scala - 获取字段键作为副产品

Question

import shapeless._
import syntax.singleton._

val book = ("author" ->> "Benjamin Pierce") ::
  ("title"  ->> "Types and Programming Languages") ::
  ("id"     ->>  "foo") ::
  ("price"  ->>  "bar") ::
  HNil

我想找到映射到 "foo" 的字段键(在本例中为 "id")并检索它，输入为 4 单例的联积字符串。

我该怎么做？

Answer 1

我建议使用自定义类型类以不同方式(并且更通用)来执行此操作:

import shapeless._
import shapeless.labelled.FieldType

trait RecSearch[R <: HList, A] {
  type Keys <: Coproduct

  def find(r: R, a: A): Option[Keys]
}

object RecSearch extends LowPriorityRecSearchInstances {
  implicit def hnilRecSearch[A]: Aux[HNil, A, CNil] =
    new RecSearch[HNil, A] {
      type Keys = CNil

      def find(r: HNil, v: A): Option[CNil] = None
    }

  implicit def cconsRecSearch1[K, V, T <: HList](implicit
    trs: RecSearch[T, V],
    wit: Witness.Aux[K]
  ): Aux[FieldType[K, V] :: T, V, K :+: trs.Keys] =
    new RecSearch[FieldType[K, V] :: T, V] {
      type Keys = K :+: trs.Keys

      def find(r: FieldType[K, V] :: T, a: V): Option[K :+: trs.Keys] =
        if (r.head == a)
          Some(Coproduct[K :+: trs.Keys](wit.value))
        else trs.find(r.tail, a).map(_.extendLeft[K])
    }
}

trait LowPriorityRecSearchInstances {
  type Aux[R <: HList, A, K <: Coproduct] = RecSearch[R, A] { type Keys = K }

  implicit def cconsRecSearch0[A, K, V, T <: HList](implicit
    trs: RecSearch[T, A]
  ): Aux[FieldType[K, V] :: T, A, K :+: trs.Keys] =
    new RecSearch[FieldType[K, V] :: T, A] {
      type Keys = K :+: trs.Keys

      def find(r: FieldType[K, V] :: T, a: A): Option[K :+: trs.Keys] =
        trs.find(r.tail, a).map(_.extendLeft[K])
    }
}

然后:

def search[R <: HList, A](r: R)(a: A)(implicit rs: RecSearch[R, A]): Option[rs.Keys] =
  rs.find(r, a)

最后:

import syntax.singleton._

val book = ("author" ->> "Benjamin Pierce") ::
  ("title"  ->> "Types and Programming Languages") ::
  ("id"     ->>  "foo") ::
  ("price"  ->>  "bar") ::
  HNil

search(book)("foo")

这将向您显示(非常详细的)副产品类型和正确的值:

Some(Inr(Inr(Inl(id))))

你的版本不适合我编译，所以我不能真正比较两者，但 95% 的时间我开始使用 Poly 我最终切换到一个类型类一些点。