ifdef is an experimental project to implement conditional compilation in Scala. Conditional compilation is a concept of compiling some region of source code only under certain conditions. Programming languages like C and Rust implement it via preprocessor and macro.

For example, in Rust, you can include unit tests in the same source code where the traits and functions are defined using the #[cfg(test)] annotation. Currently ifdef is a combination of an annotation library, a compiler plugin, and an sbt plugin that implements similar mechanism.

addSbtPlugin("com.eed3si9n.ifdef" % "sbt-ifdef" % "x.y.z")

First we create meta declarations using the ifDefDeclarations setting. This can be checked at compile-time using the @ifdef(...) annotation. sbt-ifdef defines compile, test, and scalaBinaryVersion:3 declarations out-of-box.

During the Test / compile, sbt-ifdef will pass in the test declaration, but not during Compile / compile.

package example

import com.eed3si9n.ifdef.ifdef

class A {
  def foo: Int = 42
}

@ifdef("test")
class ATest extends munit.FunSuite {
  test("hello") {
    val actual = new A().foo
    val expected = 42
    assertEquals(actual, expected)
  }
}

The above code combines the Compile configuration and the Test configuration into one source code.

sbt-ifdef also defines scalaBinaryVersion:<sbv> lifting the scalaBinaryVersion setting into the ifDefDeclarations setting. This allows Scala cross building using a single source code.

package example

import com.eed3si9n.ifdef.ifdef

class A {
  def foo: Int = 42

  @ifdef("scalaBinaryVersion:2.13")
  def bar: Int = 1

  @ifdef("scalaBinaryVersion:3")
  def bar: Int = 2
}

In the above, bar method will return 2 on Scala 3.x:

Welcome to Scala 3.6.4 (1.8.0_402, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> example.A().bar
val res0: Int = 2

You can add custom declarations in your build.sbt as follows:

// Add a custom definition for early access features
ThisBuild / ifDefDeclarations += "eap"

Then use them in your code:

// This method will only be available when "eap" is defined
@ifdef("eap")
def earlyAccessFeature(): Unit = {
  // Implementation of early access feature
}

ifdef is released under Apache License Version 2.0.

• ifdef in Scala via pre-typer processing, 2023
• ifdef macro in Scala, 2023
• Scala Preprocessor / Conditional Compilation scala-dev#640, 2019, Stefan Zeiger
• ThoughtWorksInc/enableIf.scala, 2016