Handling higher-kinded types with lambdas

Switch to the new scheme where higher-kinded types (and also some polymorphic
type aliases) are represented as instances of Lambda traits.

Author: odersky

src/dotty/tools/dotc/core/Definitions.scala

@@ -424,61 +424,6 @@ class Definitions {
 
   def functionArity(tp: Type) = tp.dealias.argInfos.length - 1
 
-  // ----- Higher kinds machinery ------------------------------------------
-  // tbr
-  private var _hkTraits: Set[Symbol] = Set()
-
-  /** The set of HigherKindedXYZ traits encountered so far */
-  def hkTraits: Set[Symbol] = _hkTraits
-
-  private var hkTraitOfArity = mutable.Map[List[Int], ClassSymbol]()
-
-  /** The HigherKinded trait corresponding to symbols `boundSyms` (which are assumed
-   *  to be the type parameters of a higher-kided type). This is a class symbol that
-   *  would be generated by the following schema.
-   *
-   *      class HigherKindedXYZ { type v_n _$hk$0; ...; type v_n _$Hk$n }
-   *
-   *  Here:
-   *
-   *   - XYZ is a string with one letter for each variant of a bound symbols,
-   *     using `P` (positive variance), `N` (negative variance), `I` (invariant).
-   *   - v_i are the variances of the bound symbols (i.e. +, -, or empty).
-   *   - _$hk$i are higher-kinded parameter names, which are specially treated in type application.
-   */
-  def hkTrait(vcs: List[Int]) = {
-
-    def varianceFlags(v: Int) = v match {
-      case -1 => Contravariant
-      case  0 => Covariant
-      case  1 => EmptyFlags
-    }
-
-    val completer = new LazyType  {
-      def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
-        val cls = denot.asClass.classSymbol
-        val paramDecls = newScope
-        for (i <- 0 until vcs.length)
-          newTypeParam(cls, tpnme.higherKindedParamName(i), EmptyFlags, paramDecls)
-        denot.info = ClassInfo(ScalaPackageClass.thisType, cls, List(ObjectClass.typeRef), paramDecls)
-      }
-    }
-
-    val traitName = tpnme.higherKindedTraitName(vcs)
-
-    def createTrait = {
-      val cls = newClassSymbol(
-        ScalaPackageClass,
-        traitName,
-        Trait | Interface | Synthetic,
-        completer)
-      _hkTraits += cls
-      cls
-    }
-
-    hkTraitOfArity.getOrElseUpdate(vcs, createTrait)
-  }
-
   // ----- LambdaXYZ traits ------------------------------------------
 
   private var myLambdaTraits: Set[Symbol] = Set()

src/dotty/tools/dotc/core/NameOps.scala

@@ -88,14 +88,13 @@ object NameOps {
     }
 
     /** Is this the name of a higher-kinded type parameter of a Lambda? */
-    def isLambdaArgName = name.startsWith(tpnme.LAMBDA_ARG_PREFIX)
-    def isHkParamName: Boolean = name(0) == '_' && name.startsWith(HK_PARAM_PREFIX) // tbr
+    def isLambdaArgName =
+      name(0) == tpnme.LAMBDA_ARG_PREFIXhead && name.startsWith(tpnme.LAMBDA_ARG_PREFIX)
 
     /** The index of the higher-kinded type parameter with this name.
      *  Pre: isLambdaArgName.
      */
     def lambdaArgIndex: Int = name.drop(name.lastIndexOf('$') + 1).toString.toInt
-    def hkParamIndex: Int = name.drop(name.lastIndexOf('$') + 1).toString.toInt // tbr
 
     /** If the name ends with $nn where nn are
       * all digits, strip the $ and the digits.
@@ -179,19 +178,6 @@ object NameOps {
       }
     }
 
-    /** The variances of the higherKinded parameters of the trait named
-     *  by this name.
-     *  @pre The name is a higher-kinded trait name, i.e. it starts with HK_TRAIT_PREFIX
-     */
-    def hkVariances: List[Int] = { // tbr
-      def varianceOfSuffix(suffix: Char): Int = {
-        val idx = tpnme.varianceSuffixes.indexOf(suffix)
-        assert(idx >= 0)
-        idx - 1
-      }
-      name.drop(tpnme.HK_TRAIT_PREFIX.length).toList.map(varianceOfSuffix)
-    }
-
     /** The name of the generic runtime operation corresponding to an array operation */
     def genericArrayOp: TermName = name match {
       case nme.apply => nme.array_apply

src/dotty/tools/dotc/core/StdNames.scala

@@ -167,6 +167,7 @@ object StdNames {
     final val REIFY_TREECREATOR_PREFIX: N       = "$treecreator"
     final val REIFY_TYPECREATOR_PREFIX: N       = "$typecreator"
     final val LAMBDA_ARG_PREFIX: N              = "$hkArg$"
+    final val LAMBDA_ARG_PREFIXhead: Char       = LAMBDA_ARG_PREFIX.head
 
     final val Any: N             = "Any"
     final val AnyVal: N          = "AnyVal"
@@ -250,8 +251,6 @@ object StdNames {
     val SKOLEM: N                   = "<skolem>"
     val SPECIALIZED_INSTANCE: N     = "specInstance$"
     val THIS: N                     = "_$this"
-    val HK_PARAM_PREFIX: N          = "_$hk$"          // tbr
-    val HK_TRAIT_PREFIX: N          = "$HigherKinded$" // tbr
 
     final val Nil: N                = "Nil"
     final val Predef: N             = "Predef"
@@ -647,9 +646,6 @@ object StdNames {
     def syntheticTypeParamNames(num: Int): List[TypeName] =
       (0 until num).map(syntheticTypeParamName)(breakOut)
 
-    def higherKindedTraitName(vcs: List[Int]): TypeName = HK_TRAIT_PREFIX ++ vcs.map(varianceSuffix).mkString // tbr
-    def higherKindedParamName(n: Int) = HK_PARAM_PREFIX ++ n.toString //tbr
-
     def lambdaTraitName(vcs: List[Int]): TypeName = LambdaPrefix ++ vcs.map(varianceSuffix).mkString
     def lambdaArgName(n: Int) = LAMBDA_ARG_PREFIX ++ n.toString