This trait provides support for importers, a facility to migrate reflection artifacts between universes.
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.
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]
...
}
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