scala - 如何使用shapeless检测字段类型注释

Question

我正在尝试使用 shapeless 在编译时收集具有特定注释的案例类的字段.我尝试使用以下代码段，但它没有按预期工作(不输出任何内容而不是打印“i”)。我怎样才能让它工作？

import shapeless._
import shapeless.labelled._

final class searchable() extends scala.annotation.StaticAnnotation
final case class Foo(@searchable i: Int, s: String)

trait Boo[A] {
  def print(a: A): Unit
}
sealed trait Boo0 {
  implicit def hnil = new Boo[HNil] { def print(hnil: HNil): Unit = () }
  implicit def hlist[K <: Symbol, V, RL <: HList](implicit b: Boo[RL]): Boo[FieldType[K, V] :: RL] =
    new Boo[FieldType[K, V] :: RL] {
      def print(a: FieldType[K, V] :: RL): Unit = {
        b.print(a.tail)
      }
    }
}
sealed trait Boo1 extends Boo0 {
  implicit def hlist1[K <: Symbol, V, RL <: HList](implicit annot: Annotation[searchable, K], witness: Witness.Aux[K], b: Boo[RL]): Boo[FieldType[K, V] :: RL] =
    new Boo[FieldType[K, V] :: RL] {
      def print(a: FieldType[K, V] :: RL): Unit = {
        Console.println(witness.value.name)
        b.print(a.tail)
      }
    }
}
object Boo extends Boo1 {
  implicit def generics[A, HL <: HList](implicit iso: LabelledGeneric.Aux[A, HL], boo: Boo[HL]): Boo[A] =
    new Boo[A] {
      def print(a: A): Unit = {
        boo.print(iso.to(a))
      }
    }
}

implicitly[Boo[Foo]].print(Foo(1, "2"))

Answer 1

看Annotation的宏，它直接拒绝不是产品或副产品的类型

val annTreeOpts =
  if (isProduct(tpe)) { ... }
  else if (isCoproduct(tpe)) { ... }
  else abort(s"$tpe is not case class like or the root of a sealed family of types")

这是非常不幸的，因为在每个字段符号级别收集类型注释有时可能非常有用。

还有一个类型类 Annotations在同一个文件中定义，该文件实际上可以将字段上的特定注释收集到 HList 中.然而问题是字段信息完全丢失。有一种笨拙的方法可以将事物组合在一起以服务于我的用例......

// A is our annotation
// B is our result type    
// C is our case class with some fields annotated with A

def empty: B = ???
def concat(b1: B, b2: B): B = ???
def func(a: A, nm: String): B = ???

object Collector extends Poly2 {
   implicit def some[K <: Symbol](implicit witness: Witness.Aux[K]) = 
     at[B, (K, Some[A])] { case (b, (_, a)) => concat(b, func(a.get, witness.value.name)) }
   implicit def none[K <: Symbol] = at[B, (K, None.type)] { case (b, _) => b }
}

def collect[HL <: HList, RL <: HList, KL <: HList, ZL <: HList](implicit 
    iso: LabelledGeneric.Aux[C, HL]
  , annot: Annotations.Aux[A, C, RL]
  , keys: Keys.Aux[HL, KL]
  , zip: Zip.Aux[KL :: RL :: HNil, ZL]
  , leftFolder: LeftFolder.Aux[ZL, B, Collector.type, B]): B = {
  zip(keys() :: annot() :: HNil).foldLeft(empty)(Collector)
}