Implement Traverse for ByColor

src/main/scala/ByColor.scala

@@ -4,6 +4,8 @@ import cats.{ Applicative, Eq, Functor, Monoid }
 import cats.syntax.all.*
 import scala.annotation.targetName
 import alleycats.Zero
+import cats.Traverse
+import cats.Eval
 
 case class ByColor[A](white: A, black: A):
 
@@ -104,6 +106,17 @@ object ByColor:
   given Functor[ByColor] with
     def map[A, B](fa: ByColor[A])(f: A => B): ByColor[B] = fa.map(f)
 
+  given Traverse[ByColor] with
+
+    override def foldLeft[A, B](fa: ByColor[A], b: B)(f: (B, A) => B): B =
+      fa.fold(b)(f)
+
+    override def foldRight[A, B](fa: ByColor[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
+      Eval.defer(f(fa.white, Eval.defer(f(fa.black, lb))))
+
+    def traverse[G[_]: Applicative, A, B](fa: ByColor[A])(f: A => G[B]): G[ByColor[B]] =
+      fa.traverse(f)
+
   extension [A](bc: ByColor[IterableOnce[A]]) def flatten: List[A] = bc.all.flatten
 
   extension [A](p: (A, A)) def asByColor: ByColor[A] = ByColor(p._1, p._2)

src/test/scala/ByColorLawsTests.scala

@@ -0,0 +1,11 @@
+package chess
+
+import cats.laws.discipline.FunctorTests
+import munit.DisciplineSuite
+import org.scalacheck.*
+import Arbitraries.given
+import cats.laws.discipline.TraverseTests
+
+class ByColorLawsTest extends DisciplineSuite:
+  checkAll("ByColor.FunctorLaws", FunctorTests[ByColor].functor[Int, Int, String])
+  checkAll("ByColor.TraverseLaws", TraverseTests[ByColor].traverse[Int, Int, Int, Int, Option, Option])