Scala example source code file (Trees.scala)

This example Scala source code file (Trees.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

compiler, doccomment, docdef, injectderivedvalue, name, nsc, reflection, selectfromarray, symbol, tree, treecopier, type, typetreewithdeferredrefcheck

The Trees.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author  Martin Odersky
 */

package scala.tools.nsc
package ast

import scala.reflect.ClassTag
import scala.reflect.internal.Flags.BYNAMEPARAM
import scala.reflect.internal.Flags.DEFAULTPARAM
import scala.reflect.internal.Flags.IMPLICIT
import scala.reflect.internal.Flags.PARAM
import scala.reflect.internal.Flags.PARAMACCESSOR
import scala.reflect.internal.Flags.PRESUPER
import scala.reflect.internal.Flags.TRAIT
import scala.compat.Platform.EOL

trait Trees extends scala.reflect.internal.Trees { self: Global =>
  // --- additional cases --------------------------------------------------------
  /** Only used during parsing */
  case class Parens(args: List[Tree]) extends Tree

  /** Documented definition, eliminated by analyzer */
  case class DocDef(comment: DocComment, definition: Tree)
       extends Tree {
    override def symbol: Symbol = definition.symbol
    override def symbol_=(sym: Symbol) { definition.symbol = sym }
    override def isDef = definition.isDef
    override def isTerm = definition.isTerm
    override def isType = definition.isType
  }

 /** Array selection `<qualifier> . <name>` only used during erasure */
  case class SelectFromArray(qualifier: Tree, name: Name, erasure: Type)
       extends RefTree with TermTree

  /** Derived value class injection (equivalent to: `new C(arg)` after erasure); only used during erasure.
   *  The class `C` is stored as a tree attachment.
   */
  case class InjectDerivedValue(arg: Tree)
       extends SymTree with TermTree

  class PostfixSelect(qual: Tree, name: Name) extends Select(qual, name)

  /** emitted by typer, eliminated by refchecks */
  case class TypeTreeWithDeferredRefCheck()(val check: () => TypeTree) extends TypTree

  // --- factory methods ----------------------------------------------------------

  /** Factory method for a primary constructor super call `super.<init>(args_1)...(args_n)`
   */
  def PrimarySuperCall(argss: List[List[Tree]]): Tree = argss match {
    case Nil        => Apply(gen.mkSuperInitCall, Nil)
    case xs :: rest => rest.foldLeft(Apply(gen.mkSuperInitCall, xs): Tree)(Apply.apply)
  }

  /** Construct class definition with given class symbol, value parameters,
   *  supercall arguments and template body.
   *
   *  @param sym        the class symbol
   *  @param constrMods the modifiers for the class constructor, i.e. as in `class C private (...)`
   *  @param vparamss   the value parameters -- if they have symbols they
   *                    should be owned by `sym`
   *  @param body       the template statements without primary constructor
   *                    and value parameter fields.
   */
  def ClassDef(sym: Symbol, constrMods: Modifiers, vparamss: List[List[ValDef]], body: List[Tree], superPos: Position): ClassDef = {
    // "if they have symbols they should be owned by `sym`"
    assert(mforall(vparamss)(_.symbol.owner == sym), (mmap(vparamss)(_.symbol), sym))

    ClassDef(sym,
      gen.mkTemplate(sym.info.parents map TypeTree,
                    if (sym.thisSym == sym || phase.erasedTypes) noSelfType else ValDef(sym.thisSym),
                    constrMods, vparamss, body, superPos))
  }

 // --- subcomponents --------------------------------------------------

  object treeInfo extends {
    val global: Trees.this.type = self
  } with TreeInfo

  // --- additional cases in operations ----------------------------------

  override protected def xtraverse(traverser: Traverser, tree: Tree): Unit = tree match {
    case Parens(ts) =>
      traverser.traverseTrees(ts)
    case DocDef(comment, definition) =>
      traverser.traverse(definition)
    case SelectFromArray(qualifier, selector, erasure) =>
      traverser.traverse(qualifier)
    case InjectDerivedValue(arg) =>
      traverser.traverse(arg)
    case TypeTreeWithDeferredRefCheck() =>
      // (and rewrap the result? how to update the deferred check? would need to store wrapped tree instead of returning it from check)
    case _ => super.xtraverse(traverser, tree)
  }

  trait TreeCopier extends super.InternalTreeCopierOps {
    def DocDef(tree: Tree, comment: DocComment, definition: Tree): DocDef
    def SelectFromArray(tree: Tree, qualifier: Tree, selector: Name, erasure: Type): SelectFromArray
    def InjectDerivedValue(tree: Tree, arg: Tree): InjectDerivedValue
    def TypeTreeWithDeferredRefCheck(tree: Tree): TypeTreeWithDeferredRefCheck
  }
  implicit val TreeCopierTag: ClassTag[TreeCopier] = ClassTag[TreeCopier](classOf[TreeCopier])

  def newStrictTreeCopier: TreeCopier = new StrictTreeCopier
  def newLazyTreeCopier: TreeCopier = new LazyTreeCopier

  class StrictTreeCopier extends super.StrictTreeCopier with TreeCopier {
    def DocDef(tree: Tree, comment: DocComment, definition: Tree) =
      new DocDef(comment, definition).copyAttrs(tree)
    def SelectFromArray(tree: Tree, qualifier: Tree, selector: Name, erasure: Type) =
      new SelectFromArray(qualifier, selector, erasure).copyAttrs(tree)
    def InjectDerivedValue(tree: Tree, arg: Tree) =
      new InjectDerivedValue(arg).copyAttrs(tree)
    def TypeTreeWithDeferredRefCheck(tree: Tree) = tree match {
      case dc@TypeTreeWithDeferredRefCheck() => new TypeTreeWithDeferredRefCheck()(dc.check).copyAttrs(tree)
    }
  }

  class LazyTreeCopier extends super.LazyTreeCopier with TreeCopier {
    def DocDef(tree: Tree, comment: DocComment, definition: Tree) = tree match {
      case t @ DocDef(comment0, definition0)
      if (comment0 == comment) && (definition0 == definition) => t
      case _ => this.treeCopy.DocDef(tree, comment, definition)
    }
    def SelectFromArray(tree: Tree, qualifier: Tree, selector: Name, erasure: Type) = tree match {
      case t @ SelectFromArray(qualifier0, selector0, _)
      if (qualifier0 == qualifier) && (selector0 == selector) => t
      case _ => this.treeCopy.SelectFromArray(tree, qualifier, selector, erasure)
    }
    def InjectDerivedValue(tree: Tree, arg: Tree) = tree match {
      case t @ InjectDerivedValue(arg0)
      if (arg0 == arg) => t
      case _ => this.treeCopy.InjectDerivedValue(tree, arg)
    }
    def TypeTreeWithDeferredRefCheck(tree: Tree) = tree match {
      case t @ TypeTreeWithDeferredRefCheck() => t
      case _ => this.treeCopy.TypeTreeWithDeferredRefCheck(tree)
    }
  }

  class Transformer extends super.Transformer {
    def transformUnit(unit: CompilationUnit) {
      try unit.body = transform(unit.body)
      catch {
        case ex: Exception =>
          log(supplementErrorMessage("unhandled exception while transforming "+unit))
          throw ex
      }
    }
  }

  // used when a phase is disabled
  object noopTransformer extends Transformer {
    override def transformUnit(unit: CompilationUnit): Unit = {}
  }

  override protected def xtransform(transformer: super.Transformer, tree: Tree): Tree = tree match {
    case DocDef(comment, definition) =>
      transformer.treeCopy.DocDef(tree, comment, transformer.transform(definition))
    case SelectFromArray(qualifier, selector, erasure) =>
      transformer.treeCopy.SelectFromArray(
        tree, transformer.transform(qualifier), selector, erasure)
    case InjectDerivedValue(arg) =>
      transformer.treeCopy.InjectDerivedValue(
        tree, transformer.transform(arg))
    case TypeTreeWithDeferredRefCheck() =>
      transformer.treeCopy.TypeTreeWithDeferredRefCheck(tree)
  }

  object resetPos extends Traverser {
    override def traverse(t: Tree) {
      if (t != EmptyTree) t.setPos(NoPosition)
      super.traverse(t)
    }
  }

  // Finally, noone resetAllAttrs it anymore, so I'm removing it from the compiler.
  // Even though it's with great pleasure I'm doing that, I'll leave its body here to warn future generations about what happened in the past.
  //
  // So what actually happened in the past is that we used to have two flavors of resetAttrs: resetAllAttrs and resetLocalAttrs.
  // resetAllAttrs destroyed all symbols and types in the tree in order to reset its state to something suitable for retypechecking
  // and/or embedding into bigger trees / different lexical scopes. (Btw here's some background on why people would want to use
  // reset attrs in the first place: https://groups.google.com/forum/#!topic/scala-internals/TtCTPlj_qcQ).
  //
  // However resetAllAttrs was more of a poison than of a treatment, because along with locally defined symbols that are the cause
  // for almost every or maybe even every case of tree corruption, it erased external bindings that sometimes could not be restored.
  // This is how we came up with resetLocalAttrs that left external bindings alone, and that was a big step forward.
  // Then slowly but steadily we've evicted all usages of resetAllAttrs from our codebase in favor of resetLocalAttrs
  // and have been living happily ever after.
  //
  // def resetAllAttrs(x: Tree, leaveAlone: Tree => Boolean = null): Tree = new ResetAttrs(localOnly = false, leaveAlone).transform(x)

  // upd. Unfortunately this didn't work out quite as we expected. The last two users of resetAllAttrs:
  // reification and typedLabelDef broke in very weird ways when we replaced resetAllAttrs with resetLocalAttrs
  // (see SI-8316 change from resetAllAttrs to resetLocalAttrs in reifiers broke Slick and
  // SI-8318 NPE in mixin in scala-continuations for more information).
  // Given that we're supposed to release 2.11.0-RC1 in less than a week, I'm temporarily reinstating resetAllAttrs
  // until we have time to better understand what's going on. In order to dissuade people from using it,
  // it now comes with a new, ridiculous name.
  /** @see ResetAttrs */
  def brutallyResetAttrs(x: Tree, leaveAlone: Tree => Boolean = null): Tree = new ResetAttrs(brutally = true, leaveAlone).transform(x)

  /** @see ResetAttrs */
  def resetAttrs(x: Tree): Tree = new ResetAttrs(brutally = false, leaveAlone = null).transform(x)

  /** A transformer which resets symbol and tpe fields of all nodes in a given tree,
   *  with special treatment of:
   *    TypeTree nodes: are replaced by their original if it exists, otherwise tpe field is reset
   *                    to empty if it started out empty or refers to local symbols (which are erased).
   *    TypeApply nodes: are deleted if type arguments end up reverted to empty
   *    This(pkg) nodes where pkg is a package: these are kept.
   *
   *  (bq:) This transformer has mutable state and should be discarded after use
   */
  private class ResetAttrs(brutally: Boolean, leaveAlone: Tree => Boolean) {
    // this used to be based on -Ydebug, but the need for logging in this code is so situational
    // that I've reverted to a hard-coded constant here.
    val debug = false
    val trace = scala.tools.nsc.util.trace when debug

    val locals = util.HashSet[Symbol](8)
    val orderedLocals = scala.collection.mutable.ListBuffer[Symbol]()
    def registerLocal(sym: Symbol) {
      if (sym != null && sym != NoSymbol) {
        if (debug && !(locals contains sym)) orderedLocals append sym
        locals addEntry sym
      }
    }

    class MarkLocals extends self.Traverser {
      def markLocal(tree: Tree) {
        if (tree.symbol != null && tree.symbol != NoSymbol) {
          val sym = tree.symbol
          registerLocal(sym)
          registerLocal(sym.sourceModule)
          registerLocal(sym.moduleClass)
          registerLocal(sym.companionClass)
          registerLocal(sym.companionModule)
          registerLocal(sym.deSkolemize)
          sym match {
            case sym: TermSymbol => registerLocal(sym.referenced)
            case _ => ;
          }
        }
      }

      override def traverse(tree: Tree) = {
        tree match {
         case _: DefTree | Function(_, _) | Template(_, _, _) =>
           markLocal(tree)
         case _ =>
           tree
        }

        super.traverse(tree)
      }
    }

    class Transformer extends self.Transformer {
      override def transform(tree: Tree): Tree = {
        if (leaveAlone != null && leaveAlone(tree))
          tree
        else
          super.transform {
            tree match {
              case tree if !tree.canHaveAttrs =>
                tree
              case tpt: TypeTree =>
                if (tpt.original != null)
                  transform(tpt.original)
                else {
                  val refersToLocalSymbols = tpt.tpe != null && (tpt.tpe exists (tp => locals contains tp.typeSymbol))
                  val isInferred = tpt.wasEmpty
                  if (refersToLocalSymbols || isInferred) {
                    tpt.duplicate.clearType()
                  } else {
                    tpt
                  }
                }
              // If one of the type arguments of a TypeApply gets reset to an empty TypeTree, then this means that:
              // 1) It isn't empty now (tpt.tpe != null), but it was empty before (tpt.wasEmpty).
              // 2) Thus, its argument got inferred during a preceding typecheck.
              // 3) Thus, all its arguments were inferred (because scalac can only infer all or nothing).
              // Therefore, we can safely erase the TypeApply altogether and have it inferred once again in a subsequent typecheck.
              // UPD: Actually there's another reason for erasing a type behind the TypeTree
              // is when this type refers to symbols defined in the tree being processed.
              // These symbols will be erased, because we can't leave alive a type referring to them.
              // Here we can only hope that everything will work fine afterwards.
              case TypeApply(fn, args) if args map transform exists (_.isEmpty) =>
                transform(fn)
              case EmptyTree =>
                tree
              case _ =>
                val dupl = tree.duplicate
                // Typically the resetAttrs transformer cleans both symbols and types.
                // However there are exceptions when we cannot erase symbols due to idiosyncrasies of the typer.
                // vetoXXX local variables declared below describe the conditions under which we cannot erase symbols.
                //
                // The first reason to not erase symbols is the threat of non-idempotency (SI-5464).
                // Here we take care of references to package classes (SI-5705).
                // There are other non-idempotencies, but they are not worked around yet.
                //
                // The second reason has to do with the fact that resetAttrs needs to be less destructive.
                // Erasing locally-defined symbols is useful to prevent tree corruption, but erasing external bindings is not,
                // therefore we want to retain those bindings, especially given that restoring them can be impossible
                // if we move these trees into lexical contexts different from their original locations.
                if (dupl.hasSymbol) {
                  val sym = dupl.symbol
                  val vetoScope = !brutally && !(locals contains sym) && !(locals contains sym.deSkolemize)
                  val vetoThis = dupl.isInstanceOf[This] && sym.isPackageClass
                  if (!(vetoScope || vetoThis)) dupl.symbol = NoSymbol
                }
                dupl.clearType()
            }
          }
      }
    }

    def transform(x: Tree): Tree = {
      new MarkLocals().traverse(x)

      if (debug) {
        assert(locals.size == orderedLocals.size)
        val msg = orderedLocals.toList filter {_ != NoSymbol} map {"  " + _} mkString EOL
        trace("locals (%d total): %n".format(orderedLocals.size))(msg)
      }

      new Transformer().transform(x)
    }
  }

  /* New pattern matching cases:

   case Parens(expr)                                               (only used during parsing)
   case DocDef(comment, defn) =>                                   (eliminated by typer)
   case TypeTreeWithDeferredRefCheck() =>                          (created and eliminated by typer)
   case SelectFromArray(_, _, _) =>                                (created and eliminated by erasure)
   case InjectDerivedValue(_) =>                                   (created and eliminated by erasure)

  */

 }