From 6c6103448bb74720a930dd370bace119e3487b10 Mon Sep 17 00:00:00 2001
From: Thanh Le <lenguyenthanh@hotmail.com>
Date: Sun, 9 Jul 2023 16:04:02 +0200
Subject: [PATCH] Make ByColor more powerful

---
 src/main/scala/ByColor.scala          | 23 +++++++++++++++++------
 src/test/scala/ByColorLawsTests.scala |  4 ++--
 src/test/scala/ByColorTest.scala      |  4 ++++
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/src/main/scala/ByColor.scala b/src/main/scala/ByColor.scala
index 593aa5f4d..1289b8974 100644
--- a/src/main/scala/ByColor.scala
+++ b/src/main/scala/ByColor.scala
@@ -1,11 +1,11 @@
 package chess
 
-import cats.{ Applicative, Eq, Eval, Functor, Monoid, Traverse }
+import cats.{ Applicative, Eq, Eval, FlatMap, Functor, Monoid, Semigroupal, Traverse }
 import cats.syntax.all.*
 import scala.annotation.targetName
 import alleycats.Zero
 
-case class ByColor[A](white: A, black: A):
+case class ByColor[A](white: A, black: A) extends ByColorSyntax:
 
   inline def apply(inline color: Color) = if color.white then white else black
 
@@ -110,10 +110,9 @@ object ByColor:
     def combine(x: ByColor[A], y: ByColor[A]) =
       ByColor(Monoid[A].combine(x.white, y.white), Monoid[A].combine(x.black, y.black))
 
-  given Functor[ByColor] with
-    def map[A, B](fa: ByColor[A])(f: A => B): ByColor[B] = fa.map(f)
+  given Functor[ByColor] with Applicative[ByColor] with Traverse[ByColor] with
 
-  given Traverse[ByColor] with
+    override def map[A, B](fa: ByColor[A])(f: A => B): ByColor[B] = fa.map(f)
 
     override def foldLeft[A, B](fa: ByColor[A], b: B)(f: (B, A) => B): B =
       fa.fold(b)(f)
@@ -124,4 +123,16 @@ object ByColor:
     def traverse[G[_]: Applicative, A, B](fa: ByColor[A])(f: A => G[B]): G[ByColor[B]] =
       fa.traverse(f)
 
-  extension [A](p: (A, A)) def asByColor: ByColor[A] = ByColor(p._1, p._2)
+    def pure[A](a: A): ByColor[A] = ByColor.fill(a)
+    def ap[A, B](ff: ByColor[A => B])(fa: ByColor[A]): ByColor[B] =
+      ByColor(ff.white(fa.white), ff.black(fa.black))
+
+trait ByColorSyntax:
+
+  extension [F[_], A](bc: ByColor[F[A]])
+
+    def mapN[Z](f: (A, A) => Z)(using functor: Functor[F], semigroupal: Semigroupal[F]): F[Z] =
+      Semigroupal.map2(bc.white, bc.black)(f)
+
+    def flatMapN[Z](f: (A, A) => F[Z])(using flatMap: FlatMap[F]): F[Z] =
+      flatMap.flatMap2(bc.white, bc.black)(f)
diff --git a/src/test/scala/ByColorLawsTests.scala b/src/test/scala/ByColorLawsTests.scala
index 15382287c..391c3e24e 100644
--- a/src/test/scala/ByColorLawsTests.scala
+++ b/src/test/scala/ByColorLawsTests.scala
@@ -1,11 +1,11 @@
 package chess
 
-import cats.laws.discipline.FunctorTests
 import munit.DisciplineSuite
 import org.scalacheck.*
 import Arbitraries.given
-import cats.laws.discipline.TraverseTests
+import cats.laws.discipline.{ ApplicativeTests, FunctorTests, 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])
+  checkAll("ByColor.ApplicativeLaws", ApplicativeTests[ByColor].applicative[Int, Int, String])
diff --git a/src/test/scala/ByColorTest.scala b/src/test/scala/ByColorTest.scala
index 288e31200..a997fc523 100644
--- a/src/test/scala/ByColorTest.scala
+++ b/src/test/scala/ByColorTest.scala
@@ -83,6 +83,10 @@ class ByColorTest extends ScalaCheckSuite:
     forAll: (bc: ByColor[Int], f: Int => Boolean) =>
       bc.exists(f) == bc.all.exists(f)
 
+  test("sequence"):
+    forAll: (bc: ByColor[Option[Int]]) =>
+      bc.sequence == (bc.white, bc.black).mapN(ByColor(_, _))
+
   test("findColor && exists"):
     forAll: (bc: ByColor[Int], f: Int => Boolean) =>
       bc.findColor(f).isDefined == bc.all.exists(f)