Scala and cats: Implicit conversion to identity monad

问题

I have a function that computes sum of squares for numeric types as shown below.

import cats.syntax.functor._
import cats.syntax.applicative._
import cats.{Id, Monad}

import scala.language.higherKinds

object PowerOfMonads {
       /**
        * Ultimate sum of squares method
        *
        * @param x First value in context
        * @param y Second value in context
        * @tparam F Monadic context
        * @tparam T Type parameter in the Monad
        * @return Sum of squares of first and second values in the Monadic context
        */
    def sumOfSquares[F[_]: Monad, A, T >: A](x: F[A], y: F[A])(implicit num: Numeric[T]) : F[T] = {
        def square(value:T): T = num.times(value, value)
        def sum(first:T, second:T): T = num.plus(first, second)

        for {
            first <- x
            second <- y
        } yield sum(square(first), square(second))
    }
}

From the client code, I would like to utilise the function as shown below

import cats.Id
import cats.instances.future._
import cats.instances.list._
import cats.instances.option._
import cats.syntax.applicative._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

println(s"Sum of squares for list ${PowerOfMonads.sumOfSquares(  List(1,2,3), List(1,2,3) )}")
println(s"Sum of squares for options ${PowerOfMonads.sumOfSquares(  Option(1), 2.pure[Option] )}")
println(s"Sum of squares for future ${PowerOfMonads.sumOfSquares(  1.pure[Future], Future(2) ).value}")
println(s"Sum of squares for id ${PowerOfMonads.sumOfSquares(1.pure[Id], 2.pure[Id])}")

Now, I would like to use implicit conversion from a numeric type T to Id[T] to invoke the function sumOfSquares as shown below

println(s"Sum of squares for int ${PowerOfMonads.sumOfSquares(1, 2)}")

using a function as shown below

import cats.syntax.applicative._
import scala.language.implicitConversions
   /**
    * Implicit conversion for any numeric type T to Id[T]
    * @param value value with type T
    * @tparam T numeric type
    * @return numeric type wrapped in the context of an Identity Monad
    */
implicit def anyNum2Id[T](value:T)(implicit num: Numeric[T]):Id[T] = value.pure[Id]

However, when executing the program, I get the following errors

could not find implicit value for evidence parameter of type cats.Monad[[A]Any] println(s"Sum of squares for int

${PowerOfMonads.sumOfSquares(1, 2)}") not enough arguments for method sumOfSquares: (implicit evidence$1: cats.Monad[[A]Any], implicit num: Numeric[T])Any. Unspecified value parameters evidence$1, num. println(s"Sum of squares for int ${PowerOfMonads.sumOfSquares(1, 2)}")

Please help me resolve the error.

回答1:

Change your imports:

  import cats.syntax.flatMap._
  import cats.syntax.functor._
  import cats.Monad

  import scala.language.higherKinds

  object PowerOfMonads {
  ...

You can help compiler to infer types:

PowerOfMonads.sumOfSquares(1: Id[Int], 2: Id[Int])

PowerOfMonads.sumOfSquares[Id, Int, Int](1, 2)

来源：https://stackoverflow.com/questions/48720903/scala-and-cats-implicit-conversion-to-identity-monad

标签

scala

monads

implicit-conversion

implicit

scala-cats