无法制作简单的无标签最终示例

Question

我读过有关无标签决赛的内容，我认为这很棒。我想建立自己的这种模式的小例子，并解决了问题。

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
  }

这段代码由于not found: value Calculator错误而无法正常工作。我该怎么做呢？

Answer 1

添加物化器：

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

最好将类型为class的实例Calculator[F[_]]（例如隐式futureCalculator）放入同一个伴侣对象Calculator，否则必须这样做import calculatorInstances._。

别忘了import cats.syntax.flatMap._和import cats.syntax.functor._。

sumin sum <- Calculator[F].sum(a, b)是类型，Either[Throwable,Int]但sumin Calculator[F].minus(sum, b)应该是Int。

也许返回的类型getPlusAndMinus应该是F[Either[Throwable, Int]]代替F[Either[String, Int]]。

解决问题的最简单方法for就是使用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
  }
}