Excerpted from https://github.com/scalaz/scalaz for instructional purposes

lawblog-monoidlaw.scala

trait Monoid[F] extends Semigroup[F] { 
    ...
    trait MonoidLaw extends SemigroupLaw {
        def leftIdentity(a: F)(implicit F: Equal[F]) = 
            F.equal(a, append(zero, a))
        def rightIdentity(a: F)(implicit F: Equal[F]) = 
            F.equal(a, append(a, zero))
    }
    ...
}