softwaremill / diffx

Pretty diffs for scala case classes

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diffx

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Pretty diffs for case classes.

The library is published for Scala 2.12 and 2.13.

Table of contents

Goals of the project

  • human-readable case class diffs
  • support for popular testing frameworks
  • OOTB collections support
  • OOTB non-case class support
  • smaller compilation overhead compared to shapless based solutions (thanks to magnolia <3)
  • programmer friendly and type safe api for partial ignore

Teaser

Add the following dependency:

"com.softwaremill.diffx" %% "diffx-core" % "0.3.29"
sealed trait Parent
case class Bar(s: String, i: Int) extends Parent
case class Foo(bar: Bar, b: List[Int], parent: Option[Parent]) extends Parent

val right: Foo = Foo(
    Bar("asdf", 5),
    List(123, 1234),
    Some(Bar("asdf", 5))
)
// right: Foo = Foo(Bar("asdf", 5), List(123, 1234), Some(Bar("asdf", 5)))

val left: Foo = Foo(
    Bar("asdf", 66),
    List(1234),
    Some(right)
)
// left: Foo = Foo(
//   Bar("asdf", 66),
//   List(1234),
//   Some(Foo(Bar("asdf", 5), List(123, 1234), Some(Bar("asdf", 5))))
// )
 

import com.softwaremill.diffx._
compare(left, right)
// res0: DiffResult = DiffResultObject(
//   "Foo",
//   ListMap(
//     "bar" -> DiffResultObject(
//       "Bar",
//       ListMap("s" -> Identical("asdf"), "i" -> DiffResultValue(66, 5))
//     ),
//     "b" -> DiffResultObject(
//       "List",
//       ListMap("0" -> DiffResultValue(1234, 123), "1" -> DiffResultMissing(1234))
//     ),
//     "parent" -> DiffResultValue("repl.MdocSession.App.Foo", "repl.MdocSession.App.Bar")
//   )
// )

Will result in:

example

Colors

When running tests through sbt, default diffx's colors work well on both dark and light backgrounds. Unfortunately Intellij Idea forces the default color to red when displaying test's error. This means that it is impossible to print something with the standard default color (either white or black depending on the color scheme).

To have better colors, external information about the desired theme is required. Specify environment variable DIFFX_COLOR_THEME and set it to either light or dark. I had to specify it in /etc/environment rather than home profile for Intellij Idea to picked it up.

If anyone has an idea how this could be improved, I am open for suggestions.

Scalatest integration

To use with scalatest, add the following dependency:

"com.softwaremill.diffx" %% "diffx-scalatest" % "0.3.29" % Test

Then, extend the com.softwaremill.diffx.scalatest.DiffMatcher trait or import com.softwaremill.diffx.scalatest.DiffMatcher._. After that you will be able to use syntax such as:

import org.scalatest.matchers.should.Matchers._
import com.softwaremill.diffx.scalatest.DiffMatcher._

left should matchTo(right)

Giving you nice error messages:

Specs2 integration

To use with specs2, add the following dependency:

"com.softwaremill.diffx" %% "diffx-specs2" % "0.3.29" % Test

Then, extend the com.softwaremill.diffx.specs2.DiffMatcher trait or import com.softwaremill.diffx.specs2.DiffMatcher._. After that you will be able to use syntax such as:

import org.specs2.matcher.MustMatchers.{left => _, right => _, _}
import com.softwaremill.diffx.specs2.DiffMatcher._

left must matchTo(right)

Utest integration

To use with utest, add following dependency:

"com.softwaremill.diffx" %% "diffx-utest" % "0.3.29" % Test

Then, mixin DiffxAssertions trait or add import com.softwaremill.diffx.utest.DiffxAssertions._ to your test code. To assert using diffx use assertEquals as follows:

import com.softwaremill.diffx.utest.DiffxAssertions._
assertEqual(left, right)

Ignoring

Fields can be excluded from comparision by calling the ignore method on the Diff instance. Since Diff instances are immutable, the ignore method creates a copy of the instance with modified logic. You can use this instance explicitly. If you still would like to use it implicitly, you first need to summon the instance of the Diff typeclass using the Derived typeclass wrapper: Derived[Diff[Person]]. Thanks to that trick, later you will be able to put your modified instance of the Diff typeclass into the implicit scope. The whole process looks as follows:

case class Person(name:String, age:Int)
implicit val modifiedDiff: Diff[Person] = Derived[Diff[Person]].ignore[Person,String](_.name)

Customization

If you'd like to implement custom matching logic for the given type, create an implicit Diff instance for that type, and make sure it's in scope when any Diff instances depending on that type are created.

If there is already a typeclass for a particular type, but you would like to use another one, you wil have to override existing instance. Because of the "exporting" mechanism the top level typeclass is Derived[Diff] rather then Diff and that's the typeclass you need to override.

To understand it better, consider following example with NonEmptyList from cats. NonEmptyList is implemented as case class so diffx will create a Derived[Diff[NonEmptyList]] typeclass instance using magnolia derivation.

Obviously that's not what we usually want. In most of the cases we would like NonEmptyList to be compared as a list. Diffx already has an instance of a typeclass for a list. One more thing to do is to use that typeclass by converting NonEmptyList to list which can be done using contramap method.

The final code looks as follows:

import cats.data.NonEmptyList
implicit def nelDiff[T: Diff]: Derived[Diff[NonEmptyList[T]]] = 
    Derived(Diff[List[T]].contramap[NonEmptyList[T]](_.toList))

And here's an example customizing the Diff instance for a child class of a sealed trait

sealed trait ABParent
case class A(id: String, name: String) extends ABParent
case class B(id: String, name: String) extends ABParent

implicit val diffA: Derived[Diff[A]] = Derived(Diff.gen[A].value.ignore[A, String](_.id))
val a1: ABParent = A("1", "X")
// a1: ABParent = A("1", "X")
val a2: ABParent = A("2", "X")
// a2: ABParent = A("2", "X")

compare(a1, a2)
// res5: DiffResult = Identical(A("1", "X"))

You may need to add -Wmacros:after Scala compiler option to make sure to check for unused implicits after macro expansion. If you get warnings from Magnolia which looks like magnolia: using fallback derivation for TYPE, you can use the Silencer compiler plugin to silent the warning with the compiler option "-P:silencer:globalFilters=^magnolia: using fallback derivation.*$"

Other 3rd party libraries support

  • com.softwaremill.common.tagging
    "com.softwaremill.diffx" %% "diffx-tagging" % "0.3.29"
    com.softwaremill.diffx.tagging.DiffTaggingSupport
  • eu.timepit.refined
    "com.softwaremill.diffx" %% "diffx-refined" % "0.3.29"    
    com.softwaremill.diffx.refined.RefinedSupport
  • org.typelevel.cats
    "com.softwaremill.diffx" %% "diffx-cats" % "0.3.29"    
    com.softwaremill.diffx.cats.DiffCatsInstances

Similar projects

There is a number of similar projects from which diffx draws inspiration.

Below is a list of some of them, which I am aware of, with their main differences:

  • xotai/diff - based on shapeless, seems not to be activly developed anymore
  • ratatool-diffy - the main purpose is to compare large data sets stored on gs or hdfs

Commercial Support

We offer commercial support for diffx and related technologies, as well as development services. Contact us to learn more about our offer!

Copyright

Copyright (C) 2019 SoftwareMill https://softwaremill.com.