An Eq typeclass implementation for the type constructor and type i.e. Int and List[_]

equals-4-all.sc

trait Eq[A] {
  def eqv(lhs: A, rhs: A): Boolean
  def neq(lhs: A, rhs: A): Boolean = !eqv(lhs, rhs)

  extension (lhs: A) {
    def ===(rhs: A): Boolean = eqv(lhs, rhs)
    def !==(rhs: A): Boolean = neq(lhs, rhs)
  }
}

object EqualForInt {
  given Eq[Int] with {
    override def eqv(lhs: Int, rhs: Int): Boolean = lhs == rhs
  }
}

object EqualForList {
  given [A: Eq as ev]: Eq[List[A]] with {
    def eqv(lhs: List[A], rhs: List[A]): Boolean =
      if lhs.length != rhs.length then false
      else lhs.zip(rhs).forall(ev.eqv(_, _))
  }
}

import EqualForList.given
import EqualForInt.given

val xs = List(1, 2, 3)
val ys = List(1, 2, 3)
val zs = List(1, 2, 4)

assert(3 === 3)
assert(xs === ys)
assert(xs !== zs)