为一个值和该值的函子编写一个隐式类

假设您有来自Scalaz或Cats的这些类型类:

import scala.language.higherKinds

trait Functor[F[_]] {
  def map[A, B](a: F[A])(f: A => B): F[B]
}

type Id[X] = X
implicit object IdFunctor extends Functor[Id] {
  def map[A, B](a: A)(f: A => B): B = f(a)
}
implicit object OptionFunctor extends Functor[Option] {
  def map[A, B](a: Option[A])(f: A => B) = a map f
}

trait EverythingIsAlwaysAFunctor[A, B, F[_]] {
  def apply(a: A): F[B]
  def functor: Functor[F]
}

object EverythingIsAlwaysAFunctor {
  implicit def functorIsFunctor[A, F[_]](implicit f: Functor[F])
  : EverythingIsAlwaysAFunctor[F[A], A, F] = {
    new EverythingIsAlwaysAFunctor[F[A], A, F] {
      def apply(fa: F[A]): F[A] = fa
      def functor: Functor[F] = f
    }
  }

  implicit def idIsAlsoAFunctor[A]
  : EverythingIsAlwaysAFunctor[A, A, Id] = {
    new EverythingIsAlwaysAFunctor[A, A, Id] {
      def apply(a: A): Id[A] = a
      def functor: Functor[Id] = implicitly[Functor[Id]]
    }
  }
}

1. A 已经成形 F[B] F ,然后使用这个函子
2. A 实际上是 Id[A]

现在你可以写你的 DoSomething doSomething 方法:

implicit class DoSomething[A, F[_]](a: A)(
  implicit eiaaf: EverythingIsAlwaysAFunctor[A, Int, F]
) {
  def doSomething: F[String] = eiaaf.functor.map(eiaaf(a))("*" * _)
}

然后它在所有情况下都有效:

val x = Option(42).doSomething
val y = 42.doSomething

println(x)
println(y)

印刷品:

Some(******************************************)
******************************************