chwthewke / auto-diff   0.6.0

Apache License 2.0 GitHub

Structural diff functions for Scala types via generic derivation

Scala versions: 2.13 2.12

Build Status Maven Central

Structural diff for Scala types

This project provides the Difference ADT, which models a structural diff of scala values. It is built (and depends) on cats and shapeless.

Differences are computed via the Diff[A] type class that provides a function of type (A, A) => Option[Difference].

The goal of this library is to provide convenient, customizable and extensible generic derivation of Diff instances, as well as a decent textual representation of Difference values.

This is intended primarily as an aid for automated testing, and was motivated by spending way too much time playing "Spot the difference" with large ADTs in test failure logs, but there may be other use cases.

case class Address( street: String, city: String )
case class Person( name: String, age: Int, address: Address )

import fr.thomasdufour.autodiff.Diff
import fr.thomasdufour.autodiff.Pretty
import fr.thomasdufour.autodiff.derived

implicit val personDiff: Diff[Person] = {

val difference = personDiff(
  Person( "Jean Martin", 29, Address( "2 rue Pasteur", "Lille" ) ),
  Person( "Jean Martin", 55, Address( "2 rue Pasteur", "Lyon" ) )

println( difference ) )



How to use auto-diff

Only Scala 2.12 and 2.13 are supported, and 2.13 support starts in version 0.4.0.

auto-diff Scala Cats
0.3.0 2.12 1.6.1
0.4.0 2.12, 2.13 2.0.0

Add some or all of the following to your build.sbt:

libraryDependencies ++= Seq(
  "fr.thomasdufour" %% "auto-diff-core" % "x.y.z",
  "fr.thomasdufour" %% "auto-diff-generic" % "x.y.z",
  "fr.thomasdufour" %% "auto-diff-enumeratum" % "x.y.z",
  "fr.thomasdufour" %% "auto-diff-scalatest" % "x.y.z" % "test"

For the current version, check the maven central badge at the top of this readme.

Obtaining Diff instances

Diff instances are available implicitly (from the companion object) for:

  • primitive types
  • String and UUID
  • java.time types
  • Scala tuples
  • Some collections and ADTs, including:
    • Option
    • Either
    • List, Vector, finite Stream and LazyList, [Sorted]Set, [Sorted]Map and a few more specialized variants
    • Iterable as a fallback
  • types including Validated, Chain and NonEmpty{Chain|List|Vector|Set}
  • enumeratum Enums when using the autodiff-enumeratum module.
import fr.thomasdufour.autodiff.Diff
import fr.thomasdufour.autodiff.Pretty

def printDiff[A: Diff]( x: A, y: A ): Unit =
  println( Pretty.colorized2.showDiff( x, y ) )

printDiff( 1, 2 )
printDiff( "abc", "def" )

printDiff( Some( "abc" ), None )
printDiff( Left( "error" ), Right( 42 ) )
printDiff[Either[String, Int]]( Right( 66 ), Right( 42 ) )

printDiff( 1 :: 2 :: Nil, 1 :: 3 :: 4 :: Nil )
printDiff( Map( "a" -> 1, "b" -> 2 ), Map( "b" -> 3, "a" -> 1 ) )


Generic derivation

Generic derivation is based on shapeless.LabelledGeneric, and is available for:

  • case classes and objects
  • sealed trait hierarchies

Like in Typelevel kittens, there are three modes of generic derivation: semi-automatic, full-automatic, and cached full-automatic.

As these modes are so directly copied from kittens, the best explanation of how they work would be found in the kittens README

The recommended mode is semi-automatic, with auto-derivation locally in scope and assigning the resulting instances to (implicit) vals.


case class Item( description: String )
case class Bag( items: List[Item] )

import fr.thomasdufour.autodiff.Diff
import fr.thomasdufour.autodiff.Pretty
import fr.thomasdufour.autodiff.derived

implicit val bagDiff: Diff[Bag] = {

    Bag( Item( "a wombat" ) :: Item( "coffee" ) :: Item( "a green fountain pen" ) :: Nil ),
    Bag( Item( "4 paperclips" ) :: Item( "coffee" ) :: Nil )
  ) )


Generic derivation should work with recursive types, including mutually recursive types and parametric recursive types. See the tests for some examples

An important note on mutually recursive types

Deriving Diff instances for mutually recursive types and declaring them implicit in the same scope can go quite badly. And by badly I mean NullPointerException- or StackOverflowError-badly.

A suggestion on how to fix this issue:

import fr.thomasdufour.autodiff.Diff
import fr.thomasdufour.autodiff.derived.semi

case class Outer( inners: Vector[Inner] )
case class Inner( outers: Vector[Outer] )

object implicits1 {
  lazy implicit val outerDiff: Diff[Outer] = semi.diff[Outer]
  lazy implicit val innerDiff: Diff[Inner] = semi.diff[Inner]

trait implicits2 {
  protected def mkOuterDiff: Diff[Outer] = {
    import auto._

  protected def mkInnerDiff: Diff[Inner] = {
    import auto._

object implicits2 extends implicits2 {
  implicit val outerDiff: Diff[Outer] = mkOuterDiff
  implicit val innerDiff: Diff[Inner] = mkInnerDiff



A Diff instance can be customized by having an implicit Diff "override" in scope for a field, for example we might consider that the order of the items in the bag is irrelevant (but without the ability to modify Bag to have a Set[Item], say):

case class Item( description: String )
case class Bag( items: List[Item] )

import fr.thomasdufour.autodiff.Diff
import fr.thomasdufour.autodiff.Pretty
import fr.thomasdufour.autodiff.derived

implicit val bagDiff: Diff[Bag] = {
  implicit val diffItems: Diff[List[Item]] = Diff.inAnyOrder


    Bag( Item( "a wombat" ) :: Nil ),
    Bag( Item( "4 paperclips" ) :: Item( "a wombat" ) :: Nil )


Manual construction

There are a number of ways to manually create Diff instances.

The most flexible (and possibly complex) would be to implement the trait directly. Other than that, we can create a diff from:

  • An equality function (and a show function): Diff.explicitEqShow[A]( eqv: ( A, A ) => Boolean, show: A => String ): Diff[A]
  • Implicit cats.Eq and cats.Show instances: Diff.implicitEqShow[A]( implicit E: Eq[A], S: Show[A] ): Diff[A]
  • The default equality and toString: Diff.defaultEqShow[A]: Diff[A]

There are also functions Diff.forProductN (1 ≤ N ≤ 22) that allow manually deconstructing types to (diffable) components.

Finally, there are Diff.ignore that never reports its arguments as different and Diff.inAnyOrder that compares collections as if they were unordered bags.


An optional, implicit Hint[A] can be used for some Diff instances related to unordered collection diffing, such as Diff.inAnyOrder or Diff[Set[A]], to indicate whether elements can be equal. This can help align mismatched elements and provide a better Difference.

(TODO: longer explanation & examples)

Future work (in no particular order)

  • Support Scala 2.13 (and cross-build for 2.12) - this is actually #1. OK, starting with 0.4.0-RC1.
  • Scala.js support, perhaps
  • Further API exploration for the "front-end", including test framework integration.
  • Improve test coverage, especially text rendering of differences.
  • Integrate cats.Show rather than having Diff also carry a show method
    • Maybe try to expand on that to have the show function drill "just deep enough" into data.


xdotai/diff for inspiration.

circe for some implementation techniques in early versions.

kittens for generic derivation guidelines.