Trait scala.reflect.api.Internals

The API of free term symbols. The main source of information about symbols is the Symbols page.

$SYMACCESSORS

The API of free type symbols. The main source of information about symbols is the Symbols page.

$SYMACCESSORS

This trait provides support for importers, a facility to migrate reflection artifacts between universes. Note: this trait should typically be used only rarely.

Reflection artifacts, such as Symbols and Types, are contained in Universes. Typically all processing happens within a single Universe (e.g. a compile-time macro Universe or a runtime reflection Universe), but sometimes there is a need to migrate artifacts from one Universe to another. For example, runtime compilation works by importing runtime reflection trees into a runtime compiler universe, compiling the importees and exporting the result back.

Reflection artifacts are firmly grounded in their Universes, which is reflected by the fact that types of artifacts from different universes are not compatible. By using Importers, however, they be imported from one universe into another. For example, to import foo.bar.Baz from the source Universe to the target Universe, an importer will first check whether the entire owner chain exists in the target Universe. If it does, then nothing else will be done. Otherwise, the importer will recreate the entire owner chain and will import the corresponding type signatures into the target Universe.

Since importers match Symbol tables of the source and the target Universes using plain string names, it is programmer's responsibility to make sure that imports don't distort semantics, e.g., that foo.bar.Baz in the source Universe means the same that foo.bar.Baz does in the target Universe.

Example

Here's how one might implement a macro that performs compile-time evaluation of its argument by using a runtime compiler to compile and evaluate a tree that belongs to a compile-time compiler:

def staticEval[T](x: T) = macro staticEval[T]

def staticEval[T](c: scala.reflect.macros.blackbox.Context)(x: c.Expr[T]) = {
  // creates a runtime reflection universe to host runtime compilation
  import scala.reflect.runtime.{universe => ru}
  val mirror = ru.runtimeMirror(c.libraryClassLoader)
  import scala.tools.reflect.ToolBox
  val toolBox = mirror.mkToolBox()

  // runtime reflection universe and compile-time macro universe are different
  // therefore an importer is needed to bridge them
  // currently mkImporter requires a cast to correctly assign the path-dependent types
  val importer0 = ru.internal.mkImporter(c.universe)
  val importer = importer0.asInstanceOf[ru.internal.Importer { val from: c.universe.type }]

  // the created importer is used to turn a compiler tree into a runtime compiler tree
  // both compilers use the same classpath, so semantics remains intact
  val imported = importer.importTree(tree)

  // after the tree is imported, it can be evaluated as usual
  val tree = toolBox.untypecheck(imported.duplicate)
  val valueOfX = toolBox.eval(imported).asInstanceOf[T]
  ...
}

Reflection API exhibits a tension inherent to experimental things: on the one hand we want it to grow into a beautiful and robust API, but on the other hand we have to deal with immaturity of underlying mechanisms by providing not very pretty solutions to enable important use cases.

In Scala 2.10, which was our first stab at reflection API, we didn't have a systematic approach to dealing with this tension, sometimes exposing too much of internals (e.g. Symbol.deSkolemize) and sometimes exposing too little (e.g. there's still no facility to change owners, to do typing transformations, etc). This resulted in certain confusion with some internal APIs living among public ones, scaring the newcomers, and some internal APIs only available via casting, which requires intimate knowledge of the compiler and breaks compatibility guarantees.

This led to creation of the internal API module for the reflection API, which provides advanced APIs necessary for macros that push boundaries of the state of the art, clearly demarcating them from the more or less straightforward rest and providing compatibility guarantees on par with the rest of the reflection API (full compatibility within minor releases, best effort towards backward compatibility within major releases, clear replacement path in case of rare incompatible changes in major releases).

The internal module itself (the value that implements InternalApi) isn't defined here, in scala.reflect.api.Universe, but is provided on per-implementation basis. Runtime API endpoint (scala.reflect.runtime.universe) provides universe.compat: InternalApi, whereas compile-time API endpoints (instances of scala.reflect.macros.Context) provide c.compat: ContextInternalApi, which extends InternalApi with additional universe-specific and context-specific functionality.

Marks underlying reference to id as boxed.

Precondition: id must refer to a captured variable A reference such marked will refer to the boxed entity, no dereferencing with `.elem` is done on it. This tree node can be emitted by macros such as reify that call referenceCapturedVariable. It is eliminated in LambdaLift, where the boxing conversion takes place.

An extractor class to create and pattern match with syntax ReferenceToBoxed(ident). This AST node does not have direct correspondence to Scala code, and is emitted by macros to reference capture vars directly without going through elem.

For example:

var x = ... fun { x }

Will emit:

Ident(x)

Which gets transformed to:

Select(Ident(x), "elem")

If ReferenceToBoxed were used instead of Ident, no transformation would be performed.