Scala example source code file (Evals.scala)

This example Scala source code file (Evals.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

evals, expr, t

The Evals.scala Scala example source code

package scala
package reflect
package macros

/**
 * <span class="badge badge-red" style="float: right;">EXPERIMENTAL</span>
 *
 *  A slice of [[scala.reflect.macros.blackbox.Context the Scala macros context]] that provides
 *  a facility to evaluate trees.
 */
trait Evals {
  self: blackbox.Context =>

  /** Takes a typed wrapper for a tree of type `T` and evaluates it to a value of type `T`.
   *
   *  Can be used to perform compile-time computations on macro arguments to the extent
   *  permitted by the shape of the arguments.
   *
   *  Known issues: because of [[https://issues.scala-lang.org/browse/SI-5748 https://issues.scala-lang.org/browse/SI-5748]]
   *  trees being evaluated first need to undergo `untypecheck`. Resetting symbols and types
   *  mutates the tree in place, therefore the conventional approach is to `duplicate` the tree first.
   *
   *  {{{
   *  scala> def impl(c: Context)(x: c.Expr[String]) = {
   *       | val x1 = c.Expr[String](c.untypecheck(x.tree.duplicate))
   *       | println(s"compile-time value is: ${c.eval(x1)}")
   *       | x
   *       | }
   *  impl: (c: Context)(x: c.Expr[String])c.Expr[String]
   *
   *  scala> def test(x: String) = macro impl
   *  test: (x: String)String
   *
   *  scala> test("x")
   *  compile-time value is: x
   *  res0: String = x
   *
   *  scala> test("x" + "y")
   *  compile-time value is: xy
   *  res1: String = xy
   *
   *  scala> val x = "x"
   *  x: String = x
   *
   *  scala> test(x + "y")
   *  compile-time value is: xy
   *  res2: String = xy
   *
   *  scala> { val x = "x"; test(x + "y") }
   *  error: exception during macro expansion:
   *  scala.tools.reflect.ToolBoxError: reflective compilation failed
   *  }}}
   *
   *  Note that in the last case evaluation has failed, because the argument of a macro
   *  refers to a runtime value `x`, which is unknown at compile time.
   */
  def eval[T](expr: Expr[T]): T
}