scala - 无法制作简单的无标签最终示例

Question

我读过关于无标签决赛的文章，我认为这很棒。我想建立我自己的这种模式的小例子并遇到了问题。

这是我的代码：

  trait Calculator[F[_]] {
    def sum(a: Int, b: Int): F[Either[Throwable, Int]]
    def minus(a: Int, b: Int): F[Either[Throwable, Int]]
  }

  object calculatorInstances {
    implicit val futureCalculator: Calculator[Future] = new Calculator[Future] {
      override def sum(a: Int, b: Int) =
        Future {
          Try(a + b).toEither
        }
      override def minus(a: Int, b: Int) =
        Future {
          Try(a - b).toEither
        }
    }
  }


  import calculatorInstances.futureCalculator
  def getPlusAndMinus[F[_]: Monad: Calculator](a: Int, b: Int): F[Either[String, Int]] = {
    for {
      sum <- Calculator[F].sum(a, b)
      res <- Calculator[F].minus(sum, b)
    } yield res
  }

not found: value Calculator由于错误，此代码不起作用。我怎样才能正确地做到这一点？

Answer 1

添加物化器：

object Calculator {
  def apply[F[_]: Calculator]: Calculator[F] = implicitly
}

最好将类型类的实例Calculator[F[_]]（如隐式futureCalculator）放在同一个伴生对象Calculator中，否则您将不得不这样做import calculatorInstances._。

不要忘记import cats.syntax.flatMap._和import cats.syntax.functor._。

suminsum <- Calculator[F].sum(a, b)是 typeEither[Throwable,Int]但suminCalculator[F].minus(sum, b)应该是Int.

也许返回类型 ofgetPlusAndMinus应该是F[Either[Throwable, Int]]而不是F[Either[String, Int]].

也许修复for理解的最简单方法是使用 monad 转换器：

def getPlusAndMinus[F[_] : Monad: Calculator](a: Int, b: Int): F[Either[Throwable, Int]] = {
  (for {
    sum <- EitherT(Calculator[F].sum(a, b))
    res <- EitherT(Calculator[F].minus(sum, b))
  } yield res).value
}

以防万一，整个代码：

import cats.data.EitherT
import cats.Monad
//import cats.syntax.flatMap._ // not necessary if we use EitherT
//import cats.syntax.functor._

import scala.concurrent.Future
import scala.language.higherKinds
import scala.util.Try
import scala.concurrent.ExecutionContext.Implicits.global

object App {

  trait Calculator[F[_]] {
    def sum(a: Int, b: Int): F[Either[Throwable, Int]]
    def minus(a: Int, b: Int): F[Either[Throwable, Int]]
  }

  object Calculator {
    def apply[F[_]: Calculator]: Calculator[F] = implicitly

    implicit val futureCalculator: Calculator[Future] = new Calculator[Future] {
      override def sum(a: Int, b: Int) =
        Future {
          Try(a + b).toEither
        }
      override def minus(a: Int, b: Int) =
        Future {
          Try(a - b).toEither
        }
    }
  }

  def getPlusAndMinus[F[_] : Monad: Calculator](a: Int, b: Int): F[Either[Throwable, Int]] = {
    (for {
      sum <- EitherT(Calculator[F].sum(a, b))
      res <- EitherT(Calculator[F].minus(sum, b))
    } yield res).value
  }
}