04.Answer

import fpinscala.testing.{Prop, Gen}
import Gen.`**`

def monoidLaws[A](m: Monoid[A], gen: Gen[A]): Prop =
  val associativity = Prop
    .forAll(gen ** gen ** gen):
      case a ** b ** c =>
        m.combine(a, m.combine(b, c)) == m.combine(m.combine(a, b), c)
    .tag("associativity")
  val identity = Prop
    .forAll(gen): a =>
      m.combine(a, m.empty) == a && m.combine(m.empty, a) == a
    .tag("identity")
  associativity && identity