Equality

equals.scala

/*
By default, there are equality tests in Scala that return false which you'd really like
to be a compile-time error.

For example:

scala> "Hello" == 42
res0: Boolean = false

scala> Some(1) == 1
<console>:12: warning: comparing values of types Some[Int] and Int using `==' will always yield false
       Some(1) == 1
       
       
Here are three ways to fix this to varying degrees...

See also:
- future language warning changes: http://dotty.epfl.ch/docs/reference/multiversal-equality.html

*/
       
       
object Scala {
  // Use "-Xfatal-warnings"

  // Option[Int] and Int are unrelated: they will most likely never compare equal
  def test1: Boolean = Option(1) == 1

  // Silent (no error or warning)
  // Presumably toString is happening, which I thought could be disabled with WartRemover
  // but apparently not yet: https://github.com/wartremover/wartremover/issues/403
  def test2: Boolean = "Hello" == 42

  // comparing values of types Int and String using `==' will always yield false
  def test3: Boolean = 42 == "Hello"
}

object SE {
  import org.scalactic.TypeCheckedTripleEquals._
  // types Some[Int] and Int do not adhere to the type constraint selected for the === and !== operators;
  // the missing implicit parameter is of type org.scalactic.CanEqual[Some[Int],Int]
  def test1: Boolean = Option(1) === 1
  def test2: Boolean = "Hello" === 42
}

object Cats {
  import cats.implicits._
  // type mismatch;
  // found   : Int(1)
  // required: Option[Int]
  def test1: Boolean = Option(1) === 1
  def test2: Boolean = "Hello" === 42
}

/*
NB:

  settings(wartremoverErrors in Compile += Wart.Equals)

...disables `==` which may be useful.
*/