Eq

Scala’s built-in == operator works for any pair of objects, no matter what types we compare. Ideally, Scala shouldn’t let us compare two types that can never be equal.

Typical problem

scala> 42 == "hello"
// <console>:1: warning: comparing values of types Int and String using `==' will always yield false
//        42 == "hello"
//           ^
// res0: Boolean = false

Eq is designed to support type-safe equality.

Type-safe equality

import cats.implicits._

42 === "hello"
// <console>:2: error: type mismatch;
//  found   : String("hello")
//  required: Int
//        42 === "hello"
//               ^

"world" === "hello"
// res3: Boolean = false

"world" =!= "hello"
// res4: Boolean = true