Evals
Takes a typed wrapper for a tree of type T and evaluates it to a value of type T.
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
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.
EXPERIMENTAL
A slice of the Scala macros context that provides a facility to evaluate trees.