Macro Projects 

Introduction 

Some common problems arise when working with macros.

1. The current macro implementation in the compiler requires that macro implementations be compiled before they are used. The solution is typically to put the macros in a subproject or in their own configuration.
2. Sometimes the macro implementation should be distributed with the main code that uses them and sometimes the implementation should not be distributed at all.

The rest of the page shows example solutions to these problems.

Defining the Project Relationships 

The macro implementation will go in a subproject in the macro/ directory. The core project in the core/ directory will depend on this subproject and use the macro. This configuration is shown in the following build definition. build.sbt:

lazy val commonSettings = Seq(
  scalaVersion := "2.12.18",
  organization := "com.example"
)
lazy val scalaReflect = Def.setting { "org.scala-lang" % "scala-reflect" % scalaVersion.value }

lazy val core = (project in file("core"))
  .dependsOn(macroSub)
  .settings(
    commonSettings,
    // other settings here
  )

lazy val macroSub = (project in file("macro"))
  .settings(
    commonSettings,
    libraryDependencies += scalaReflect.value
    // other settings here
  )

This specifies that the macro implementation goes in macro/src/main/scala/ and tests go in macro/src/test/scala/. It also shows that we need a dependency on the compiler for the macro implementation. As an example macro, we’ll use desugar from macrocosm. macro/src/main/scala/demo/Demo.scala:

package demo

import language.experimental.macros
import scala.reflect.macros.blackbox.Context

object Demo {

  // Returns the tree of `a` after the typer, printed as source code.
  def desugar(a: Any): String = macro desugarImpl

  def desugarImpl(c: Context)(a: c.Expr[Any]) = {
    import c.universe._

    val s = show(a.tree)
    c.Expr(
      Literal(Constant(s))
    )
  }
}

macro/src/test/scala/demo/Usage.scala:

package demo

object Usage {
   def main(args: Array[String]): Unit = {
      val s = Demo.desugar(List(1, 2, 3).reverse)
      println(s)
   }
}

This can be then run at the console:

$ sbt
> macroSub/Test/run
scala.collection.immutable.List.apply[Int](1, 2, 3).reverse

Actual tests can be defined and run as usual with macro/test.

The main project can use the macro in the same way that the tests do. For example,

core/src/main/scala/MainUsage.scala:

package demo

object Usage {
   def main(args: Array[String]): Unit = {
      val s = Demo.desugar(List(6, 4, 5).sorted)
      println(s)
   }
}

$ sbt
> core/run
scala.collection.immutable.List.apply[Int](6, 4, 5).sorted[Int](math.this.Ordering.Int)

Common Interface 

Sometimes, the macro implementation and the macro usage should share some common code. In this case, declare another subproject for the common code and have the main project and the macro subproject depend on the new subproject. For example, the project definitions from above would look like:

lazy val commonSettings = Seq(
  scalaVersion := "2.12.18",
  organization := "com.example"
)
lazy val scalaReflect = Def.setting { "org.scala-lang" % "scala-reflect" % scalaVersion.value }

lazy val core = (project in file("core"))
  .dependsOn(macroSub, util)
  .settings(
    commonSettings,
    // other settings here
  )

lazy val macroSub = (project in file("macro"))
  .dependsOn(util)
  .settings(
    commonSettings,
    libraryDependencies += scalaReflect.value
    // other settings here
  )

lazy util = (project in file("util"))
  .settings(
    commonSettings,
    // other setting here
  )

Code in util/src/main/scala/ is available for both the macroSub and main projects to use.

Distribution 

To include the macro code with the core code, add the binary and source mappings from the macro subproject to the core project. And also macro subproject should be removed from core project dependency in publishing. For example, the core Project definition above would now look like:

lazy val core = (project in file("core"))
  .dependsOn(macroSub % "compile-internal, test-internal")
  .settings(
    commonSettings,
    // include the macro classes and resources in the main jar
    Compile / packageBin / mappings ++= (macroSub / Compile / packageBin / mappings).value,
    // include the macro sources in the main source jar
    Compile / packageSrc / mappings ++= (macroSub / Compile / packageSrc / mappings).value
  )

You may wish to disable publishing the macro implementation. This is done by overriding publish and publishLocal to do nothing:

lazy val macroSub = (project in file("macro"))
  .settings(
    commonSettings,
    libraryDependencies += scalaReflect.value,
    publish := {},
    publishLocal := {}
  )

The techniques described here may also be used for the common interface described in the previous section.