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.

Consider following example with NonEmptyList from cats. NonEmptyList is implemented as case class, so the default behavior of diffx would be to create a 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 (for any iterable to be precise). All we need 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 com.softwaremill.diffx._
import _root_.cats.data.NonEmptyList
implicit def nelDiff[T: Diff]: Diff[NonEmptyList[T]] = 

Note: There is a diffx-cats module, so you don’t have to do this