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Diffstat (limited to 'src/reflect/scala/reflect/macros/Evals.scala')
-rw-r--r-- | src/reflect/scala/reflect/macros/Evals.scala | 47 |
1 files changed, 46 insertions, 1 deletions
diff --git a/src/reflect/scala/reflect/macros/Evals.scala b/src/reflect/scala/reflect/macros/Evals.scala index 3837d749da..6aab3d5b02 100644 --- a/src/reflect/scala/reflect/macros/Evals.scala +++ b/src/reflect/scala/reflect/macros/Evals.scala @@ -1,9 +1,54 @@ package scala.reflect package macros +/** A slice of [[scala.reflect.macros.Context the Scala macros context]] that provides + * a facility to evaluate trees. + */ trait Evals { self: 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 `resetAllAttrs`. 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.resetAllAttrs(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 }
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