pbyrne84.github.io

Scala Case Class Prettification

scala-case-class-prettification

In scalatest there is a fairly hidden way to change the rendering off diffs on failure.

e.g. a test failure on the command line looks like this

And when “clicking to see the difference” the difference is made clearer in intellij

import org.scalactic.Prettifier

object Prettifiers {
  implicit val prettifier: Prettifier = Prettifier.apply {
    case a: AnyRef if CaseClassPrettifier.shouldBeUsedInTestMatching(a) =>
      new CaseClassPrettifier().prettify(a)

    case a: Any => Prettifier.default(a)
  }
}

You may have to do a clean compile to get the implicit to compile in. If you have a look at the tests you can see it is very similar how zio renders errors

CaseClassPrettifierTest.scala

e.g.

NestedMultiLevel(
  fieldName1 = 4,
  fieldName2 = NestedBasic(
    fieldName1 = 4,
    fieldName2 = SinglePrimitive(
      fieldName1 = 4
    )
  )
)

With intellij or something similar when it fails the diff is easily comparable. It doesn’t try and do anything fancy such as maximum line length as that would potentially break the ease of compare.

I do put this in any project using scalatest as games of spot the difference on failure are needlessly tiring. Especially when dealing with theSameElementsAs on collections of case classes. There can have too many things wrong in the match and then the simplest thing is to sort then do the diff. There is probably a way to write a matcher that auto sorts on the way in to fix the hoopla that matcher causes. theSameElementsAs and things like it are a bit of a bug bear as the user is usually thinking of the success and not the potentially anti-social failure.