Complete ch06

.scalafmt.conf

@@ -5,5 +5,6 @@ runner.dialect = scala3
 assumeStandardLibraryStripMargin = true
 # https://github.com/scalameta/scalameta/issues/4090
 project.excludePaths = [
-    "glob:**/ch04/src/**.scala"
+    "glob:**/ch04/src/**.scala",
+    "glob:**/ch06/src/Cat.scala"
   ]

README.md

@@ -13,6 +13,7 @@ The older code is available in branches.
 3. [Objects as Codata](ch03)
 4. [Contextual Abstraction](ch04)
 5. [Reified Interpreters](ch05)
+6. [Using Cats](ch06)
 
 ## Running tests
 ```

build.mill

@@ -33,6 +33,10 @@ trait CatsModule extends ScalaModule with Cross.Module[String] with ScalafmtModu
     "-source", "future",
   )
 
+  def ivyDeps = Agg(
+    ivy"org.typelevel::cats-core:${v.catsVersion}"
+  )
+
   object test extends ScalaTests with TestModule.ScalaTest {
    val commonDeps = Seq(
      ivy"org.scalatest::scalatest:${v.scalatestVersion}",

ch06/src/Cat.scala

@@ -0,0 +1,32 @@
+package ch06
+
+import cats.Show
+import cats.instances.int.catsStdShowForInt
+import cats.instances.string.catsStdShowForString
+import cats.syntax.show.toShow
+import cats.Eq
+import cats.syntax.eq.catsSyntaxEq
+
+final case class Cat(name: String, age: Int, color: String)
+
+object Cat:
+  /*
+  6.2.1.1 Exercise: Cat Show
+  Re-implement the Cat application from Section 4.5.1 using Show instead of Display.
+ */
+  given Show[Cat]:
+    override def show(cat: Cat): String =
+      val name  = cat.name.show
+      val age   = cat.age.show
+      val color = cat.color.show
+      s"$name is a $age year-old $color cat."
+
+  /*
+  6.3.4.1 Exercise: Equality, Liberty, and Felinity
+  Implement an instance of Eq for our running Cat example.
+  */
+  given Eq[Cat]:
+    override def eqv(x: Cat, y: Cat): Boolean =
+      (x.name === y.name) &&
+        (x.age === y.age) &&
+        (x.color === y.color)

ch06/test/src/CatSpec.scala

@@ -0,0 +1,24 @@
+package ch06
+
+import org.scalatest.funspec.AnyFunSpec
+import org.scalatest.matchers.should.Matchers.shouldBe
+import cats.syntax.show.toShow
+import cats.syntax.eq.catsSyntaxEq
+
+class CatSpec extends AnyFunSpec:
+    describe("Cat"):
+        it("Show"):
+            Cat("Garfield", 41, "ginger and black").show `shouldBe` "Garfield is a 41 year-old ginger and black cat."
+
+        it("Eq"):
+            val cat1 = Cat("Garfield", 38, "orange and black")
+            val cat2 = Cat("Heathcliff", 32, "orange and black")
+
+            cat1 === cat2 `shouldBe` false
+            cat1 =!= cat2 `shouldBe` true
+
+            val optionCat1 = Option(cat1)
+            val optionCat2 = Option.empty[Cat]
+
+            optionCat1 === optionCat2 `shouldBe` false
+            optionCat1 =!= optionCat2 `shouldBe` true

versions.mill

@@ -3,3 +3,4 @@ package build
 val scalaVersion = "3.6.2"
 val scalatestVersion = "3.2.19"
 val scalacheckVersion = "3.2.19.0"
+val catsVersion = "2.12.0"