Created
August 5, 2020 19:09
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Cats .widen (covariant functors) and .narrow (contravariant functors): both are for upcasts, and thus safe
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package example | |
import cats._ | |
import cats.implicits._ | |
sealed trait List[+A] | |
case object Nil extends List[Nothing] | |
final case class Cons[A](head: A, tail: List[A]) extends List[A] | |
final case class Co[I, A](run : I => A) | |
object Co { | |
implicit def cofunctor[I]: Functor[Co[I, *]] = new Functor[Co[I, *]]() { | |
override def map[A,B](x: Co[I, A])(f: A => B): Co[I, B] = | |
Co(x.run andThen f) | |
} | |
} | |
final case class Contra[O, A](run : A => O) | |
object Contra { | |
implicit def contrafunctor[O]: Contravariant[Contra[O, *]] = new Contravariant[Contra[O, *]]() { | |
override def contramap[A,B](x : Contra[O, A])(f: B => A): Contra[O, B] = | |
Contra(f andThen x.run) | |
} | |
} | |
object Example { | |
import Co._ | |
def singleton[A]: Co[A, Cons[A]] = | |
Co { x => Cons(x, Nil) } | |
def single[A]: Co[A, List[A]] = | |
singleton[A].widen | |
def badHead[A]: Contra[A, List[A]] = | |
Contra { xs => | |
xs match { | |
case Nil => throw new IllegalArgumentException() | |
case Cons(x, _) => x | |
} | |
} | |
def head[A]: Contra[A, Cons[A]] = | |
badHead[A].narrow | |
} |
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