1.4 MEET CATS

1.5 Example Eq
1.6 Controlling Instance Selection

build.sbt

@@ -11,7 +11,8 @@ lazy val root = project
     libraryDependencies ++= Seq(
       "org.scalameta" %% "munit" % "0.7.29" % Test,
       "org.scalactic" %% "scalactic" % "3.2.17",
+      "org.typelevel" %% "cats-core" % "2.10.0",
       "org.scalatest" %% "scalatest" % "3.2.17" % "test",
-      "org.scalatest" %% "scalatest-flatspec" % "3.2.17" % "test"
+      "org.scalatest" %% "scalatest-flatspec" % "3.2.17" % "test",
     )
   )

src/main/scala/_1_Theory/_4_MEET_CATS/CatsShow.scala

@@ -0,0 +1,6 @@
+package _1_Theory._4_MEET_CATS
+
+import cats.Show
+
+val showInt: Show[Int] = Show[Int]
+val showString: Show[String] = Show[String]

src/main/scala/_1_Theory/_4_MEET_CATS/CustomShowInstances.scala

@@ -0,0 +1,8 @@
+package _1_Theory._4_MEET_CATS
+
+import cats.Show
+
+import java.util.Date
+
+implicit val dateShow: Show[Date] =
+  (date: Date) => s"${date.getTime}ms since the epoch."

src/main/scala/_1_Theory/_6_Controlling_Instance_Selection/SomeType.scala

@@ -0,0 +1,20 @@
+package _1_Theory._6_Controlling_Instance_Selection
+
+sealed trait A
+final case object B extends A
+final case object C extends A
+
+trait Printer[-T]:
+  def print(t: T): Unit
+
+object PrinterInstances:
+
+  implicit val aPrinter: Printer[A] = (t: A) => println(s"a printer print $t")
+
+  implicit val bPrinter: Printer[B.type] = (t: B.type) => println(s"b printer print $t")
+
+// implicit val cPrinter: Printer[C.type] = (t: C.type) => println(s"c printer print $t")
+
+object PrinterUse:
+  def print[T](t: T)(using printer: Printer[T]): Unit =
+    printer.print(t)

src/test/scala/_1_Theory/_3_Exercise_Printable_Library/PrintableUse.scala

@@ -3,28 +3,22 @@ package _1_Theory._3_Exercise_Printable_Library
 import org.scalatest.flatspec.AnyFlatSpec
 
 
-
 class PrintableUse extends AnyFlatSpec:
 
-  "cat" should "be printable with implicit instance" in:
+  final case class Cat(name: String, age: Int, color: String)
 
-    final case class Cat(name: String, age: Int, color: String)
+  given Printable[Cat] with
+    def format(cat: Cat) = s"${cat.name} is a ${cat.age} year-old ${cat.color} cat."
 
-    given Printable[Cat] with
-      def format(cat: Cat) = s"${cat.name} is a ${cat.age} year-old ${cat.color} cat."
+  "cat" should "be printable with implicit instance" in :
 
     val cat = Cat("Garfield", 35, "orange and black")
     assert(Printable.format(cat) == "Garfield is a 35 year-old orange and black cat.")
     Printable.print(cat)
 
-  "cat" should "be printable with extension method" in:
-
-      final case class Cat(name: String, age: Int, color: String)
+  "cat" should "be printable with extension method" in :
 
-      import PrintableSyntax._
+    import PrintableSyntax._
 
-      given Printable[Cat] with
-        def format(cat: Cat) = s"${cat.name} is a ${cat.age} year-old ${cat.color} cat."
-
-      val cat = Cat("Garfield", 35, "orange and black")
-      cat.print
+    val cat = Cat("Garfield", 35, "orange and black")
+    cat.print

src/test/scala/_1_Theory/_4_MEET_CATS/CatsShowTest.scala

@@ -0,0 +1,22 @@
+package _1_Theory._4_MEET_CATS
+
+import org.scalatest.flatspec.AnyFlatSpec
+
+class CatsShowTest extends AnyFlatSpec:
+  "showInt.show(123)" should "create string '123'" in:
+    val intAsString = showInt.show(123)
+    assert(intAsString == "123")
+
+  "showString.show('abc')" should "create string 'abc'" in:
+    val stringAsString = showString.show("abc")
+    assert(stringAsString == "abc")
+
+  "123.show" should "create string '123'" in:
+    import cats.syntax.show._
+    val intAsString = 123.show
+    assert(intAsString == "123")
+
+  "abc.show" should "create string 'abc'" in:
+    import cats.syntax.show._
+    val stringAsString = "abc".show
+    assert(stringAsString == "abc")

src/test/scala/_1_Theory/_4_MEET_CATS/CustomShowInstancesTest.scala

@@ -0,0 +1,13 @@
+package _1_Theory._4_MEET_CATS
+
+import org.scalatest.flatspec.AnyFlatSpec
+
+class CustomShowInstancesTest extends AnyFlatSpec:
+
+  "new Date().show" should "return a string representation of the date" in:
+    import java.util.Date
+    import _1_Theory._4_MEET_CATS.dateShow
+    import cats.syntax.show._
+
+    val date = new Date()
+    assert(date.show == s"${date.getTime.toString}ms since the epoch.")

src/test/scala/_1_Theory/_4_MEET_CATS/README.md

@@ -0,0 +1,3 @@
+* Normally, we can simply import cats as:
+  * `import cats._` imports all of Cats’ type classes in one go;
+  * `import cats.implicits._` imports all of the standard type class in‐ stances and all of the syntax in one go.

src/test/scala/_1_Theory/_4_MEET_CATS/_6_Exercise_Cat_Show/PrintableCatWithCats.scala

@@ -0,0 +1,16 @@
+package _1_Theory._4_MEET_CATS._6_Exercise_Cat_Show
+
+import org.scalatest.flatspec.AnyFlatSpec
+
+class PrintableCatWithCats extends AnyFlatSpec:
+  final case class Cat(name: String, age: Int, color: String)
+
+  "PrintableCatWithCats" should "print cat" in:
+    import cats.Show
+    import cats.syntax.show._
+
+    implicit val catShow: Show[Cat] =
+      Show.show(cat => s"${cat.name} is a ${cat.age} year-old ${cat.color} cat.")
+
+    val cat = Cat("Garfield", 38, "ginger and black")
+    assert(cat.show == "Garfield is a 38 year-old ginger and black cat.")

src/test/scala/_1_Theory/_5_Example_Eq/_5_Exercise_Equality_Liberty_and_Felinity/CatEqTest.scala

@@ -0,0 +1,31 @@
+package _1_Theory._5_Example_Eq._5_Exercise_Equality_Liberty_and_Felinity
+
+import cats.Eq
+import org.scalatest.flatspec.AnyFlatSpec
+
+class CatEqTest extends AnyFlatSpec:
+
+  final case class Cat(name: String, age: Int, color: String)
+
+  import cats.syntax.eq._
+  import cats.syntax.option._
+
+  val cat1: Cat = Cat("Garfield", 38, "orange and black")
+  val cat2: Cat = Cat("Garfield", 38, "orange and black")
+  val cat3: Cat = Cat("Heathcliff", 33, "orange and black")
+
+  implicit val catEq: Eq[Cat] = (cat1: Cat, cat2: Cat) =>
+      cat1.name == cat2.name
+        && cat1.age == cat2.age
+        && cat1.color == cat2.color
+
+  "CatEq" should "return true for equal cats" in:
+    assert(cat1 eqv cat2)
+
+  it should "return false for non-equal cats" in:
+    assert(cat1 =!= cat3)
+
+  it should "return true for equal cats with option" in:
+    val optCat1 = Option(cat1)
+    val optCat2 = Option(cat2)
+    assert(optCat1 eqv optCat2)

src/test/scala/_1_Theory/_6_Controlling_Instance_Selection/ContravariantTest.scala

@@ -0,0 +1,16 @@
+package _1_Theory._6_Controlling_Instance_Selection
+
+import org.scalatest.flatspec.AnyFlatSpec
+
+class ContravariantTest extends AnyFlatSpec:
+
+  "contravariant" should "work fine on both issues on scala3" in:
+    import PrinterInstances._
+
+    // issue1:
+    PrinterUse.print(C)
+    // a printer print C
+
+    // issue2:
+    PrinterUse.print(B)
+    // b printer print B

src/test/scala/_1_Theory/_6_Controlling_Instance_Selection/Untitled.md

@@ -0,0 +1,23 @@
+* Covariance:
+  Covariance means that the type F[B] is a subtype of the type F[A] if B is a subtype of A. This is useful for modelling many types, including collections like List and Option:
+
+  ```scala
+  trait List[+A]
+  trait Option[+A]
+  ```
+
+* Contravariance:
+  Perhaps confusingly, contravariance means that the type F[B] is a subtype of F[A] if A is a subtype of B.This is useful for modelling types that represent inputs, like our JsonWriter type class above:
+
+  ```scala
+  trait JsonWriter[-A] {
+  def write(value: A): Json
+  }
+  ```
+
+* Invariance:
+  This means the types F[A] and F[B] are never subtypes of one another, no matter what the relationship between A and B. This is the default semantics for Scala type constructors.
+
+
+
+* Pay Attention: The two issues arisen in this section can be achieved in scala3, see the test in the same folder.