ch08 makin' objects

Chapter08.scala

package fp_tdd

import cats.Apply
import cats.syntax.apply._
import org.scalacheck.Prop.forAll
import org.scalacheck.{Arbitrary, Gen, Properties}

object Chapter08 extends Properties("Ch08") {
  // ======== TODO ========
  // $5 + 10 CHF = $10 if rate is 2:1
  // Money rounding?
  // hashCode
  // Equal null
  // Equal object
  // Dollar/Farnc duplication
  // Common times
  // Currency
  // ======== DONE ========
  // 1. $5 * 2 = $10
  // 2. Dollar side effect
  // 2. times -> `*`
  // 3. Equality
  // 5. 5CHF * 2 = 10CHF
  // 6. Common equals
  // 7. Compare Francs to Dollars
  // 8. add `*` on Money

// product code -------------------------------------------------------------------
  trait Money {
    protected val amount: Int
    def *(multiplier: Int): Money
    override def equals(a: Any): Boolean =
      a.asInstanceOf[Money].amount == amount && this.getClass == a.getClass
  }
  object Money {
    def dollar(amount: Int): Money = Dollar(amount)
    def franc(amount: Int):  Money = Franc(amount)
  }
  class Dollar(protected val amount: Int) extends Money {
    def *(multiplier: Int): Money = Dollar(amount * multiplier)
  }
  object Dollar { def apply(amount: Int) = new Dollar(amount) }
  class Franc(protected val amount: Int) extends Money {
    def *(multiplier: Int): Money = Franc(amount * multiplier)
  }
  object Franc { def apply(amount: Int) = new Franc(amount) }

// test code --------------------------------------------------------------------
  import Money.{dollar, franc}
  import org.scalacheck.Gen._

  implicit val intGen: Gen[Int] = Gen.chooseNum(-1000000, 1000000)
  implicit val consGen: Gen[Int => Money] = Gen.oneOf(Dollar(_:Int): Money, Franc(_:Int): Money)

  def double[G[_]: Apply, A](ga: G[A]): G[(A, A)]    = (ga, ga).mapN((_, _))
  def triple[G[_]: Apply, A](ga: G[A]): G[(A, A, A)] = (ga, ga, ga).mapN((_, _, _))
  implicit val genApply: Apply[Gen] = new Apply[Gen] {
    override def ap[A, B](gf: Gen[A => B])(ga: Gen[A]): Gen[B] = gf flatMap ga.map
    override def map[A, B](ga: Gen[A])(f: A => B): Gen[B]      = ga map f
  }

  implicit val moneyGen:     Gen[Money]        = (consGen, intGen).mapN(_(_))
  implicit val moneyConsGen: Gen[Int => Money] = oneOf("USD", "CHF") map constructor

  implicit val arbCons: Arbitrary[Int => Money] = Arbitrary(moneyConsGen)
  implicit val arbMoney: Arbitrary[Money]       = Arbitrary(moneyGen)

  def constructor(currency: String)(n: Int): Money = currency match {
    case "USD" => dollar(n)
    case "CHF" => franc(n)
  }
  implicit val unique3: Gen[(Money, Money, Money)] =
    triple(moneyGen).suchThat { case (a, b, c) => a != b && b != c && c != a }

// properties -------------------------------------------------------------------
  property("m(0) * x = m(0)") = forAll { (x: Int, m: Int => Money) =>
    m(0) * x == m(0)
  }
  property("m(1) * x = m(x)") = forAll { (x: Int, m: Int => Money) =>
    m(1) * x == m(x)
  }
  property("m(a) * b = m(b) * a") = forAll { (a: Int, b: Int, m: Int => Money) =>
    m(a) * b == m(b) * a
  }
  property("(m(a) * b) * c = m(a) * (b * c)") = forAll { (a: Int, b: Int, c: Int, m: Int => Money) =>
    (m(a) * b) * c == m(a) * (b * c)
  }

  property("m(a) == m(b) <=> a == b") = forAll(double(intGen), moneyConsGen) { case ((a, b), m) =>
    (m(a) == m(b)) == (a == b)
  }
  property("m(a) == m(b) <=> m(a) equals m(b)") = forAll(double(moneyGen)) { case (a: Money, b: Money) =>
    (a equals b) == (a == b)
  }
  property("equals is reflexive") = forAll { (a: Money) =>
    a equals a
  }
  property("equals is symmetric") = forAll(double(intGen)) { case (a, b) =>
    (a equals b) == (b equals a)
  }
  property("equals is transitive") = forAll( for {
    oneOf3 <- unique3.map { case (a, b, c)=> Gen.oneOf(a, b, c) }
    x      <- oneOf3
    y      <- oneOf3.suchThat(_ equals x)
    z      <- oneOf3.retryUntil(_ equals y)
  } yield (x, z)) { case ((x, z)) => x equals z }

  property("(m1 == m2) == (m1.getClass == m2.getClass)") =
      forAll(double(consGen), intGen) { case ((c1, c2), n) =>
    val m1 = c1(n)
    val m2 = c2(n)
    (m1 == m2) == (m1.getClass == m2.getClass)
  }
}