embracing-nondeterminism-code/src/test/scala/green/thisfieldwas/embracingnondeterminism/data/SemigroupLaws.scala

package green.thisfieldwas.embracingnondeterminism.data

import green.thisfieldwas.embracingnondeterminism.util._
import org.scalacheck.Arbitrary
import org.scalacheck.Arbitrary.arbitrary

import scala.reflect.runtime.universe.TypeTag

/** Include this trait to check that your context `F[_]` conforms to the
  * Semigroup laws. Call the `checkSemigroupLaws()` function within your laws
  * spec to register the associated properties.
  */
trait SemigroupLaws { this: Laws =>

  import green.thisfieldwas.embracingnondeterminism.syntax.semigroup._

  /** Defined per Semigroup laws taken from the Haskell wiki:
    * [[https://wiki.haskell.org/Data.Semigroup#Semigroup_Laws]]
    *
    * @tparam S
    *   The Semigroup type being tested.
    */
  def checkSemigroupLaws[S: Semigroup: Arbitrary]()(implicit TT: TypeTag[S]): Unit = {
    // Combining three values can be performed in any order and produce the
    // same result.
    property(s"${TT.name} Semigroup is associative") {
      forAll(for {
        a <- arbitrary[S]
        b <- arbitrary[S]
        c <- arbitrary[S]
      } yield (a, b, c)) { case (a, b, c) =>
        a |+| (b |+| c) mustBe (a |+| b) |+| c
      }
    }
  }
}