Select 字段（按类型）

Question

我正在尝试创建一个类型 class，允许 select 给定类型的字段。这是我到目前为止所做的，但编译器无法找到 Selector.Aux

case class AddressKey(street: String, city: String)

trait AddressKeySelector[A] {
  def addressKey(v: A): AddressKey
  def addressKeyColumnName: String
}

object AddressKeySelector {

  implicit def typeAddressKeySelector[A, Repr <: HList, K](
      implicit gen: Generic.Aux[A, Repr],
      reprSelector: Selector.Aux[Repr, K, AddressKey]): AddressKeySelector[A] =
    new AddressKeySelector[A] {
      override def addressKey(v: A): AddressKey = reprSelector(gen.to(v))

      override def addressKeyColumnName: String = ???
    }
}

会这样使用

case class MyType(addressKeyField: AddressKey, otherField: String)
val data = MyType(AddressKey("addr", "city"), "other")
val addrKey = AddressKeySelector[MyType].addressKey(data) 
// addrKey: AddressKey = AddressKey(addr,city) 
val addrField = AddressKeySelector[MyType].addressKeyColumnName(data)
// addrField: String = addressKeyField

当只有一个字段的类型为 AddressKey 时，这应该才有效。关于如何实施它的任何想法？

Answer 1

ops.hlist.Selector 没有 Aux，所以我假设您正在使用那个。鉴于您想要提取您可能想要使用 LabelledGeneric 和 ops.record.Selector 的字段名称。但是 Selector.Aux[Repr, K, AddressKey] 仍然不起作用，因为 Selector 只能按 "key" (K) 搜索，而不能按 "value" (AddressKey) 搜索，而你正在按值搜索键。

理想情况下，我认为你会像这样实现它：

import shapeless._, ops.record._

trait AddressKeySelector[A] {
  def addressKey(v: A): AddressKey
  def addressKeyColumnName: String
}

object AddressKeySelector {
  def apply[A](implicit sel: AddressKeySelector[A]): sel.type = sel

  implicit def typeAddressKeySelector[A, Repr <: HList, K <: Symbol, Swapped <: HList](
      implicit 
      gen: LabelledGeneric.Aux[A, Repr],
      swap: SwapRecord.Aux[Repr, Swapped],
      swappedSelector: Selector.Aux[Swapped, AddressKey, K],
      reprSelector: Selector.Aux[Repr, K, AddressKey]
  ): AddressKeySelector[A] =
    new AddressKeySelector[A] {
      override def addressKey(v: A): AddressKey = reprSelector(gen.to(v))

      override def addressKeyColumnName: String = swappedSelector(swap()).name
    }
}

但是对于 some reason 这不起作用。

所以我认为您最好的选择是自己实施按值搜索。它相对简单。您可以从 ops.hlist.Selector 中获得灵感。请记住，LabelledGeneric 发出的 Hlist 中的每个条目都被编码为 labelled.FieldType[Key,Value] 并且您可以获得 Key 的运行时值（或者换句话说：字段名称）与 Witness.Aux[Key].

Select 字段（按类型）

Select field by type

scala

shapeless

scala-implicits