EXPERIMENTAL

A trait that defines strongly-typed tree wrappers and operations on them for use in Scala Reflection.

Expr wraps an abstract syntax tree (scala.reflect.api.Trees#Tree) and tags it with its type (scala.reflect.api.Types#Type).

Usually Exprs are created via scala.reflect.api.Universe#reify, in which case a compiler produces a scala.reflect.api.TreeCreator for the provided expression and also creates a complementary scala.reflect.api.TypeTags#WeakTypeTag that corresponds to the type of that expression.

Exprs can also be created manually via the Expr companion object, but then the burden of providing a TreeCreator lies on the programmer. Compile-time reflection via macros, as described in scala.reflect.macros.Aliases, provides an easier way to instantiate exprs manually. Manual creation, however, is very rarely needed when working with runtime reflection.

Expr can be migrated from one mirror to another by using the in method. Migration means that all symbolic references to classes/objects/packages in the expression are re-resolved within the new mirror (typically using that mirror's classloader). The default universe of an Expr is typically scala.reflect.runtime#universe, the default mirror is typically scala.reflect.runtime#currentMirror.