我有一个计算数字类型的平方和的函数,如下所示。
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))
}
}
从客户端代码中,我想使用如下所示的功能
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])}")
现在,我想使用从数字类型 T 到 Id[T] 的隐式转换来调用函数 sumOfSquares ,如下所示
println(s"Sum of squares for int ${PowerOfMonads.sumOfSquares(1, 2)}")
使用如下所示的函数
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]
但是,在执行程序时,出现以下错误
找不到类型证据参数的隐式值
cats.Monad[[A]Any] println(s"int 的平方和
${PowerOfMonads.sumOfSquares(1, 2)}") 方法参数不足
sumOfSquares:(隐式证据$1:cats.Monad[[A]Any],隐式num:
数字[T])任意。未指定的值参数evidence$1,num。
println(s"int ${PowerOfMonads.sumOfSquares(1,
2)}")
请帮助我解决该错误。