add cats submodule

Author: vaddya

README.md

@@ -7,3 +7,4 @@
 * `capstone` - [Functional Programming in Scala Capstone](https://www.coursera.org/learn/scala-capstone), [Certificate](https://www.coursera.org/account/accomplishments/certificate/FLQGR3W83U9W)
 * `reactive` - [Programming Reactive Systems](https://www.edx.org/course/programming-reactive-systems)
 * `effective` - [Effective Programming in Scala](https://www.coursera.org/learn/effective-scala)
+* `cats` - [Scala with Cats Book](https://underscore.io/books/scala-with-cats/)

build.sbt

@@ -14,7 +14,8 @@ lazy val global = project
     spark,
     capstone,
     reactive,
-    effective
+    effective,
+    cats
   )
 
 lazy val principles = sparkProject("principles")
@@ -24,6 +25,7 @@ lazy val spark = sparkProject("spark")
 lazy val capstone = sparkProject("capstone")
 lazy val reactive = sparkProject("reactive")
 lazy val effective = sparkProject("effective")
+lazy val cats = sparkProject("cats")
 
 def sparkProject(dir: String) = Project(dir, file(dir))
   .settings(

cats/build.sbt

@@ -0,0 +1,3 @@
+libraryDependencies ++= Seq(
+  "org.typelevel" %% "cats-core" % "2.7.0"
+)

cats/src/main/scala/com/vaddya/fpscala/cats/intro/Cat.scala

@@ -0,0 +1,3 @@
+package com.vaddya.fpscala.cats.intro
+
+final case class Cat(name: String, age: Int, color: String)

cats/src/main/scala/com/vaddya/fpscala/cats/intro/Eq.scala

@@ -0,0 +1,18 @@
+package com.vaddya.fpscala.cats.intro
+
+import cats.Eq
+import cats.syntax.eq.*
+
+object Eq {
+  implicit val catEq: Eq[Cat] = (cat1, cat2) =>
+    cat1 == cat2
+}
+
+@main def runEq(): Unit =
+  import Eq.*
+
+  val cat1 = Option(Cat("Busya", 6, "Black"))
+  val cat2 = Option.empty[Cat]
+
+  println(cat1 === cat1)
+  println(cat1 =!= cat2)

cats/src/main/scala/com/vaddya/fpscala/cats/intro/Printable.scala

@@ -0,0 +1,48 @@
+package com.vaddya.fpscala.cats.intro
+
+trait Printable[A]:
+  def format(value: A): String
+
+object Printable:
+  // Scala 2
+  implicit val stringPrintable: Printable[String] = (value: String) => value
+
+  // Scala 3
+  given intPrintable: Printable[Int] with
+    override def format(value: Int): String = value.toString
+
+object PrintableInstances:
+  // Scala 2
+  def format[A](value: A)(implicit printable: Printable[A]): String =
+    printable.format(value)
+
+  // Scala 3
+  def print[A](value: A)(using printable: Printable[A]): Unit =
+    println(format(value))
+
+object PrintableSyntax:
+  // Scala 2
+  implicit class PrintableOps[A](value: A):
+    def format(implicit printable: Printable[A]): String =
+      printable.format(value)
+
+  // Scala 3
+  extension[A] (value: A)
+    def print(using printable: Printable[A]): Unit =
+      println(summon[Printable[A]].format(value))
+
+@main def runPrintable(): Unit =
+  import PrintableInstances.print as printing
+  import PrintableSyntax.*
+
+  given Printable[Cat] with
+    override def format(value: Cat): String =
+      val name = PrintableInstances.format(value.name)
+      val age = PrintableInstances.format(value.age)
+      val color = PrintableInstances.format(value.color)
+
+      s"$name is a $age year-old $color cat."
+
+  val cat = Cat("Busya", 6, "Black")
+  printing(cat)
+  cat.print

cats/src/main/scala/com/vaddya/fpscala/cats/intro/Show.scala

@@ -0,0 +1,19 @@
+package com.vaddya.fpscala.cats.intro
+
+import cats.Show
+import cats.implicits.toShow
+import cats.instances.all.*
+
+object Show:
+  implicit val catShow: Show[Cat] = (cat: Cat) =>
+    val name = cat.name.show
+    val age = cat.age.show
+    val color = cat.color.show
+
+    s"$name is a $age year-old $color cat."
+
+@main def runShow(): Unit =
+  import Show.*
+
+  val cat = Cat("Busya", 6, "Black")
+  println(cat.show)