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Scala example source code file (TypeConstraints.scala)

This example Scala source code file (TypeConstraints.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

boolean, collection, list, nil, notype, type, typeconstraint, typeconstraints, typevar, undopair, undopairs

The TypeConstraints.scala Scala example source code

package scala
package reflect
package internal
package tpe

import scala.collection.{ generic }
import generic.Clearable

private[internal] trait TypeConstraints {
  self: SymbolTable =>
  import definitions._

  /** A log of type variable with their original constraints. Used in order
    *  to undo constraints in the case of isSubType/isSameType failure.
    */
  private lazy val _undoLog = new UndoLog
  def undoLog = _undoLog

  import TypeConstraints.UndoPair
  class UndoLog extends Clearable {
    type UndoPairs = List[UndoPair[TypeVar, TypeConstraint]]
    //OPT this method is public so we can do `manual inlining`
    var log: UndoPairs = List()

    // register with the auto-clearing cache manager
    perRunCaches.recordCache(this)

    /** Undo all changes to constraints to type variables upto `limit`. */
    //OPT this method is public so we can do `manual inlining`
    def undoTo(limit: UndoPairs) {
      assertCorrectThread()
      while ((log ne limit) && log.nonEmpty) {
        val UndoPair(tv, constr) = log.head
        tv.constr = constr
        log = log.tail
      }
    }

    /** No sync necessary, because record should only
      *  be called from within an undo or undoUnless block,
      *  which is already synchronized.
      */
    private[reflect] def record(tv: TypeVar) = {
      log ::= UndoPair(tv, tv.constr.cloneInternal)
    }

    def clear() {
      if (settings.debug)
        self.log("Clearing " + log.size + " entries from the undoLog.")
      log = Nil
    }

    // `block` should not affect constraints on typevars
    def undo[T](block: => T): T = {
      val before = log
      try block
      finally undoTo(before)
    }
  }

  /** @PP: Unable to see why these apparently constant types should need vals
    *  in every TypeConstraint, I lifted them out.
    */
  private lazy val numericLoBound = IntTpe
  private lazy val numericHiBound = intersectionType(List(ByteTpe, CharTpe), ScalaPackageClass)

  /** A class expressing upper and lower bounds constraints of type variables,
    * as well as their instantiations.
    */
  class TypeConstraint(lo0: List[Type], hi0: List[Type], numlo0: Type, numhi0: Type, avoidWidening0: Boolean = false) {
    def this(lo0: List[Type], hi0: List[Type]) = this(lo0, hi0, NoType, NoType)
    def this(bounds: TypeBounds) = this(List(bounds.lo), List(bounds.hi))
    def this() = this(List(), List())

    /*  Syncnote: Type constraints are assumed to be used from only one
     *  thread. They are not exposed in api.Types and are used only locally
     *  in operations that are exposed from types. Hence, no syncing of any
     *  variables should be ncessesary.
     */

    /** Guard these lists against AnyClass and NothingClass appearing,
      *  else loBounds.isEmpty will have different results for an empty
      *  constraint and one with Nothing as a lower bound.  [Actually
      *  guarding addLoBound/addHiBound somehow broke raw types so it
      *  only guards against being created with them.]
      */
    private var lobounds = lo0 filterNot typeIsNothing
    private var hibounds = hi0 filterNot typeIsAny
    private var numlo = numlo0
    private var numhi = numhi0
    private var avoidWidening = avoidWidening0

    def loBounds: List[Type] = if (numlo == NoType) lobounds else numlo :: lobounds
    def hiBounds: List[Type] = if (numhi == NoType) hibounds else numhi :: hibounds
    def avoidWiden: Boolean = avoidWidening

    def addLoBound(tp: Type, isNumericBound: Boolean = false) {
      // For some reason which is still a bit fuzzy, we must let Nothing through as
      // a lower bound despite the fact that Nothing is always a lower bound.  My current
      // supposition is that the side-effecting type constraint accumulation mechanism
      // depends on these subtype tests being performed to make forward progress when
      // there are mutally recursive type vars.
      // See pos/t6367 and pos/t6499 for the competing test cases.
      val mustConsider = tp.typeSymbol match {
        case NothingClass => true
        case _            => !(lobounds contains tp)
      }
      if (mustConsider) {
        if (isNumericBound && isNumericValueType(tp)) {
          if (numlo == NoType || isNumericSubType(numlo, tp))
            numlo = tp
          else if (!isNumericSubType(tp, numlo))
            numlo = numericLoBound
        }
        else lobounds ::= tp
      }
    }

    def checkWidening(tp: Type) {
      if(tp.isStable) avoidWidening = true
      else tp match {
        case HasTypeMember(_, _) => avoidWidening = true
        case _ =>
      }
    }

    def addHiBound(tp: Type, isNumericBound: Boolean = false) {
      // My current test case only demonstrates the need to let Nothing through as
      // a lower bound, but I suspect the situation is symmetrical.
      val mustConsider = tp.typeSymbol match {
        case AnyClass => true
        case _        => !(hibounds contains tp)
      }
      if (mustConsider) {
        checkWidening(tp)
        if (isNumericBound && isNumericValueType(tp)) {
          if (numhi == NoType || isNumericSubType(tp, numhi))
            numhi = tp
          else if (!isNumericSubType(numhi, tp))
            numhi = numericHiBound
        }
        else hibounds ::= tp
      }
    }

    def instWithinBounds = instValid && isWithinBounds(inst)

    def isWithinBounds(tp: Type): Boolean = (
         lobounds.forall(_ <:< tp)
      && hibounds.forall(tp <:< _)
      && (numlo == NoType || (numlo weak_<:< tp))
      && (numhi == NoType || (tp weak_<:< numhi))
    )

    var inst: Type = NoType // @M reduce visibility?

    def instValid = (inst ne null) && (inst ne NoType)

    def cloneInternal = {
      val tc = new TypeConstraint(lobounds, hibounds, numlo, numhi, avoidWidening)
      tc.inst = inst
      tc
    }

    override def toString = {
      val boundsStr = {
        val lo = loBounds filterNot typeIsNothing match {
          case Nil       => ""
          case tp :: Nil => " >: " + tp
          case tps       => tps.mkString(" >: (", ", ", ")")
        }
        val hi = hiBounds filterNot typeIsAny match {
          case Nil       => ""
          case tp :: Nil => " <: " + tp
          case tps       => tps.mkString(" <: (", ", ", ")")
        }
        lo + hi
      }
      if (inst eq NoType) boundsStr
      else boundsStr + " _= " + inst.safeToString
    }
  }

  /** Solve constraint collected in types `tvars`.
    *
    *  @param tvars      All type variables to be instantiated.
    *  @param tparams    The type parameters corresponding to `tvars`
    *  @param variances  The variances of type parameters; need to reverse
    *                    solution direction for all contravariant variables.
    *  @param upper      When `true` search for max solution else min.
    */
  def solve(tvars: List[TypeVar], tparams: List[Symbol], variances: List[Variance], upper: Boolean, depth: Depth): Boolean = {

    def solveOne(tvar: TypeVar, tparam: Symbol, variance: Variance) {
      if (tvar.constr.inst == NoType) {
        val up = if (variance.isContravariant) !upper else upper
        tvar.constr.inst = null
        val bound: Type = if (up) tparam.info.bounds.hi else tparam.info.bounds.lo
        //Console.println("solveOne0(tv, tp, v, b)="+(tvar, tparam, variance, bound))
        var cyclic = bound contains tparam
        foreach3(tvars, tparams, variances)((tvar2, tparam2, variance2) => {
          val ok = (tparam2 != tparam) && (
            (bound contains tparam2)
              ||  up && (tparam2.info.bounds.lo =:= tparam.tpeHK)
              || !up && (tparam2.info.bounds.hi =:= tparam.tpeHK)
            )
          if (ok) {
            if (tvar2.constr.inst eq null) cyclic = true
            solveOne(tvar2, tparam2, variance2)
          }
        })
        if (!cyclic) {
          if (up) {
            if (bound.typeSymbol != AnyClass) {
              debuglog(s"$tvar addHiBound $bound.instantiateTypeParams($tparams, $tvars)")
              tvar addHiBound bound.instantiateTypeParams(tparams, tvars)
            }
            for (tparam2 <- tparams)
              tparam2.info.bounds.lo.dealias match {
                case TypeRef(_, `tparam`, _) =>
                  debuglog(s"$tvar addHiBound $tparam2.tpeHK.instantiateTypeParams($tparams, $tvars)")
                  tvar addHiBound tparam2.tpeHK.instantiateTypeParams(tparams, tvars)
                case _ =>
              }
          } else {
            if (bound.typeSymbol != NothingClass && bound.typeSymbol != tparam) {
              debuglog(s"$tvar addLoBound $bound.instantiateTypeParams($tparams, $tvars)")
              tvar addLoBound bound.instantiateTypeParams(tparams, tvars)
            }
            for (tparam2 <- tparams)
              tparam2.info.bounds.hi.dealias match {
                case TypeRef(_, `tparam`, _) =>
                  debuglog(s"$tvar addLoBound $tparam2.tpeHK.instantiateTypeParams($tparams, $tvars)")
                  tvar addLoBound tparam2.tpeHK.instantiateTypeParams(tparams, tvars)
                case _ =>
              }
          }
        }
        tvar.constr.inst = NoType // necessary because hibounds/lobounds may contain tvar

        //println("solving "+tvar+" "+up+" "+(if (up) (tvar.constr.hiBounds) else tvar.constr.loBounds)+((if (up) (tvar.constr.hiBounds) else tvar.constr.loBounds) map (_.widen)))
        val newInst = (
          if (up) {
            if (depth.isAnyDepth) glb(tvar.constr.hiBounds)
            else glb(tvar.constr.hiBounds, depth)
          }
          else {
            if (depth.isAnyDepth) lub(tvar.constr.loBounds)
            else lub(tvar.constr.loBounds, depth)
          }
          )

        debuglog(s"$tvar setInst $newInst")
        tvar setInst newInst
        //Console.println("solving "+tvar+" "+up+" "+(if (up) (tvar.constr.hiBounds) else tvar.constr.loBounds)+((if (up) (tvar.constr.hiBounds) else tvar.constr.loBounds) map (_.widen))+" = "+tvar.constr.inst)//@MDEBUG
      }
    }

    // println("solving "+tvars+"/"+tparams+"/"+(tparams map (_.info)))
    foreach3(tvars, tparams, variances)(solveOne)

    def logBounds(tv: TypeVar) = log {
      val what = if (!tv.instValid) "is invalid" else s"does not conform to bounds: ${tv.constr}"
      s"Inferred type for ${tv.originString} (${tv.inst}) $what"
    }

    tvars forall (tv => tv.instWithinBounds || util.andFalse(logBounds(tv)))
  }
}

private[internal] object TypeConstraints {
  // UndoPair is declared in companion object to not hold an outer pointer reference
  final case class UndoPair[TypeVar <: SymbolTable#TypeVar,
    TypeConstraint <: TypeConstraints#TypeConstraint](tv: TypeVar, tConstraint: TypeConstraint)
}

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