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Plugin nits
src/main/scala Dropped an obsolete test. Expanded annotation reach. Privacy: Readme


Caveat: this is only at the early stages of experimentation, and since it may alter the Scala semantics of your code, well... you've been warned :-)

Scalaxy/Privacy is a Scala compiler plugin that:

  • Changes default visibility from public to private[this] (public requires a @public annotation).

    @public object Foo {
      val privateByDefault = 10
      @public val explicitlyPublic = 12
  • Warns about non-trivial public methods and values without type annotations

    object Foo {
      // Warning: public `f` method has a non-trivial return type without type annotation.
      @public def f(x: Int) = if (x < 0) "1" else 2

Why, oh why??

The default public visibility of Scala definitions is often at odds with the principle of encapsulation. That's why some coding guidelines like Twitter's excellent Effective Scala recommend to mark methods as private[this] when they don't need to be public (which may have fringe performance benefits as well).

While this is great practice, it can quickly lead to lots of private[this] boilerplate, which this compiler plugin aims to remove the need for. The bet here is that with Scalaxy/Privacy:

  • You'll pay more attention to what needs to be @public, because you'll need to make that explicit: your code will be better encapsulated throughout.
  • You'll type less @public annotations than you would've typed private or private[this] modifiers, which means less code to write and read.
  • You'll be able to migrate your code away from Scalaxy/Privacy when you're bored of it / whenever I die or fail to update the plugin on time for the subsequent Scala version, using Scalaxy/Privacy's migration diff generator.

Also, with the extra warnings about missing type annotations for public members with non-trivial bodies:

  • You'll make your code more readable,
  • You'll spot unintentional propagations of weird types (no more returning Unit or MyPrivateLocalType three levels deep by mistake)

Now well... one may consider this as a language fork, which it might well be, somehow, but don't you worry: even if you convert all your code to using @public instead of private, the plugin will (soon) allow you to migrate all your code back to its equivalent traditional Scala, by generating the diffs you'll need to apply to it.

Longer examples

@public object Foo {
  // Any val, def, class or object not marked with @public is assumed to be private[this].
  val privateByDefault = 10

  // The @public annotation is removed by the compiler plugin.
  @public val explicitlyPublic = 12

  // Warning: public `f` method has a non-trivial return type without type annotation.
  @public def f(x: Int) = if (x < 0) "1" else 2

  // Ok.
  @public def g(x: Int): Int = f(x)

println(Foo.privateByDefault) // Error: Scalaxy/Privacy made that one private[this].
println(Foo.explicitlyPublic) // Ok.

// Regular Scala visibility rules apply within elements tagged with @noprivacy
@noprivacy object Bar {
  val publicByDefault = 10
  private val explicitlyPrivate = 12

println(Bar.publicByDefault)   // Ok.
println(Bar.explicitlyPrivate) // Error.

If that wasn't long enough, have a look at the following tests:

More details

Private by default

  • protected, private, override and abstract definitions are unmodified
  • Accessors of case classes' canonical fields are unmodified:

    case class Foo(thisIsPublic: Int) {
      val thisIsPrivate = 10
  • Code that compiles with the plugin will most likely compile without, unless there's name clashes due to wildcard imports (without the plugin, more names will be publicly visible). This means that IDEs can grok code annotated with @public and @noprivacy without the need to run the plugin, because the code they'll see will also be valid Scala.

Warnings on missing type annotations for public declarations

  • vals and defs which body is considered trivial are not required to have a type annotation. For instance, def x = List(1, 2) and def y = List[Int]() obviously have a return type of List[Int], so no warning is needed. However, def z = List(1, "2") is less trivial, so the plugin will warn about it. The justification for these special cases (which affect List, Set, Seq, Array, Iterable, Traversable and to a lesser extent, Map) is that annotating return type of trivial methods brings no value to users, and the plugin aims to make your code more maintainable while cutting the boilerplate.
  • Method and value overrides don't need to have type annotations: the risk of returning an unexpected type is very low, since the parent definition is likely to have a proper type annotation.


If you're using sbt 0.13.0+, just put the following lines in build.sbt:

scalaVersion := "2.11.6"

autoCompilerPlugins := true

// This compile-time dependency brings @public and @noprivacy, which are only needed to avoid confusing IDEs.
libraryDependencies += "com.nativelibs4java" %% "scalaxy-privacy" % "0.3-SNAPSHOT" % "provided"

// Scalaxy/Privacy compiler plugin.
addCompilerPlugin("com.nativelibs4java" %% "scalaxy-privacy-plugin" % "0.3-SNAPSHOT")

// Ensure Scalaxy/Privacy's plugin is used.
scalacOptions += "-Xplugin-require:scalaxy-privacy"

// Scalaxy/Privacy snapshots are published on the Sonatype repository.
resolvers += Resolver.sonatypeRepo("snapshots")


  • Fix issue of double modifiers:

    @public object Foo { val x = 10 }
    class Bar(x: Int, y: Int) { val x = 10 }
  • Check there aren't weird corner cases


If you want to build / test / hack on this project:

  • Make sure to use paulp's sbt script with sbt 0.12.2+
  • Use the following commands to checkout the sources and build the tests continuously:

    git clone git://
    cd Scalaxy
    sbt "project scalaxy-privacy-plugin" "; clean ; ~test"
  • Test with:

    git clone git://
    cd Scalaxy
    sbt "project scalaxy-privacy-plugin" "run examples/Test.scala"
  • You can also use plain scalac directly, once Scalaxy/Privacy's JAR is cached by sbt / Ivy:

    git clone git://
    cd Scalaxy
    sbt update
    cd Privacy
    scalac -Xplugin:$HOME/.ivy2/cache/com.nativelibs4java/scalaxy-privacy-plugin_2.10/jars/scalaxy-privacy-plugin_2.10-0.3-SNAPSHOT.jar examples/Test.scala
    scalac examples/Test.scala
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