Scala conditional compilation

Question

I\'m writing a Scala program and I want it to work with two version of a big library.

This big library\'s version 2 changes the API very slightly (only

Answer 1

1. C++ preprocessors can be used with Java/Scala if you run cpp before javac or scalac (also there is Manifold).

---

2. If you really want to have conditional compilation in Scala you can use macro annotation (expanding at compile time)

macros/src/main/scala/extendsAPIClass.scala

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("enable macro paradise")
class extendsAPIClass extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro ExtendsAPIClassMacro.impl
}

object ExtendsAPIClassMacro {
  def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._
    annottees match {
      case q"$mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents { $self => ..$stats }" :: tail => 
        def updateParents(parents: Seq[Tree], args: Seq[Tree]) = 
          q"""${tq"APIClass"}(..$args)""" +: parents.filter { case tq"scala.AnyRef" => false; case _ => true }

        val parents1 = sys.env.get("LIB_VERSION") match {
          case Some("1") => updateParents(parents, Seq(q""" "x" """, q"1"))
          case Some("2") => updateParents(parents, Seq(q""" "x" """, q"1", q""" "y" """))
          case None      => parents
        }

        q"""
          $mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents1 { $self => ..$stats }
          ..$tail
        """
    }
  }
}

core/src/main/scala/MyClass.scala (if LIB_VERSION=2)

@extendsAPIClass
class MyClass

//Warning:scalac: {
//  class MyClass extends APIClass("x", 1, "y") {
//    def () = {
//      super.();
//      ()
//    }
//  };
//  ()
//}

build.sbt

ThisBuild / name := "macrosdemo"

lazy val commonSettings = Seq(
  scalaVersion := "2.13.2",
  organization := "com.example",
  version := "1.0.0",
  scalacOptions ++= Seq(
    "-Ymacro-debug-lite",
    "-Ymacro-annotations",
  ),
)

lazy val macros: Project = (project in file("macros")).settings(
  commonSettings,
  libraryDependencies ++= Seq(
    scalaOrganization.value % "scala-reflect" % scalaVersion.value,
  )
)

lazy val core: Project = (project in file("core")).aggregate(macros).dependsOn(macros).settings(
  commonSettings,
  )
)

---

3. Alternatively you can use Scalameta for code generation (at the time before compile time)

build.sbt

ThisBuild / name := "scalametacodegendemo"

lazy val commonSettings = Seq(
  scalaVersion := "2.13.2",
  organization := "com.example",
  version := "1.0.0",
)

lazy val common = project
  .settings(
    commonSettings,
  )

lazy val in = project
  .dependsOn(common)
  .settings(
    commonSettings,
  )

lazy val out = project
  .dependsOn(common)
  .settings(
    sourceGenerators in Compile += Def.task {
      Generator.gen(
        inputDir = sourceDirectory.in(in, Compile).value,
        outputDir = sourceManaged.in(Compile).value
      )
    }.taskValue,
    commonSettings,
  )

project/build.sbt

libraryDependencies += "org.scalameta" %% "scalameta" % "4.3.10"

project/Generator.scala

import sbt._

object Generator {
  def gen(inputDir: File, outputDir: File): Seq[File] = {
    val finder: PathFinder = inputDir ** "*.scala"

    for(inputFile <- finder.get) yield {
      val inputStr = IO.read(inputFile)
      val outputFile = outputDir / inputFile.toURI.toString.stripPrefix(inputDir.toURI.toString)
      val outputStr = Transformer.transform(inputStr)
      IO.write(outputFile, outputStr)
      outputFile
    }
  }
}

project/Transformer.scala

import scala.meta._

object Transformer {
  def transform(input: String): String = {
    val (v1on, v2on) = sys.env.get("LIB_VERSION") match {
      case Some("1") => (true, false)
      case Some("2") => (false, true)
      case None      => (false, false)
    }
    var v1 = false
    var v2 = false
    input.tokenize.get.filter(_.text match {
      case "// Lib v1" =>
        v1 = true
        false
      case "// End Lib v1" =>
        v1 = false
        false
      case "// Lib v2" =>
        v2 = true
        false
      case "// End Lib v2" =>
        v2 = false
        false
      case _ => (v1on && v1) || (v2on && v2) || (!v1 && !v2)
    }).mkString("")
  }
}

common/src/main/scala/com/api/APIClass.scala

package com.api

class APIClass(a: String, b: Integer, c: String)

in/src/main/scala/com/example/MyClass.scala

package com.example

import com.api.APIClass

// Lib v1
class MyClass extends APIClass("x", 1)
// End Lib v1

// Lib v2
class MyClass extends APIClass("x", 1, "y")
// End Lib v2

out/target/scala-2.13/src_managed/main/scala/com/example/MyClass.scala

(after sbt out/compile if LIB_VERSION=2)

package com.example

import com.api.APIClass

class MyClass extends APIClass("x", 1, "y")

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