I am doing the Functor exercise from the "Scala with Cats" book. One of the exercises is to define a functor for a binary tree.
Here is my attempt (I put this code in a scala worksheet):
import cats.Functor
sealed trait Tree[+A]
final case class Branch[A](left: Tree[A], right: Tree[A]) extends Tree[A]
final case class Leaf[A](value: A) extends Tree[A]
object Tree {
def branch[A](left: Tree[A], right: Tree[A]): Tree[A] = {
Branch(left, right)
}
def leaf[A](value: A): Tree[A] = {
Leaf(value)
}
}
implicit val treeFunctor: Functor[Tree] = new Functor[Tree] {
def map[A, B](value: Tree[A])(func: A => B): Tree[B] = {
value match {
case l: Leaf[A] => Leaf(func(l.value))
case b: Branch[A] => Branch(map(b.left)(func), map(b.right)(func))
}
}
}
Tree.branch(Tree.leaf(10), Tree.leaf(20)).map(_ * 2)
This fails, it says "cannot resolve symbol map" in the final line.
What is going on and how can I fix it? As far as I am aware, I have an equivalent solution to the one provide in the book.
import cats.implicits._
is missing which gives you Ops
extension methods
import cats.implicits._
Tree.branch(Tree.leaf(10), Tree.leaf(20)).map(_ * 2)
// res0: Tree[Int] = Branch(Leaf(20),Leaf(40))
this works because it expands to
toFunctorOps(Tree.branch(Tree.leaf(10), Tree.leaf(20)))(treeFunctor).map(_ * 2)
Without extension methods in scope you could summon treeFunctor
instance using companion object with main method convention Functor.apply[Tree]
val tree = Tree.branch(Tree.leaf(10), Tree.leaf(20))
Functor[Tree].map(tree)(_ * 2)
// res0: Tree[Int] = Branch(Leaf(20),Leaf(40))