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使用无形状更新案例类字段

shapeless scala

simao · 技术社区 · 7 年前

我定义了以下类型:

sealed trait Refreshable {
  val expireAt: Instant
  val version: Int
}

case class A(id: Int, expireAt: Instant, version: Int) extends Refreshable

case class B(name: String, expireAt: Instant, version: Int) extends Refreshable

我想定义一种方法 refresh 这会更新这些类型的两个字段,但我找不到我需要的隐式。看起来我应该可以用shapeless做这样的事情?

def refresh[T <: RefreshableClass])(v: T)(implicit ev: LabelledGeneric[T]) = {
  val gen = ev.from(v)

  gen.updateWith('expire)(_.plus(...)).updateWith('version)(_+1)
}

2 回复 | 直到 7 年前

Oleg Pyzhcov 7 年前

可能最简单的方法是使用不成形透镜。首先,一些进口:

import java.time._
import shapeless._

case class A(id: Int, expireAt: Instant, version: Int)
case class B(name: String, expireAt: Instant, version: Int)

创建路径,选择标记为expireAt/version的产品元素

val (expirePath, versionPath) = (^.expireAt, ^.version)

def refresh[T](t: T)(implicit
                    expireL: expirePath.Lens[T, Instant],
                    versionL: versionPath.Lens[T, Int]): T = {
  val t1 = expireL().set(t)(Instant.now())
  val t2 = versionL().modify(t1)(_ + 1)

  t2
}

使用它:

println(refresh(A(1, null, 0)))
println(refresh(B("The B", null, 4)))

Dmytro Mitin 7 年前

~~把你的特质封存起来。~~

sealed trait Refreshable {
  val expireAt: Instant
  val version: Int
}

具有 Typelevel Scala

def refresh[T <: Refreshable, R <: HList](v: T)(implicit
  generic: LabelledGeneric.Aux[T, R],
  modifier: Modifier.Aux[R, Symbol @@ "version", Int, Int, R],
  modifier1: Modifier.Aux[R, Symbol @@ "expireAt", Instant, Instant, R]): R = {
  val rec = generic.to(v)
  val rec1 = modifier(rec, _ + 1)
  modifier1(rec1, _.plus(1L, ChronoUnit.MILLIS))
}

def refresh[T <: Refreshable, R <: HList](v: T)(implicit
  generic: LabelledGeneric.Aux[T, R],
  modifier: Modifier.Aux[R, Witness.`'version`.T, Int, Int, R],
  modifier1: Modifier.Aux[R, Witness.`'expireAt`.T, Instant, Instant, R]): R = {
  val rec = generic.to(v)
  val rec1 = modifier(rec, _ + 1)
  modifier1(rec1, _.plus(1L, ChronoUnit.MILLIS))
}

如果你喜欢与 updateWith 这也是可能的:

def refresh[T <: Refreshable, R <: HList](v: T)(implicit
  generic: LabelledGeneric.Aux[T, R],
  modifier: Modifier.Aux[R, Witness.`'version`.T, Int, Int, R],
  modifier1: Modifier.Aux[R, Witness.`'expireAt`.T, Instant, Instant, R],
  selector: Selector.Aux[R, Witness.`'version`.T, Int],
  selector1: Selector.Aux[R, Witness.`'expireAt`.T, Instant]): R = {
  val rec = generic.to(v)
  rec.updateWith('expireAt)(_.plus(1L, ChronoUnit.MILLIS))
    .updateWith('version)(_ + 1)
}

通常不需要指定类型参数 T R ,应推断:

refresh(A(1, Instant.now, 1)) //1 :: 2017-09-20T18:53:34.039Z :: 2 :: HNil
refresh(B("a", Instant.now, 1)) // a :: 2017-09-20T18:53:34.074Z :: 2 :: HNil

但如果您确实需要,可以指定它们:

refresh[A, Record.`'id -> Int, 'expireAt -> Instant, 'version -> Int`.T](A(1, Instant.now, 1))
refresh[B, Record.`'name -> String, 'expireAt -> Instant, 'version -> Int`.T](B("a", Instant.now, 1))