Scala 2.x uses universal equality, meaning that any two values of any two types can be compared with each other using == and !=. However, this can sometimes lead to buggy programs where we assume that we are comparing two values of the same type but in fact the types are different. In this case, because of universal equality, the compiler validates the buggy code but at runtime the comparison will always return the same result regardless of the two values being compared — it will always return false in the case of ==, and always true in the case of !=. This is almost certainly not the desired outcome!

This kind of bug often occurs after a refactoring that modifies the types in a program — e.g. like the modification we have just finished to convert a plain type alias to an opaque type alias.

Scala 3 introduces multiversal equality to help deal with this problem. We can explicitly "opt in" to the new, safer multiversal equality, either with an import in the relevant source files or with the addition of a compiler option. Once enabled, multiversal equality checks for the presence of an Eql ("equality") typeclass instance that allows the comparison of the types of the two values being compared with == or !=. If no valid Eql instance is found, then the comparison does not type-check and the compilation fails with an error.

• Enable multiversal equality at the project level by adding -language:strictEquality to the Scala compiler options in project/Build.scala

• Use the course notes on Multiversal Equality and your experience from the exercise on givens to make the code compile with the flag -language:strictEquality enabled

  • Hint: The simplest solution will probably involve using CanEqual.derived

• Run the provided tests by executing the test command from the sbt prompt and verify that all tests pass

After successfully completing the tasks in this exercise, move to the next one by running the cmtc next-exercise from the command line.

scalafmt based source code formatting is in place in this project. scalafmt supports both Scala 2 and Scala 3. You can [re]format the code by running scalafmtAll from the sbt prompt. As we switch from Scala 2 to Scala 3, you need to make sure that a matching scalafmt configuration is in place. In any of the exercises, you can run cmtc pull-template .scalafmt.conf to "pull-in" the correct configuration file.

The font size can be a bit too small for the taste of some people. You can change the Markdown zoom setting in IntelliJ by pasting the following CSS snippet in the markdown setting in " Settings" -> "Languages & Frameworks" -> "Custom CSS -> CSS rules" and adjust the font-size setting to your liking:

body {
  font-size: 120% !important;
  }