scala cats applicative failed to compile

Question

I try to follow https://typelevel.org/cats/typeclasses/applicative.html

trait Applicative[F[_]] extends Functor[F] {
    def product[A, B](fa: F[A], fb: F[B]): F[(A, B)]

    def pure[A](a: A): F[A]
  }

  // Example implementation for right-biased Either
  implicit def applicativeForEither[L]: Applicative[Either[L, *]] = new Applicative[Either[L, *]] {
    def product[A, B](fa: Either[L, A], fb: Either[L, B]): Either[L, (A, B)] = (fa, fb) match {
      case (Right(a), Right(b)) => Right((a, b))
      case (Left(l) , _       ) => Left(l)
      case (_       , Left(l) ) => Left(l)
    }

    def pure[A](a: A): Either[L, A] = Right(a)

    def map[A, B](fa: Either[L, A])(f: A => B): Either[L, B] = fa match {
      case Right(a) => Right(f(a))
      case Left(l)  => Left(l)
    }
  }

It fails to compile with error:

not found: type * implicit def applicativeForEither[L]: Applicative[Either[L, *]] = new Applicative[Either[L, *]] {

In cat's it uses '?' instead of '*' (e.g. EitherTFunctor) but it also not compile when I copy-paste it.

What should I do to fix it?

Answer 1

To make your code compilable with asterics in type params you should add kind-projector plugin into your build.sbt file or plugins.sbt file:

addCompilerPlugin("org.typelevel" % "kind-projector" % "0.13.2" cross CrossVersion.full)

read more about kind-projector at the README.MD