Trait with Higher-Kinded Types

Question

Given the following trait (from this helpful shapeless talk):

scala> trait NatT[F[_], G[_]] { def apply[T](f: F[T]): G[T] } warning: there were two feature warnings; re-run with -feature for details defined trait NatT

I believe this means that NatT accepts two higher-kinded parameters: F and G.

With this assumption, I tried to make an instance where F and G have type Option:

scala> case object Maybe extends NatT[Option, Option] { | override def apply(f: Option[Int]) = f | } <console>:8: error: object creation impossible, since method apply in trait NatT of type [T](f: Option[T])Option[T] is not de fined case object Maybe extends NatT[Option, Option] { ^ <console>:9: error: method apply overrides nothing. Note: the super classes of object Maybe contain the following, non final members named apply: def apply[T](f: Option[T]): Option[T] override def apply(f: Option[Int]) = f ^

How can I fix this attempt at making a Maybe instance?

Michael Zajac · Accepted Answer · 2015-04-06 14:11:08Z

Your apply method is missing the type parameter. Simple as that.

case object Maybe extends NatT[Option, Option] { def apply[A](f: Option[A]): Option[A] = f }

Your attempts to define apply without a type parameter are seen as a different method, so it appears that apply is unimplemented. Given that F and G are supposed to be higher-kinds, it doesn't really make sense to try to fix them to Option[Int].

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