Scala example source code file (Metalevels.scala)

This example Scala source code file (Metalevels.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

cannotreifyruntimesplice, collection, freeref, metalevels, nosymbol, reflection, reifier, reify, t, transformer, tree, treesplice

The Metalevels.scala Scala example source code

package scala.reflect.reify
package phases

import scala.collection.{ mutable }

trait Metalevels {
  self: Reifier =>

  import global._

  /**
   *  Makes sense of cross-stage bindings.
   *
   *  ----------------
   *
   *  Analysis of cross-stage bindings becomes convenient if we introduce the notion of metalevels.
   *  Metalevel of a tree is a number that gets incremented every time you reify something and gets decremented when you splice something.
   *  Metalevel of a symbol is equal to the metalevel of its definition.
   *
   *  Example 1. Consider the following snippet:
   *
   *    reify {
   *      val x = 2             // metalevel of symbol x is 1, because it's declared inside reify
   *      val y = reify{x}      // metalevel of symbol y is 1, because it's declared inside reify
   *                            // metalevel of Ident(x) is 2, because it's inside two reifies
   *      y.splice              // metalevel of Ident(y) is 0, because it's inside a designator of a splice
   *    }
   *
   *  Cross-stage bindings are introduced when symbol.metalevel != curr_metalevel.
   *  Both bindings introduced in Example 1 are cross-stage.
   *
   *  Depending on what side of the inequality is greater, the following situations might occur:
   *
   *  1) symbol.metalevel < curr_metalevel. In this case reifier will generate a free variable
   *  that captures both the name of the symbol (to be compiled successfully) and its value (to be run successfully).
   *  For example, x in Example 1 will be reified as follows: Ident(newFreeVar("x", IntTpe, x))
   *
   *  2) symbol.metalevel > curr_metalevel. This leads to a metalevel breach that violates intuitive perception of splicing.
   *  As defined in macro spec, splicing takes a tree and inserts it into another tree - as simple as that.
   *  However, how exactly do we do that in the case of y.splice? In this very scenario we can use dataflow analysis and inline it,
   *  but what if y were a var, and what if it were calculated randomly at runtime?
   *
   *  This question has a genuinely simple answer. Sure, we cannot resolve such splices statically (i.e. during macro expansion of `reify`),
   *  but now we have runtime toolboxes, so noone stops us from picking up that reified tree and evaluating it at runtime
   *  (in fact, this is something that `Expr.splice` does transparently).
   *
   *  This is akin to early vs late binding dilemma.
   *  The prior is faster, plus, the latter (implemented with reflection) might not work because of visibility issues or might be not available on all platforms.
   *  But the latter still has its uses, so I'm allowing metalevel breaches, but introducing the -Xlog-runtime-evals to log them.
   *
   *  upd. We no longer do that. In case of a runaway `splice` inside a `reify`, one will get a static error.
   *  Why? Unfortunately, the cute idea of transparently converting between static and dynamic splices has failed.
   *  1) Runtime eval that services dynamic splices requires scala-compiler.jar, which might not be on library classpath
   *  2) Runtime eval incurs a severe performance penalty, so it'd better to be explicit about it
   *
   *  ----------------
   *
   *  As we can see, the only problem is the fact that lhs'es of `splice` can be code blocks that can capture variables from the outside.
   *  Code inside the lhs of an `splice` is not reified, while the code from the enclosing reify is.
   *
   *  Hence some bindings become cross-stage, which is not bad per se (in fact, some cross-stage bindings have sane semantics, as in the example above).
   *  However this affects freevars, since they are delicate inter-dimensional beings that refer to both current and next planes of existence.
   *  When splicing tears the fabric of the reality apart, some freevars have to go single-dimensional to retain their sanity.
   *
   *  Example 2. Consider the following snippet:
   *
   *    reify {
   *      val x = 2
   *      reify{x}.splice
   *    }
   *
   *  Since the result of the inner reify is wrapped in a splice, it won't be reified
   *  together with the other parts of the outer reify, but will be inserted into that result verbatim.
   *
   *  The inner reify produces an Expr[Int] that wraps Ident(freeVar("x", IntTpe, x)).
   *  However the freevar the reification points to will vanish when the compiler processes the outer reify.
   *  That's why we need to replace that freevar with a regular symbol that will point to reified x.
   *
   *  Example 3. Consider the following fragment:
   *
   *    reify {
   *      val x = 2
   *      val y = reify{x}
   *      y.splice
   *    }
   *
   *  In this case the inner reify doesn't appear next to splice, so it will be reified together with x.
   *  This means that no special processing is needed here.
   *
   *  Example 4. Consider the following fragment:
   *
   *    reify {
   *      val x = 2
   *      {
   *        val y = 2
   *        val z = reify{reify{x + y}}
   *        z.splice
   *      }.splice
   *    }
   *
   *  The reasoning from Example 2 still holds here - we do need to inline the freevar that refers to x.
   *  However, we must not touch anything inside the splice'd block, because it's not getting reified.
   */
  val metalevels = new Transformer {
    var insideSplice = false
    val inlineableBindings = mutable.Map[TermName, Tree]()

    def withinSplice[T](op: => T) = {
      val old = insideSplice
      insideSplice = true
      try op
      finally insideSplice = old
    }

    // Q: here we deal with all sorts of reified trees. what about ReifiedType(_, _, _, _, _, _)?
    // A: nothing. reified trees give us problems because they sometimes create dimensional rifts as described above
    //    to the contrast, reified types (i.e. synthetic typetags materialized by Implicits.scala) always stay on the same metalevel as their enclosing code
    override def transform(tree: Tree): Tree = tree match {
      case TreeSplice(ReifiedTree(universe, mirror, symtab, rtree, tpe, rtpe, concrete)) =>
        if (reifyDebug) println("entering inlineable splice: " + tree)
        val inlinees = symtab.syms filter (_.isLocalToReifee)
        inlinees foreach (inlinee => symtab.symAliases(inlinee) foreach (alias => inlineableBindings(alias) = symtab.symBinding(inlinee)))
        val symtab1 = symtab -- inlinees
        if (reifyDebug) println("trimmed %s inlineable free defs from its symbol table: %s".format(inlinees.length, inlinees map (inlinee => symtab.symName(inlinee)) mkString(", ")))
        withinSplice { super.transform(TreeSplice(ReifiedTree(universe, mirror, symtab1, rtree, tpe, rtpe, concrete))) }
      case TreeSplice(splicee) =>
        if (reifyDebug) println("entering splice: " + splicee)
        val breaches = splicee filter (sub => sub.hasSymbolField && sub.symbol != NoSymbol && sub.symbol.metalevel > 0)
        if (!insideSplice && breaches.nonEmpty) {
          // we used to convert dynamic splices into runtime evals transparently, but we no longer do that
          // why? see comments above
          // if (settings.logRuntimeSplices.value) reporter.echo(tree.pos, "this splice cannot be resolved statically")
          // withinSplice { super.transform(tree) }
          if (reifyDebug) println("metalevel breach in %s: %s".format(tree, (breaches map (_.symbol)).distinct mkString ", "))
          CannotReifyRuntimeSplice(tree)
        } else {
          withinSplice { super.transform(tree) }
        }
      // todo. also inline usages of `inlineableBindings` in the symtab itself
      // e.g. a free$Foo can well use free$x, if Foo is path-dependent w.r.t x
      // FreeRef(_, _) check won't work, because metalevels of symbol table and body are different, hence, freerefs in symbol table look different from freerefs in body
      case FreeRef(_, name) if inlineableBindings contains name =>
        if (reifyDebug) println("inlineable free ref: %s in %s".format(name, showRaw(tree)))
        val inlined = reify(inlineableBindings(name))
        if (reifyDebug) println("verdict: inlined as %s".format(showRaw(inlined)))
        inlined
      case _ =>
        super.transform(tree)
    }
  }
}