使用Scalaz(或Shapeless)为每个子类创建Monoids

发布时间：2020-12-16 18:23:01 所属栏目：安全来源：网络整理

导读：是否有可能为每个子类创建幺半群？例如, package currency final case class GBP[A: Monoid](amount: A)object Implicits { implicit class CurrencyOps[A: Monoid](a: A) { def GBP = currency.GBP(a) } implicit def gbpMonoid[A: Monoid]: Monoid[GBP[A]]

是否有可能为每个子类创建幺半群？例如,

package currency 

final case class GBP[A: Monoid](amount: A)

object Implicits {
  implicit class CurrencyOps[A: Monoid](a: A) {
    def GBP = currency.GBP(a)
  }

  implicit def gbpMonoid[A: Monoid]: Monoid[GBP[A]] = new Monoid[GBP[A]] {
    override def zero =
      GBP(Monoid[A].zero)

    override def append(f1: GBP[A],f2: => GBP[A]): GBP[A] =
      GBP(Semigroup[A].append(f1.amount,f2.amount))
  }
}

test("GBP support plus") {
  1.GBP |+| 2.GBP shouldBe 3.GBP // passed
}

我想添加更多代表货币的案例类(例如USD,EUR,..)

sealed trait Currency
final case class GBP[A: Monoid](amount: A) extends Currency
final case class USD[A: Monoid](amount: A) extends Currency
final case class EUR[A: Monoid](amount: A) extends Currency

因此,我必须为新案例类实现monoids.它有点样板.

implicit class CurrencyOps[A: Monoid](a: A) {
  def GBP = currency.GBP(a)
  def EUR = currency.EUR(a)
  def USD = currency.USD(a)
}

implicit def gbpMonoid[A: Monoid]: Monoid[GBP[A]] = new Monoid[GBP[A]] {
  override def zero =
    GBP(Monoid[A].zero)

  override def append(f1: GBP[A],f2: => GBP[A]): GBP[A] =
    GBP(Semigroup[A].append(f1.amount,f2.amount))
}

implicit def usdMonoid[A: Monoid]: Monoid[USD[A]] = new Monoid[USD[A]] {
  override def zero =
    USD(Monoid[A].zero)

  override def append(f1: USD[A],f2: => USD[A]): USD[A] =
    USD(Semigroup[A].append(f1.amount,f2.amount))
}

implicit def eurMonoid[A: Monoid]: Monoid[EUR[A]] = new Monoid[EUR[A]] {
  override def zero =
    EUR(Monoid[A].zero)

  override def append(f1: EUR[A],f2: => EUR[A]): EUR[A] =
    EUR(Semigroup[A].append(f1.amount,f2.amount))
}

解决方法

小建议

首先,我想建议从案例类中删除Monoid需求,因为它们将在每个Currency实例中携带隐式值.如果没有这个要求,你的包装器可以更高效,甚至可以实现为value classes：

sealed trait Currency extends Any
  final case class GBP[A](amount: A) extends AnyVal with Currency
  final case class USD[A](amount: A) extends AnyVal with Currency
  final case class EUR[A](amount: A) extends AnyVal with Currency

无形的实施

从这里,您可以根据需要通过无形构建简单的实现：

import scalaz._
import shapeless._

implicit def monoidCurrency[A,C[_] <: Currency]
(implicit monoid: Monoid[A],gen: Generic.Aux[C[A],A :: HNil]) =
  new Monoid[C[A]] {
    def zero: C[A] = gen.from(monoid.zero :: HNil)
    def append(f1: C[A],f2: => C[A]): C[A] = {
      val x = gen.to(f1).head
      val y = gen.to(f2).head
      gen.from(monoid.append(x,y) :: HNil)
    }
  }

并验证它

import scalaz.syntax.monoid._
import scalaz.std.anyVal._

println(2.USD |+| 3.USD) // USD(5)

进一步改进

你可以摆脱没有形状.考虑这样的实现：

trait CurrencyUnit{
  def show(amounts: String) = s"$amounts $this"
}

final case class Currency[A,U <: CurrencyUnit](amount: A) extends AnyVal

CurrencyUnit现在不是类的问题,它只是编译时类型标记

implicit case object GBP extends CurrencyUnit
implicit case object USD extends CurrencyUnit{
  override def show(amounts: String) = s"$$$amounts "
}
implicit case object EUR extends CurrencyUnit

implicit class CurrencyOps[A](a: A) {
  def GBP = Currency[A,GBP.type](a)
  def EUR = Currency[A,EUR.type](a)
  def USD = Currency[A,USD.type](a)
}

您可以根据需要进行配置

import scalaz.syntax.show._

implicit def currencyShow[A: Show,U <: CurrencyUnit](implicit unit: U) =
  new Show[Currency[A,U]] {
    override def shows(f: Currency[A,U]) = unit.show(f.amount.shows)
  }

最重要的是通过scalaz.Isomorphism.Iso功能很容易派生类型类：

import Isomorphism._

implicit def currencyIso[A,U <: CurrencyUnit] = new (Currency[A,U] <=> A) {
  def to: (Currency[A,U]) => A = _.amount
  def from: (A) => Currency[A,U] = Currency[A,U]
}

implicit def currencyMonoid[A: Monoid,U <: CurrencyUnit] =
  new IsomorphismMonoid[Currency[A,U],A] {
    def G: Monoid[A] = implicitly
    def iso: Currency[A,U] <=> A = implicitly
  }

最后,您也可以验证此解决方案

import scalaz.syntax.monoid._
import scalaz.std.anyVal._

println((2.USD |+| 3.USD).shows) // $5

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!