],
    ser: Lazy[Modifier[HL]],
    annots: Annotations.Aux[MyAnnotation, T, AL],
    zip: Zip.Aux[HL :: AL :: HNil, ZL],
    rightFolder: RightFolder[ZL, HNil.type, Collector.type]
  ): Modifier[T] = new Modifier[T] {
    override def modify(t: T): T = {
      val generic = gen.to(t)
      println(generic)
      val annotations = annots()
      println(annotations)
      val zipped = zip(generic :: annotations :: HNil)
      println(zipped)
      val modified = zipped.foldRight(HNil)(Collector)
      println(modified)

      val typed = gen.from(generic) // temporary
      typed
    }
  }
}

上面的代码编译通过.但是，在测试中实例化 Modifier 时:

The code above compiles. However, when instanciating a Modifier in a test:

  case class Test(a: String, @MyAnnotation("sha1") b: String)

  val test = Test("A", "B")
  val modifier: Modifier[Test] = implicitly

测试文件无法编译并给出以下错误:

the test file does not compile and give the following error:

  [error] ambiguous implicit values:
  [error]  both value StringCanBuildFrom in object Predef of type => 
           scala.collection.generic.CanBuildFrom[String,Char,String]
  [error]  and method $conforms in object Predef of type [A]=> <:<[A,A]
  [error]  match expected type T
  [error]       val ser1: Modifier[Test] = implicitly

问题似乎来自正确的文件夹定义:当从 genericModifier 中的隐式列表中删除 rightFolder 时，它起作用了:

The problem seems to come from the right folder definition: when removing rightFolder from the list of implicits in genericModifier, then it works:

  implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](
    implicit
    gen: Generic.Aux[T, HL],
    ser: Lazy[Modifier[HL]],
    annots: Annotations.Aux[MyAnnotation, T, AL],
    zip: Zip.Aux[HL :: AL :: HNil, ZL]/*,
    rightFolder: RightFolder[ZL, HNil.type, Collector.type]*/
  ): Modifier[T] = new Modifier[T] {
    override def modify(t: T): T = {
      val generic = gen.to(t)
      println(generic)
      val annotations = annots()
      println(annotations)
      val zipped = zip(generic :: annotations :: HNil)
      println(zipped)
      /*val modified = zipped.foldRight(HNil)(Collector)
      println(modified)*/

      val typed = gen.from(generic) // temporary
      typed
    }
  }

怎么了?

推荐答案

你的代码有几个错误:

• 只为 Option 定义 Poly 太粗糙了(模式匹配是在运行时执行的，编译器应该知道 Some 和None 在编译时)

• defining Poly just for Option is too rough (pattern matching is performed at runtime and compiler should know definitions for Some and None at compile time)

HNil 应该代替 HNil.type 和 HNil : HNil 而不是 HNil (HNil 和 HNil.type 类型不同)

HNil should be instead of HNil.type and HNil : HNil instead of HNil (types HNil and HNil.type are different)

编译器不知道 RightFolder 实际上返回原始的 HList 类型，因此您应该使用 RightFolder.Aux 类型.

compiler doesn't know that RightFolder actually returns the original HList type, so you should use RightFolder.Aux type.

正确的代码是

import shapeless.ops.hlist.{RightFolder, Zip}
import shapeless.{::, Annotations, Generic, HList, HNil, Lazy, Poly2}
import scala.annotation.StaticAnnotation

object App {
  case class MyAnnotation(func: String) extends StaticAnnotation

  object Collector extends Poly2 {
//    implicit def myCase[ACC <: HList, E] = at[(E, Option[PII]), ACC] {
//      case ((e, None), acc) => e :: acc
//      case ((e, Some(MyAnnotation(func))), acc) => {
//        println(func)
//        e :: acc
//      }
//    }

    implicit def someCase[ACC <: HList, E]: Case.Aux[(E, Some[MyAnnotation]), ACC, E :: ACC] = at {
      case ((e, Some(MyAnnotation(func))), acc) =>
        println(func)
        e :: acc
    }

    implicit def noneCase[ACC <: HList, E]: Case.Aux[(E, None.type), ACC, E :: ACC] = at {
      case ((e, None), acc) => e :: acc
    }
  }

  trait Modifier[T] {
    def modify(t: T): T
  }

  implicit def hListModifier[HL <: HList]: Modifier[HL] = identity(_) 
  // added as an example, you should replace this with your Modifier for HList

  implicit def genericModifier[T, HL <: HList, AL <: HList, ZL <: HList](implicit
    gen: Generic.Aux[T, HL],
    ser: Lazy[Modifier[HL]],
    annots: Annotations.Aux[MyAnnotation, T, AL],
    zip: Zip.Aux[HL :: AL :: HNil, ZL],
    rightFolder: RightFolder.Aux[ZL, HNil/*.type*/, Collector.type, HL /*added*/]
    ): Modifier[T] = new Modifier[T] {
    override def modify(t: T): T = {
      val generic = gen.to(t)
      println(generic)
      val annotations = annots()
      println(annotations)
      val zipped = zip(generic :: annotations :: HNil)
      println(zipped)
      val modified = zipped.foldRight(HNil : HNil /*added*/)(Collector)
      println(modified)

      val typed = gen.from(modified)
      typed
    }
  }

  case class Test(a: String, @MyAnnotation("sha1") b: String)

  val test = Test("A", "B")
  val modifier: Modifier[Test] = implicitly[Modifier[Test]]

  def main(args: Array[String]): Unit = {
    val test1 = modifier.modify(test) // prints "sha1"
    println(test1) // Test(A,B)
  }
}

这篇关于无形与注释的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！