path: root/src/compiler/scala/tools/nsc/typechecker/EtaExpansion.scala

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

package scala.tools.nsc
package typechecker

import scala.collection.mutable.ListBuffer
import symtab.Flags._

/** This trait ...
 *
 *  @author  Martin Odersky
 *  @version 1.0
 */
trait EtaExpansion { self: Analyzer =>

  import global._

  object etaExpansion {
    private def isMatch(vparam: ValDef, arg: Tree) = arg match {
      case Ident(name)  => vparam.name == name
      case _            => false
    }

    def unapply(tree: Tree): Option[(List[ValDef], Tree, List[Tree])] = tree match {
      case Function(vparams, Apply(fn, args)) if (vparams corresponds args)(isMatch) =>
        Some((vparams, fn, args))
      case _ =>
        None
    }
  }

  /** <p>
   *    Expand partial function applications of type `type`.
   *  </p><pre>
   *  p.f(es_1)...(es_n)
   *     ==>  {
   *            <b>private synthetic val</b> eta$f   = p.f   // if p is not stable
   *            ...
   *            <b>private synthetic val</b> eta$e_i = e_i    // if e_i is not stable
   *            ...
   *            (ps_1 => ... => ps_m => eta$f([es_1])...([es_m])(ps_1)...(ps_m))
   *          }</pre>
   *  <p>
   *    tree is already attributed
   *  </p>
   */
  def etaExpand(unit : CompilationUnit, tree: Tree, typer: Typer): Tree = {
    val tpe = tree.tpe
    var cnt = 0 // for NoPosition
    def freshName() = {
      cnt += 1
      unit.freshTermName("eta$" + (cnt - 1) + "$")
    }
    val defs = new ListBuffer[Tree]

    /** Append to `defs` value definitions for all non-stable
     *  subexpressions of the function application `tree`.
     */
    def liftoutPrefix(tree: Tree): Tree = {
      def liftout(tree: Tree, byName: Boolean): Tree =
        if (treeInfo.isExprSafeToInline(tree)) tree
        else {
          val vname: Name = freshName()
          // Problem with ticket #2351 here
          defs += atPos(tree.pos) {
            val rhs = if (byName) {
              val res = typer.typed(Function(List(), tree))
              new ChangeOwnerTraverser(typer.context.owner, res.symbol) traverse tree // SI-6274
              res
            } else tree
            ValDef(Modifiers(SYNTHETIC), vname.toTermName, TypeTree(), rhs)
          }
          atPos(tree.pos.focus) {
            if (byName) Apply(Ident(vname), List()) else Ident(vname)
          }
        }
      val tree1 = tree match {
        // a partial application using named arguments has the following form:
        // { val qual$1 = qual
        //   val x$1 = arg1
        //   [...]
        //   val x$n = argn
        //   qual$1.fun(x$1, ..)..(.., x$n) }
        // Eta-expansion has to be performed on `fun`
        case Block(stats, fun) =>
          defs ++= stats
          liftoutPrefix(fun)
        case Apply(fn, args) =>
          val byName: Int => Option[Boolean] = fn.tpe.params.map(p => definitions.isByNameParamType(p.tpe)).lift
          val newArgs = mapWithIndex(args) { (arg, i) =>
            // with repeated params, there might be more or fewer args than params
            liftout(arg, byName(i).getOrElse(false))
          }
          treeCopy.Apply(tree, liftoutPrefix(fn), newArgs) setType null
        case TypeApply(fn, args) =>
          treeCopy.TypeApply(tree, liftoutPrefix(fn), args) setType null
        case Select(qual, name) =>
          treeCopy.Select(tree, liftout(qual, false), name) setSymbol NoSymbol setType null
        case Ident(name) =>
          tree
      }
      if (tree1 ne tree) tree1 setPos tree1.pos.makeTransparent
      tree1
    }

    /** Eta-expand lifted tree.
     */
    def expand(tree: Tree, tpe: Type): Tree = tpe match {
      case mt @ MethodType(paramSyms, restpe) if !mt.isImplicit =>
        val params: List[(ValDef, Boolean)] = paramSyms.map {
          sym =>
            val origTpe = sym.tpe
            val isRepeated = definitions.isRepeatedParamType(origTpe)
            // SI-4176 Don't leak A* in eta-expanded function types. See t4176b.scala
            val droppedStarTpe = if (settings.etaExpandKeepsStar.value) origTpe else dropIllegalStarTypes(origTpe)
            val valDef = ValDef(Modifiers(SYNTHETIC | PARAM), sym.name.toTermName, TypeTree(droppedStarTpe), EmptyTree)
            (valDef, isRepeated)
        }
        atPos(tree.pos.makeTransparent) {
          val args = params.map {
            case (valDef, isRepeated) => gen.paramToArg(Ident(valDef.name), isRepeated)
          }
          Function(params.map(_._1), expand(Apply(tree, args), restpe))
        }
      case _ =>
        tree
    }

    val tree1 = liftoutPrefix(tree)
    atPos(tree.pos)(Block(defs.toList, expand(tree1, tpe)))
  }
}