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

package green.thisfieldwas.embracingnondeterminism.stdlib

import org.scalacheck.Arbitrary.arbitrary
import org.scalatest.matchers.must.Matchers
import org.scalatest.propspec.AnyPropSpec
import org.scalatestplus.scalacheck.ScalaCheckPropertyChecks

/** Proves that Scala's Either conforms to the following typeclasses:
  * - Functor
  * - Monad
  */
class EitherSpec extends AnyPropSpec with ScalaCheckPropertyChecks with Matchers {

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

  property("Either.map() preserves function composition") {
    forAll(for {
      fa <- arbitrary[Either[Exception, 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"Either.flatMap() preserves left identity") {
    forAll(for {
      a <- arbitrary[Int]
      h <- arbitrary[Int => Either[Exception, String]]
    } yield (a, h)) { case (a, h) =>
      val leftIdentity = (x: Int) => Right(x).flatMap(h)
      leftIdentity(a) mustBe h(a)
    }
  }

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

  property(s"Either.flatMap() is associative") {
    forAll(for {
      a <- arbitrary[Double]
      f <- arbitrary[Double => Either[Exception, String]]
      g <- arbitrary[String => Either[Exception, Int]]
      h <- arbitrary[Int => Either[Exception, 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)
    }
  }
}