Can't make simple tagless final example

Question

I've read about tagless final and I think it's great. I wanted to build my own small example of this pattern and got the problem.

This is my code:

  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
  }

This code doesn't work because of not found: value Calculator error. How can I do it properly?

Answer 1

Add materializer:

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

It's better to put instances of type class Calculator[F[_]] (like implicit futureCalculator) to the same companion object Calculator, otherwise you'll have to import calculatorInstances._.

Don't forget to import cats.syntax.flatMap._ and import cats.syntax.functor._.

sum in sum <- Calculator[F].sum(a, b) is of type Either[Throwable,Int] but sum in Calculator[F].minus(sum, b) is expected to be Int.

Maybe returning type of getPlusAndMinus should be F[Either[Throwable, Int]] instead of F[Either[String, Int]].

Maybe the easiest way to fix for comprehension is to use monad transformer:

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
}

Just in case, the whole code:

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
  }
}