在不同类型的案例类之间安全地复制字段

Question

假设您有如下案例类

case class Test1(a:String,b:Int,c:Char)

case class Test2(a:String,b:Int)

然后你用以下变量实例化类

val test1 = Test1("first",2,'3')

val test2 = Test2("1st",20)

有没有办法使用.copy 方法（或其他方法），将Test2 中的变量应用于Test1，类似于

val test3 = test1.copy(test2) //Note this isn't valid scala code
// Result should be ("1st",20,'3')

如果这在纯 scala 中是不可能的，如何在 Shapeless 1/2 中完成（当前代码在 Shapeless 1 中，但我们计划在某个时间点升级到 Shapeless 2）

Answer 1

在 shapeless 2.0.0 中，可以这样做，

scala> import shapeless._
import shapeless._

scala> case class Test1(a: String, b: Int, c: Char)
defined class Test1

scala> case class Test2(a: String, b: Int)
defined class Test2

scala> val test1 = Test1("first", 2, '3')
test1: Test1 = Test1(first,2,3)

scala> val test2 = Test2("1st", 20)
test2: Test2 = Test2(1st,20)

scala> val test1Gen = Generic[Test1]
test1Gen: ... = $1$$1@6aac621f

scala> val test2Gen = Generic[Test2]
test2Gen: ... = $1$$1@5aa4954c

scala> val test3 = test1Gen.from(test2Gen.to(test2) :+ test1.c)
test3: Test1 = Test1(1st,20,3)

请注意，这是对每个案例类中字段顺序的假设，而不是使用字段标签信息。如果您有多个相同类型的字段，这可能很容易出错：类型可能排列在一起，但潜在语义可能会改变。

我们可以通过使用 shapeless 的 LabelledGeneric 来解决这个问题。 LabelledGeneric 将案例类值映射到无形的可扩展记录，这些记录除了捕获字段值的类型外，还通过相应的 Scala Symbol 的单例类型对类型中的字段名称进行编码。通过一些额外的基础设施（我将很快将其添加到 shapeless 2.1.0），这使我们能够以最少的样板安全地在案例类之间进行映射，

import shapeless._, record._, syntax.singleton._, ops.hlist.Remove

/**
 * This will be in shapeless 2.1.0 ...
 *
 * Permute the elements of the supplied `HList` of type `L` into the same order
 * as the elements of the `HList` of type `M`.
 */
trait Align[L <: HList, M <: HList] extends (L => M) {
  def apply(l: L): M
}

object Align {
  def apply[L <: HList, M <: HList]
    (implicit alm: Align[L, M]): Align[L, M] = alm

  implicit val hnilAlign: Align[HNil, HNil] = new Align[HNil, HNil] {
    def apply(l: HNil): HNil = l
  }

  implicit def hlistAlign[L <: HList, MH, MT <: HList, R <: HList]
    (implicit
      select: Remove.Aux[L, MH, (MH, R)],
      alignTail: Align[R, MT]): Align[L, MH :: MT] = new Align[L, MH :: MT] {
    def apply(l: L): MH :: MT = {
      val (h, t) = l.removeElem[MH]
      h :: alignTail(t)
    }
  }
}

/**
 * This, or something like it, will be in shapeless 2.1.0 ...
 *
 * Utility trait intended for inferring a field type from a sample value and
 * unpacking it into its key and value types.
 */
trait Field {
  type K
  type V
  type F = FieldType[K, V]
}

object Field {
  def apply[K0, V0](sample: FieldType[K0, V0]) =
    new Field { type K = K0; type V = V0 }
}

object OldWineNewBottle {
  case class From(s1: String, s2: String)
  case class To(s2: String, i: Int, s1: String)

  val from = From("foo", "bar")

  val fromGen = LabelledGeneric[From]
  val toGen = LabelledGeneric[To]

  // Define the type of the i field by example
  val iField = Field('i ->> 0)

  val align = Align[iField.F :: fromGen.Repr, toGen.Repr]

  // extend the fields of From with a field for 'i', permute into
  // the correct order for To and create a new instance ...
  val to = toGen.from(align('i ->> 23 :: fromGen.to(from)))

  assert(to == To("bar", 23, "foo"))
}