embracing-nondeterminism-code/src/test/scala/green/thisfieldwas/embracingnondeterminism/stdlib/ListSpec.scala

package green.thisfieldwas.embracingnondeterminism.stdlib

import green.thisfieldwas.embracingnondeterminism.data.{Monoid, MonoidLaws, SemigroupLaws}
import green.thisfieldwas.embracingnondeterminism.util._
import org.scalacheck.Arbitrary.arbitrary
import org.scalacheck.{Arbitrary, Gen}

/** Proves that Scala's List conforms to the following typeclasses:
  *   - Functor
  *   - Monad
  *   - Semigroup
  *   - Monoid
  */
class ListSpec extends Laws with SemigroupLaws with MonoidLaws {

  property("List.map() preserves identity functions") {
    forAll(arbitrary[List[Int]]) { fa =>
      fa.map(identity) mustBe identity(fa)
    }
  }

  property("List.map() preserves function composition") {
    forAll(for {
      fa <- arbitrary[List[Double]]
      f <- arbitrary[Double => String]
      g <- arbitrary[String => Int]
    } yield (fa, f, g)) { case (fa, f, g) =>
      fa.map(g compose f) mustBe fa.map(f).map(g)
    }
  }

  property(s"List.flatMap() preserves left identity") {
    forAll(for {
      a <- arbitrary[Int]
      h <- arbitrary[Int => List[String]]
    } yield (a, h)) { case (a, h) =>
      val leftIdentity = (x: Int) => List(x).flatMap(h)
      leftIdentity(a) mustBe h(a)
    }
  }

  property(s"List.flatMap() preserves right identity") {
    forAll(for {
      a <- arbitrary[Int]
      h <- arbitrary[Int => List[String]]
    } yield (a, h)) { case (a, h) =>
      val rightIdentity = h(_: Int).flatMap(List(_))
      rightIdentity(a) mustBe h(a)
    }
  }

  property(s"List.flatMap() is associative") {
    forAll(for {
      a <- arbitrary[Double]
      f <- arbitrary[Double => List[String]]
      g <- arbitrary[String => List[Int]]
      h <- arbitrary[Int => List[Boolean]]
    } yield (a, f, g, h)) { case (a, f, g, h) =>
      val assocLeft = f(_: Double).flatMap(g).flatMap(h)
      val assocRight = f(_: Double).flatMap(g(_).flatMap(h))
      assocLeft(a) mustBe assocRight(a)
    }
  }

  implicit def arbitraryList[A: Arbitrary]: Arbitrary[List[A]] = Arbitrary {
    for {
      length <- Gen.sized(Gen.choose(0, _))
      list <- Gen.listOfN(length, arbitrary[A])
    } yield list
  }

  /** List forms a Semigroup under concatenation and a Monoid with an identity
    * value of an empty List().
    *
    * @tparam A
    *   Any type held by the List.
    * @return
    *   The Semigroup instance for List.
    */
  implicit def listMonoid[A: Arbitrary]: Monoid[List[A]] = new Monoid[List[A]] {

    override def empty: List[A] = List()

    override def combine(left: List[A], right: List[A]): List[A] = left ++ right
  }

  checkSemigroupLaws[List[Int]]()
  checkMonoidLaws[List[Int]]()
}