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trait InvariantFunctor[F[_]] {
def xmap[A, B](ma: F[A], f: A => B, g: B => A): F[B]
}
object InvariantFunctor {
import Endo._
implicit val EndoInvariantFunctor = new InvariantFunctor[Endo] {
def xmap[A, B](ma: Endo[A], f: A => B, g: B => A): Endo[B] = (b: B) => f(ma(g(b)))
}
// The definition about fails with:
// anon-type.scala:9: error: not found: value b
// def xmap[A, B](ma: Endo[A], f: A => B, g: B => A): Endo[B] = (b: B) => f(ma(g(b)))
// ^
// anon-type.scala:8: error: not found: type $anon
// implicit val EndoInvariantFunctor = new InvariantFunctor[Endo] {
// ^
// These both work:
// implicit val EndoInvariantFunctorAscribed: InvariantFunctor[Endo] = new InvariantFunctor[Endo] {
// def xmap[A, B](ma: Endo[A], f: A => B, g: B => A): Endo[B] = (b: B) => f(ma(g(b)))
// }
//
// implicit val EndoInvariantFunctorStubbed = new InvariantFunctor[Endo] {
// def xmap[A, B](ma: Endo[A], f: A => B, g: B => A): Endo[B] = error("stub")
// }
}
trait Endo[X]
object Endo {
implicit def EndoTo[A](f: A => A): Endo[A] = new Endo[A] {
def apply(a: A) = f(a)
}
implicit def EndoFrom[A](e: Endo[A]): A => A = e.apply(_)
}
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