ignoring

Fields can be excluded from comparison 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.

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

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] = Diff.derived[Person].ignore(_.age)

compare(Person("bob", 25), Person("bob", 30))
// res1: DiffResult = DiffResultObject(
//   name = "Person",
//   fields = ListMap(
//     "name" -> IdenticalValue(value = "bob"),
//     "age" -> IdenticalValue(value = "<ignored>")
//   )
// )

Starting from diffx 0.5.5 it is possible to globally customize how ignoring works. By default, an instance of Diff under a particular path gets replaced by Diff.ignored instance. Diff.ignored is configured to always produce identical results with fixed placeholder "<ignored>" no-matter what it gets. To customize that behavior one has to create an implicit instance of DiffConfiguration with desired behavior. Below is an example of how to include results of original comparison into ignored output:

implicit val conf: DiffConfiguration = DiffConfiguration(makeIgnored =
  (original: Diff[Any]) =>
    (left: Any, right: Any, context: DiffContext) => {
      IdenticalValue(s"Ignored but was: ${original.apply(left, right, context).show()(ShowConfig.noColors)}")
    }
)
// conf: DiffConfiguration = DiffConfiguration(makeIgnored = <function1>)
val d = Diff[Person].ignore(_.age)
// d: Diff[Person] = com.softwaremill.diffx.Diff$$anon$1@71ed624c
d(Person("bob", 25), Person("bob", 30)) 
// res2: DiffResult = DiffResultObject(
//   name = "Person",
//   fields = ListMap(
//     "name" -> IdenticalValue(value = "bob"),
//     "age" -> IdenticalValue(value = "Ignored but was: 25 -> 30")
//   )
// )