Scala example source code file (Variances.scala)

This example Scala source code file (Variances.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

annotation, bivariant, collection, list, methodtype, polytype, refinedtype, symbol, type, typeref, validatevariancemap, variance

The Variances.scala Scala example source code

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

package scala
package reflect
package internal

import Variance._
import scala.collection.{ mutable, immutable }
import scala.annotation.tailrec

/** See comments at scala.reflect.internal.Variance.
 */
trait Variances {
  self: SymbolTable =>

  /** Used in Refchecks.
   *  TODO - eliminate duplication with varianceInType
   */
  class VarianceValidator extends Traverser {
    private val escapedLocals = mutable.HashSet[Symbol]()
    // A flag for when we're in a refinement, meaning method parameter types
    // need to be checked.
    private var inRefinement = false
    @inline private def withinRefinement(body: => Type): Type = {
      val saved = inRefinement
      inRefinement = true
      try body finally inRefinement = saved
    }

    /** Is every symbol in the owner chain between `site` and the owner of `sym`
     *  either a term symbol or private[this]? If not, add `sym` to the set of
     *  esacped locals.
     *  @pre  sym.isLocalToThis
     */
    @tailrec final def checkForEscape(sym: Symbol, site: Symbol) {
      if (site == sym.owner || site == sym.owner.moduleClass || site.hasPackageFlag) () // done
      else if (site.isTerm || site.isPrivateLocal) checkForEscape(sym, site.owner) // ok - recurse to owner
      else escapedLocals += sym
    }

    protected def issueVarianceError(base: Symbol, sym: Symbol, required: Variance): Unit = ()

    // Flip occurrences of type parameters and parameters, unless
    //  - it's a constructor, or case class factory or extractor
    //  - it's a type parameter of tvar's owner.
    def shouldFlip(sym: Symbol, tvar: Symbol) = (
         sym.isParameter
      && !(tvar.isTypeParameterOrSkolem && sym.isTypeParameterOrSkolem && tvar.owner == sym.owner)
    )
    // return Bivariant if `sym` is local to a term
    // or is private[this] or protected[this]
    def isLocalOnly(sym: Symbol) = !sym.owner.isClass || (
         sym.isTerm // ?? shouldn't this be sym.owner.isTerm according to the comments above?
      && (sym.isLocalToThis || sym.isSuperAccessor) // super accessors are implicitly local #4345
      && !escapedLocals(sym)
    )

    private object ValidateVarianceMap extends TypeMap(trackVariance = true) {
      private var base: Symbol = _

      /** The variance of a symbol occurrence of `tvar` seen at the level of the definition of `base`.
       *  The search proceeds from `base` to the owner of `tvar`.
       *  Initially the state is covariant, but it might change along the search.
       *
       *  A local alias type is treated as Bivariant;
       *  this is OK because we always expand aliases for variance checking.
       *  However, for an alias which might be externally visible, we must assume Invariant,
       *  because there may be references to the type parameter that are not checked,
       *  leading to unsoundness (see SI-6566).
       */
      def relativeVariance(tvar: Symbol): Variance = {
        def nextVariance(sym: Symbol, v: Variance): Variance = (
          if (shouldFlip(sym, tvar)) v.flip
          else if (isLocalOnly(sym)) Bivariant
          else if (sym.isAliasType) (
            // Unsound pre-2.11 behavior preserved under -Xsource:2.10
            if (settings.isScala211 || sym.isOverridingSymbol) Invariant
            else {
              deprecationWarning(sym.pos, s"Construct depends on unsound variance analysis and will not compile in scala 2.11 and beyond")
              Bivariant
            }
          )
          else v
        )
        def loop(sym: Symbol, v: Variance): Variance = (
          if (sym == tvar.owner || v.isBivariant) v
          else loop(sym.owner, nextVariance(sym, v))
        )
        loop(base, Covariant)
      }
      def isUncheckedVariance(tp: Type) = tp match {
        case AnnotatedType(annots, _)    => annots exists (_ matches definitions.uncheckedVarianceClass)
        case _                           => false
      }

      private def checkVarianceOfSymbol(sym: Symbol) {
        val relative = relativeVariance(sym)
        val required = relative * variance
        if (!relative.isBivariant) {
          def sym_s  = s"$sym (${sym.variance}${sym.locationString})"
          def base_s = s"$base in ${base.owner}" + (if (base.owner.isClass) "" else " in " + base.owner.enclClass)
          log(s"verifying $sym_s is $required at $base_s")
          if (sym.variance != required)
            issueVarianceError(base, sym, required)
        }
      }
      override def mapOver(decls: Scope): Scope = {
        decls foreach (sym => withVariance(if (sym.isAliasType) Invariant else variance)(this(sym.info)))
        decls
      }
      private def resultTypeOnly(tp: Type) = tp match {
        case mt: MethodType => !inRefinement
        case pt: PolyType   => true
        case _              => false
      }

      /** For PolyTypes, type parameters are skipped because they are defined
       *  explicitly (their TypeDefs will be passed here.) For MethodTypes, the
       *  same is true of the parameters (ValDefs) unless we are inside a
       *  refinement, in which case they are checked from here.
       */
      def apply(tp: Type): Type = tp match {
        case _ if isUncheckedVariance(tp)                    => tp
        case _ if resultTypeOnly(tp)                         => this(tp.resultType)
        case TypeRef(_, sym, _) if sym.isAliasType           => this(tp.normalize)
        case TypeRef(_, sym, _) if !sym.variance.isInvariant => checkVarianceOfSymbol(sym) ; mapOver(tp)
        case RefinedType(_, _)                               => withinRefinement(mapOver(tp))
        case ClassInfoType(parents, _, _)                    => parents foreach this ; tp
        case mt @ MethodType(_, result)                      => flipped(mt.paramTypes foreach this) ; this(result)
        case _                                               => mapOver(tp)
      }
      def validateDefinition(base: Symbol) {
        val saved = this.base
        this.base = base
        try apply(base.info)
        finally this.base = saved
      }
    }

    /** Validate variance of info of symbol `base` */
    private def validateVariance(base: Symbol) {
      ValidateVarianceMap validateDefinition base
    }

    override def traverse(tree: Tree) {
      def sym = tree.symbol
      // No variance check for object-private/protected methods/values.
      // Or constructors, or case class factory or extractor.
      def skip = (
           sym == NoSymbol
        || sym.isLocalToThis
        || sym.owner.isConstructor
        || sym.owner.isCaseApplyOrUnapply
      )
      tree match {
        case defn: MemberDef if skip =>
          debuglog(s"Skipping variance check of ${sym.defString}")
        case ClassDef(_, _, _, _) | TypeDef(_, _, _, _) =>
          validateVariance(sym)
          super.traverse(tree)
        // ModuleDefs need not be considered because they have been eliminated already
        case ValDef(_, _, _, _) =>
          validateVariance(sym)
        case DefDef(_, _, tparams, vparamss, _, _) =>
          validateVariance(sym)
          traverseTrees(tparams)
          traverseTreess(vparamss)
        case Template(_, _, _) =>
          super.traverse(tree)
        case CompoundTypeTree(templ) =>
          super.traverse(tree)

        // SI-7872 These two cases make sure we don't miss variance exploits
        // in originals, e.g. in `foo[({type l[+a] = List[a]})#l]`
        case tt @ TypeTree() if tt.original != null =>
          super.traverse(tt.original)
        case tt : TypTree =>
          super.traverse(tt)

        case _ =>
      }
    }
  }

  /** Compute variance of type parameter `tparam` in all types `tps`. */
  def varianceInTypes(tps: List[Type])(tparam: Symbol): Variance =
    fold(tps map (tp => varianceInType(tp)(tparam)))

  /** Compute variance of type parameter `tparam` in type `tp`. */
  def varianceInType(tp: Type)(tparam: Symbol): Variance = {
    def inArgs(sym: Symbol, args: List[Type]): Variance = fold(map2(args, sym.typeParams)((a, p) => inType(a) * p.variance))
    def inSyms(syms: List[Symbol]): Variance            = fold(syms map inSym)
    def inTypes(tps: List[Type]): Variance              = fold(tps map inType)

    def inSym(sym: Symbol): Variance = if (sym.isAliasType) inType(sym.info).cut else inType(sym.info)
    def inType(tp: Type): Variance   = tp match {
      case ErrorType | WildcardType | NoType | NoPrefix => Bivariant
      case ThisType(_) | ConstantType(_)                => Bivariant
      case TypeRef(_, `tparam`, _)                      => Covariant
      case BoundedWildcardType(bounds)                  => inType(bounds)
      case NullaryMethodType(restpe)                    => inType(restpe)
      case SingleType(pre, sym)                         => inType(pre)
      case TypeRef(pre, _, _) if tp.isHigherKinded      => inType(pre)                 // a type constructor cannot occur in tp's args
      case TypeRef(pre, sym, args)                      => inType(pre)                 & inArgs(sym, args)
      case TypeBounds(lo, hi)                           => inType(lo).flip             & inType(hi)
      case RefinedType(parents, defs)                   => inTypes(parents)            & inSyms(defs.toList)
      case MethodType(params, restpe)                   => inSyms(params).flip         & inType(restpe)
      case PolyType(tparams, restpe)                    => inSyms(tparams).flip        & inType(restpe)
      case ExistentialType(tparams, restpe)             => inSyms(tparams)             & inType(restpe)
      case AnnotatedType(annots, tp)                    => inTypes(annots map (_.atp)) & inType(tp)
    }

    inType(tp)
  }
}