isosubtyping.scala

import cats.{Applicative, Contravariant, Functor}
import cats.implicits._

implicit def covariance[F[_]: Functor, A, B >: A](fa: F[A]): F[B] =
  fa.widen

implicit def contravariance[F[_]: Contravariant, A, B <: A](fa: F[A]): F[B] =
  fa.narrow

def test[F[_]: Applicative, A](a: A) = {
  if (42 > 24)
    Some(a).pure[F]
  else
    None.pure[F]
}

Very nice! Unfortunately it still compiles without the covariance implicit, but the return type of test is Any rather than F[Option[A]] :-)

That is quite correct that it compiles without the implicit, but obviously Any isn't a useful type here. :-) Normally you would hit this case with a type annotation already on test. Without covariance, the type of the if/else is inferred to Any and then the type annotation F[Option[A]] would be an error. The implicit fixes this.

The only reason I didn't put the annotation on here is I wanted to test that the type inference was able to piece together the correct thing without any help. There are a number of situations where type inference uses expected result type in working out what a type should be, particularly when resolving implicits. I just wanted to avoid that situation entirely.