Scala 2 Support

Currently, Decisions4s is published only for Scala 3. Publishing it for Scala 2 is not feasible due to its reliance on the latest Scala features.

We believe the Scala ecosystem should migrate to Scala 3 as soon as possible. However, we understand that business constraints may require maintaining Scala 2 codebases for the foreseeable future.

Using Decisions4s from Scala 2 is possible but comes with challenges.

We recommend the following approach:

• Create a separate module, your-app-decisions, that uses Scala 3 and depends only on Decisions4s. Define your decisions within this module.
  • This setup is necessary to leverage typeclass derivation based on macros, which can only be invoked from a Scala 3 module. Additionally, rule definitions rely on context functions, which are not available in Scala 2.
• Take advantage of the compatibility between Scala 2 and Scala 3 by using .dependsOn() to link the new Scala 3 module to your main Scala 2 module. This allows you to evaluate and visualize decisions from within your Scala 2 codebase.
• If you're using decisions4s-cats-effect, exclude the cats-effect dependency to avoid conflicts where both Scala 3 and Scala 2 versions end up on the classpath. Although this isn't entirely safe in theory, it should work in practice. Ensure that you thoroughly test your decision-evaluation code to confirm compatibility.

Example setup

val myServiceDecisions = project
  .in(file("my-service-decisions"))
  .settings(
    scalaVersion := "3.3.3",
    libraryDependencies ++= Seq(
      "org.business4s" %% "decisions4s-core" % "xxx",
      ("some.other" %% "scala-lib-published-for-scala-2" % "yyy").cross(CrossVersion.for3Use2_13),
    ),
  )

val myService = project
  .in(file("my-service"))
  .settings(
    scalaVersion := "2.13.14",
    libraryDependencies ++= Seq(
      ("org.business4s" %% "decisions4s-cats-effect" % "xxx").cross(CrossVersion.for2_13Use3).excludeAll("org.typelevel"),
      ("org.business4s" %% "decisions4s-dmn" % "xxx").cross(CrossVersion.for2_13Use3),
    ),
  )
  .depenedsOn(myServiceDecisions)