diff --git a/src/dotty/tools/backend/jvm/DottyBackendInterface.scala b/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
index b6adba85a28b..5776cc8e234f 100644
--- a/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
+++ b/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
@@ -32,7 +32,7 @@ import NameOps._
 import StdNames.nme
 import NameOps._
 
-class DottyBackendInterface()(implicit ctx: Context) extends BackendInterface{
+class DottyBackendInterface(outputDirectory: AbstractFile)(implicit ctx: Context) extends BackendInterface{
   type Symbol          = Symbols.Symbol
   type Type            = Types.Type
   type Tree            = tpd.Tree
@@ -734,7 +734,7 @@ class DottyBackendInterface()(implicit ctx: Context) extends BackendInterface{
     def setter(clz: Symbol): Symbol = decorateSymbol(sym).setter
 
     def moduleSuffix: String = "" // todo: validate that names already have $ suffix
-    def outputDirectory: AbstractFile = new PlainDirectory(new Directory(new JFile(ctx.settings.d.value)))
+    def outputDirectory: AbstractFile = DottyBackendInterface.this.outputDirectory
     def pos: Position = sym.pos
 
     def throwsAnnotations: List[Symbol] = Nil
diff --git a/src/dotty/tools/backend/jvm/GenBCode.scala b/src/dotty/tools/backend/jvm/GenBCode.scala
index e8d196ce73c4..2d444d3beb9e 100644
--- a/src/dotty/tools/backend/jvm/GenBCode.scala
+++ b/src/dotty/tools/backend/jvm/GenBCode.scala
@@ -29,6 +29,7 @@ import scala.tools.asm.tree._
 import dotty.tools.dotc.util.{Positions, DotClass}
 import tpd._
 import StdNames._
+import scala.reflect.io.{Directory, PlainDirectory, AbstractFile}
 
 import scala.tools.nsc.backend.jvm.opt.LocalOpt
 
@@ -37,9 +38,11 @@ class GenBCode extends Phase {
   private val entryPoints = new mutable.HashSet[Symbol]()
   def registerEntryPoint(sym: Symbol) = entryPoints += sym
 
+  def outputDir(implicit ctx: Context): AbstractFile =
+    new PlainDirectory(new Directory(new JFile(ctx.settings.d.value)))
 
   def run(implicit ctx: Context): Unit = {
-    new GenBCodePipeline(entryPoints.toList, new DottyBackendInterface()(ctx))(ctx).run(ctx.compilationUnit.tpdTree)
+    new GenBCodePipeline(entryPoints.toList, new DottyBackendInterface(outputDir)(ctx))(ctx).run(ctx.compilationUnit.tpdTree)
     entryPoints.clear()
   }
 }
diff --git a/src/dotty/tools/dotc/Bench.scala b/src/dotty/tools/dotc/Bench.scala
index 47b5fd6dd64f..2fc38d78c640 100644
--- a/src/dotty/tools/dotc/Bench.scala
+++ b/src/dotty/tools/dotc/Bench.scala
@@ -12,7 +12,7 @@ object Bench extends Driver {
 
   @sharable private var numRuns = 1
 
-  def newCompiler(): Compiler = new Compiler
+  def newCompiler(implicit ctx: Context): Compiler = new Compiler
 
   private def ntimes(n: Int)(op: => Reporter): Reporter =
     (emptyReporter /: (0 until n)) ((_, _) => op)
diff --git a/src/dotty/tools/dotc/Driver.scala b/src/dotty/tools/dotc/Driver.scala
index 7f22fc77465f..3437b86fcf9b 100644
--- a/src/dotty/tools/dotc/Driver.scala
+++ b/src/dotty/tools/dotc/Driver.scala
@@ -10,7 +10,7 @@ abstract class Driver extends DotClass {
 
   val prompt = "\ndotc> "
 
-  protected def newCompiler(): Compiler
+  protected def newCompiler(implicit ctx: Context): Compiler
 
   protected def emptyReporter: Reporter = new StoreReporter(null)
 
@@ -90,7 +90,7 @@ abstract class Driver extends DotClass {
    */
   def process(args: Array[String], rootCtx: Context): Reporter = {
     val (fileNames, ctx) = setup(args, rootCtx)
-    doCompile(newCompiler(), fileNames)(ctx)
+    doCompile(newCompiler(ctx), fileNames)(ctx)
   }
 
   def main(args: Array[String]): Unit = {
diff --git a/src/dotty/tools/dotc/FromTasty.scala b/src/dotty/tools/dotc/FromTasty.scala
index d8d8b8b1e13e..8f29c882c8f7 100644
--- a/src/dotty/tools/dotc/FromTasty.scala
+++ b/src/dotty/tools/dotc/FromTasty.scala
@@ -30,7 +30,7 @@ import ast.tpd._
  *    scala dotty.tools.dotc.FromTasty -Xprint:front extMethods.T
  */
 object FromTasty extends Driver {
-  override def newCompiler(): Compiler = new TASTYCompiler
+  override def newCompiler(implicit ctx: Context): Compiler = new TASTYCompiler
 
   class TASTYCompiler extends Compiler {
 
diff --git a/src/dotty/tools/dotc/Main.scala b/src/dotty/tools/dotc/Main.scala
index 699a572349e2..6c473d8bbb44 100644
--- a/src/dotty/tools/dotc/Main.scala
+++ b/src/dotty/tools/dotc/Main.scala
@@ -2,10 +2,9 @@ package dotty.tools
 package dotc
 
 import core.Contexts.Context
-import reporting.Reporter
 
 /* To do:
  */
 object Main extends Driver {
-  override def newCompiler(): Compiler = new Compiler
+  override def newCompiler(implicit ctx: Context): Compiler = new Compiler
 }
diff --git a/src/dotty/tools/dotc/Resident.scala b/src/dotty/tools/dotc/Resident.scala
index 3ae369f277cf..18bb2ff4fd4e 100644
--- a/src/dotty/tools/dotc/Resident.scala
+++ b/src/dotty/tools/dotc/Resident.scala
@@ -25,7 +25,7 @@ class Resident extends Driver {
 
   object residentCompiler extends Compiler
 
-  override def newCompiler(): Compiler = ???
+  override def newCompiler(implicit ctx: Context): Compiler = ???
 
   override def sourcesRequired = false
 
diff --git a/src/dotty/tools/dotc/Run.scala b/src/dotty/tools/dotc/Run.scala
index ba86e3e70de2..39fd42a64d72 100644
--- a/src/dotty/tools/dotc/Run.scala
+++ b/src/dotty/tools/dotc/Run.scala
@@ -82,6 +82,9 @@ class Run(comp: Compiler)(implicit ctx: Context) {
     compileSources(List(new SourceFile(virtualFile)))
   }
 
+  /** The context created for this run */
+  def runContext = ctx
+
   /** Print summary; return # of errors encountered */
   def printSummary(): Reporter = {
     ctx.runInfo.printMaxConstraint()
diff --git a/src/dotty/tools/dotc/config/ScalaSettings.scala b/src/dotty/tools/dotc/config/ScalaSettings.scala
index 035b20130fd7..65bc9ba23965 100644
--- a/src/dotty/tools/dotc/config/ScalaSettings.scala
+++ b/src/dotty/tools/dotc/config/ScalaSettings.scala
@@ -89,7 +89,7 @@ class ScalaSettings extends Settings.SettingGroup {
   val showPhases = BooleanSetting("-Xshow-phases", "Print a synopsis of compiler phases.")
   val sourceReader = StringSetting("-Xsource-reader", "classname", "Specify a custom method for reading source files.", "")
   val XnoValueClasses = BooleanSetting("-Xno-value-classes", "Do not use value classes. Helps debugging.")
-
+  val XreplLineWidth = IntSetting("-Xrepl-line-width", "Maximial number of columns per line for REPL output", 390)
   val XoldPatmat = BooleanSetting("-Xoldpatmat", "Use the pre-2.10 pattern matcher. Otherwise, the 'virtualizing' pattern matcher is used in 2.10.")
   val XnoPatmatAnalysis = BooleanSetting("-Xno-patmat-analysis", "Don't perform exhaustivity/unreachability analysis. Also, ignore @switch annotation.")
   val XfullLubs = BooleanSetting("-Xfull-lubs", "Retains pre 2.10 behavior of less aggressive truncation of least upper bounds.")
diff --git a/src/dotty/tools/dotc/core/Denotations.scala b/src/dotty/tools/dotc/core/Denotations.scala
index 2af3f463dde6..b52c1120195e 100644
--- a/src/dotty/tools/dotc/core/Denotations.scala
+++ b/src/dotty/tools/dotc/core/Denotations.scala
@@ -590,7 +590,10 @@ object Denotations {
         } while (d ne denot)
         this
       case _ =>
-        if (coveredInterval.containsPhaseId(ctx.phaseId)) staleSymbolError
+        if (coveredInterval.containsPhaseId(ctx.phaseId)) {
+          if (ctx.debug) ctx.traceInvalid(this)
+          staleSymbolError
+        }
         else NoDenotation
     }
 
diff --git a/src/dotty/tools/dotc/core/Phases.scala b/src/dotty/tools/dotc/core/Phases.scala
index 970a9297ac2e..83ac64d536e9 100644
--- a/src/dotty/tools/dotc/core/Phases.scala
+++ b/src/dotty/tools/dotc/core/Phases.scala
@@ -347,6 +347,13 @@ object Phases {
     override def toString = phaseName
   }
 
+  /** Replace all instances of `oldPhaseClass` in `current` phases
+   *  by the result of `newPhases` applied to the old phase.
+   */
+  def replace(oldPhaseClass: Class[_ <: Phase], newPhases: Phase => List[Phase], current: List[List[Phase]]): List[List[Phase]] =
+    current.map(_.flatMap(phase =>
+      if (oldPhaseClass.isInstance(phase)) newPhases(phase) else phase :: Nil))
+
   /** Dotty deviation: getClass yields Class[_], instead of [Class <: <type of receiver>].
    *  We can get back the old behavior using this decorator. We should also use the same
    *  trick for standard getClass.
diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index 4495b40961af..705e1a4fa884 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -45,19 +45,54 @@ trait SymDenotations { this: Context =>
     if (denot.is(ValidForever) || denot.isRefinementClass) true
     else {
       val initial = denot.initial
-      if (initial ne denot)
-        ctx.withPhase(initial.validFor.firstPhaseId).stillValid(initial.asSymDenotation)
-      else try {
-        val owner = denot.owner.denot
-        stillValid(owner) && (
-          !owner.isClass
-          || owner.isRefinementClass
-          || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
-          || denot.isSelfSym)
-      } catch {
-        case ex: StaleSymbol => false
+      val firstPhaseId = initial.validFor.firstPhaseId.max(ctx.typerPhase.id)
+      if ((initial ne denot) || ctx.phaseId != firstPhaseId)
+        ctx.withPhase(firstPhaseId).stillValidInOwner(initial.asSymDenotation)
+      else
+        stillValidInOwner(denot)
+    }
+
+  private[SymDenotations] def stillValidInOwner(denot: SymDenotation): Boolean = try {
+    val owner = denot.owner.denot
+    stillValid(owner) && (
+      !owner.isClass
+      || owner.isRefinementClass
+      || (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol)
+      || denot.isSelfSym)
+  } catch {
+    case ex: StaleSymbol => false
+  }
+
+  /** Explain why symbol is invalid; used for debugging only */
+  def traceInvalid(denot: Denotation): Boolean = {
+    def show(d: Denotation) = s"$d#${d.symbol.id}"
+    def explain(msg: String) = {
+      println(s"${show(denot)} is invalid at ${this.period} because $msg")
+      false
+    }
+    denot match {
+      case denot: SymDenotation =>
+        def explainSym(msg: String) = explain(s"$msg\n defined = ${denot.definedPeriodsString}")
+        if (denot.is(ValidForever) || denot.isRefinementClass) true
+        else {
+          implicit val ctx: Context = this
+          val initial = denot.initial
+          if ((initial ne denot) || ctx.phaseId != initial.validFor.firstPhaseId) {
+            ctx.withPhase(initial.validFor.firstPhaseId).traceInvalid(initial.asSymDenotation)
+          } else try {
+            val owner = denot.owner.denot
+            if (!traceInvalid(owner)) explainSym("owner is invalid")
+            else if (!owner.isClass || owner.isRefinementClass || denot.isSelfSym) true
+            else if (owner.unforcedDecls.lookupAll(denot.name) contains denot.symbol) true
+            else explainSym(s"decls of ${show(owner)} are ${owner.unforcedDecls.lookupAll(denot.name).toList}, do not contain ${denot.symbol}")
+          } catch {
+            case ex: StaleSymbol => explainSym(s"$ex was thrown")
+          }
       }
+      case _ =>
+        explain("denotation is not a SymDenotation")
     }
+  }
 }
 
 object SymDenotations {
diff --git a/src/dotty/tools/dotc/repl/AbstractFileClassLoader.scala b/src/dotty/tools/dotc/repl/AbstractFileClassLoader.scala
new file mode 100644
index 000000000000..a3a463717311
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/AbstractFileClassLoader.scala
@@ -0,0 +1,31 @@
+package dotty.tools
+package dotc
+package repl
+
+import io.AbstractFile
+
+/**
+ * A class loader that loads files from a {@link scala.tools.nsc.io.AbstractFile}.
+ *
+ * @author Lex Spoon
+ */
+class AbstractFileClassLoader(root: AbstractFile, parent: ClassLoader)
+extends ClassLoader(parent)
+{
+  override def findClass(name: String): Class[_] = {
+    var file: AbstractFile = root
+    val pathParts = name.split("[./]").toList
+    for (dirPart <- pathParts.init) {
+      file = file.lookupName(dirPart, true)
+      if (file == null) {
+        throw new ClassNotFoundException(name)
+      }
+    }
+    file = file.lookupName(pathParts.last+".class", false)
+    if (file == null) {
+      throw new ClassNotFoundException(name)
+    }
+    val bytes = file.toByteArray
+    defineClass(name, bytes, 0, bytes.length)
+  }
+}
diff --git a/src/dotty/tools/dotc/repl/CompilingInterpreter.scala b/src/dotty/tools/dotc/repl/CompilingInterpreter.scala
new file mode 100644
index 000000000000..7d1da141966c
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/CompilingInterpreter.scala
@@ -0,0 +1,811 @@
+package dotty.tools
+package dotc
+package repl
+
+import java.io.{File, PrintWriter, StringWriter, Writer}
+import java.lang.{Class, ClassLoader}
+import java.net.{URL, URLClassLoader}
+
+import scala.collection.immutable.ListSet
+import scala.collection.mutable
+import scala.collection.mutable.{ListBuffer, HashSet, ArrayBuffer}
+
+//import ast.parser.SyntaxAnalyzer
+import io.{PlainFile, VirtualDirectory}
+import scala.reflect.io.{PlainDirectory, Directory}
+import reporting.{ConsoleReporter, Reporter}
+import core.Flags
+import util.{SourceFile, NameTransformer}
+import io.ClassPath
+import ast.Trees._
+import parsing.Parsers._
+import core._
+import dotty.tools.backend.jvm.GenBCode
+import Symbols._, Types._, Contexts._, StdNames._, Names._, NameOps._
+import Decorators._
+import scala.util.control.NonFatal
+
+/** An interpreter for Scala code which is based on the `dotc` compiler.
+ *
+ *  The overall approach is based on compiling the requested code and then
+ *  using a Java classloader and Java reflection to run the code
+ *  and access its results.
+ *
+ *  In more detail, a single compiler instance is used
+ *  to accumulate all successfully compiled or interpreted Scala code.  To
+ *  "interpret" a line of code, the compiler generates a fresh object that
+ *  includes the line of code and which has public definition(s) to export
+ *  all variables defined by that code.  To extract the result of an
+ *  interpreted line to show the user, a second "result object" is created
+ *  which imports the variables exported by the above object and then
+ *  exports a single definition named "result".  To accommodate user expressions
+ *  that read from variables or methods defined in previous statements, "import"
+ *  statements are used.
+ *
+ * This interpreter shares the strengths and weaknesses of using the
+ *  full compiler-to-Java.  The main strength is that interpreted code
+ *  behaves exactly as does compiled code, including running at full speed.
+ *  The main weakness is that redefining classes and methods is not handled
+ *  properly, because rebinding at the Java level is technically difficult.
+ *
+ * @author Moez A. Abdel-Gawad
+ * @author Lex Spoon
+ * @author Martin Odersky
+ *
+ * @param out   The output to use for diagnostics
+ * @param ictx  The context to use for initialization of the interpreter,
+ *              needed to access the current classpath.
+ */
+class CompilingInterpreter(out: PrintWriter, ictx: Context) extends Compiler with Interpreter {
+  import ast.untpd._
+  import CompilingInterpreter._
+
+  /** directory to save .class files to */
+  val virtualDirectory =
+    if (ictx.settings.d.isDefault(ictx)) new VirtualDirectory("(memory)", None)
+    else new PlainDirectory(new Directory(new java.io.File(ictx.settings.d.value(ictx)))) // for now, to help debugging
+
+  /** A GenBCode phase that uses `virtualDirectory` for its output */
+  private class REPLGenBCode extends GenBCode {
+    override def outputDir(implicit ctx: Context) = virtualDirectory
+  }
+
+  /** Phases of this compiler use `REPLGenBCode` instead of `GenBCode`. */
+  override def phases = Phases.replace(
+    classOf[GenBCode], _ => new REPLGenBCode :: Nil, super.phases)
+
+  /** whether to print out result lines */
+  private var printResults: Boolean = true
+
+  /** Temporarily be quiet */
+  override def beQuietDuring[T](operation: => T): T = {
+    val wasPrinting = printResults
+    try {
+      printResults = false
+      operation
+    } finally {
+      printResults = wasPrinting
+    }
+  }
+
+  private def newReporter = new ConsoleReporter(Console.in, out) {
+    override def printMessage(msg: String) = {
+      out.print(/*clean*/(msg) + "\n")
+        // Suppress clean for now for compiler messages
+        // Otherwise we will completely delete all references to
+        // line$object$ module classes. The previous interpreter did not
+        // have the project because the module class was written without the final `$'
+        // and therefore escaped the purge. We can turn this back on once
+        // we drop the final `$' from module classes.
+      out.flush()
+    }
+  }
+
+  /** the previous requests this interpreter has processed */
+  private val prevRequests = new ArrayBuffer[Request]()
+
+  /** the compiler's classpath, as URL's */
+  val compilerClasspath: List[URL] = ictx.platform.classPath(ictx).asURLs
+
+  protected def parentClassLoader: ClassLoader = classOf[Interpreter].getClassLoader
+
+  /* A single class loader is used for all commands interpreted by this Interpreter.
+     It would also be possible to create a new class loader for each command
+     to interpret.  The advantages of the current approach are:
+
+       - Expressions are only evaluated one time.  This is especially
+         significant for I/O, e.g. "val x = Console.readLine"
+
+     The main disadvantage is:
+
+       - Objects, classes, and methods cannot be rebound.  Instead, definitions
+         shadow the old ones, and old code objects refer to the old
+         definitions.
+  */
+  /** class loader used to load compiled code */
+  val classLoader: ClassLoader = {
+    val parent = new URLClassLoader(compilerClasspath.toArray, parentClassLoader)
+    new AbstractFileClassLoader(virtualDirectory, parent)
+  }
+
+  // Set the current Java "context" class loader to this interpreter's class loader
+  Thread.currentThread.setContextClassLoader(classLoader)
+
+  /** Parse a line into a sequence of trees. Returns None if the input is incomplete. */
+  private def parse(line: String)(implicit ctx: Context): Option[List[Tree]] = {
+    var justNeedsMore = false
+    val reporter = newReporter
+    reporter.withIncompleteHandler { _ => _ => justNeedsMore = true } {
+      // simple parse: just parse it, nothing else
+      def simpleParse(code: String)(implicit ctx: Context): List[Tree] = {
+        val source = new SourceFile("<console>", code.toCharArray())
+        val parser = new Parser(source)
+        val (selfDef, stats) = parser.templateStatSeq
+        stats
+      }
+      val trees = simpleParse(line)(ctx.fresh.setReporter(reporter))
+      if (reporter.hasErrors) {
+        Some(Nil) // the result did not parse, so stop
+      } else if (justNeedsMore) {
+        None
+      } else {
+        Some(trees)
+      }
+    }
+  }
+
+  /** Compile a SourceFile.  Returns the root context of the run that compiled the file.
+   */
+  def compileSources(sources: List[SourceFile])(implicit ctx: Context): Context = {
+    val reporter = newReporter
+    val run = newRun(ctx.fresh.setReporter(reporter))
+    run.compileSources(sources)
+    run.runContext
+  }
+
+  /** Compile a string.  Returns true if there are no
+   *  compilation errors, or false otherwise.
+   */
+  def compileString(code: String)(implicit ctx: Context): Boolean = {
+    val runCtx = compileSources(List(new SourceFile("<script>", code.toCharArray)))
+    !runCtx.reporter.hasErrors
+  }
+
+  override def interpret(line: String)(implicit ctx: Context): Interpreter.Result = {
+    // if (prevRequests.isEmpty)
+    //  new Run(this) // initialize the compiler // (not sure this is needed)
+    // parse
+    parse(indentCode(line)) match {
+      case None => Interpreter.Incomplete
+      case Some(Nil) => Interpreter.Error // parse error or empty input
+      case Some(tree :: Nil) if tree.isTerm && !tree.isInstanceOf[Assign] =>
+        interpret(s"val $newVarName =\n$line")
+      case Some(trees) =>
+        val req = new Request(line, newLineName)
+        if (!req.compile())
+          Interpreter.Error // an error happened during compilation, e.g. a type error
+        else {
+          val (interpreterResultString, succeeded) = req.loadAndRun()
+          if (printResults || !succeeded)
+            out.print(clean(interpreterResultString))
+          if (succeeded) {
+            prevRequests += req
+            Interpreter.Success
+          }
+          else Interpreter.Error
+        }
+    }
+  }
+
+  /** Trait collecting info about one of the statements of an interpreter request */
+  private trait StatementInfo {
+    /** The statement */
+    def statement: Tree
+
+    /** The names defined previously and referred to in the statement */
+    def usedNames: List[Name]
+
+    /** The names defined in the statement */
+    val boundNames: List[Name]
+
+    /** Statement is an import that contains a wildcard */
+    val importsWildcard: Boolean
+
+    /** The names imported by the statement (if it is an import clause) */
+    val importedNames: Seq[Name]
+
+    /** Statement defines an implicit calue or method */
+    val definesImplicit: Boolean
+  }
+
+  /** One line of code submitted by the user for interpretation */
+  private class Request(val line: String, val lineName: String)(implicit ctx: Context) {
+    private val trees = parse(line) match {
+      case Some(ts) => ts
+      case None => Nil
+    }
+
+    /** name to use for the object that will compute "line" */
+    private def objectName = lineName + INTERPRETER_WRAPPER_SUFFIX
+
+    /** name of the object that retrieves the result from the above object */
+    private def resultObjectName = "RequestResult$" + objectName
+
+    private def chooseHandler(stat: Tree): StatementHandler = stat match {
+      case stat: DefDef => new DefHandler(stat)
+      case stat: ValDef => new ValHandler(stat)
+      case stat @ Assign(Ident(_), _) => new AssignHandler(stat)
+      case stat: ModuleDef => new ModuleHandler(stat)
+      case stat: TypeDef if stat.isClassDef => new ClassHandler(stat)
+      case stat: TypeDef => new TypeAliasHandler(stat)
+      case stat: Import => new ImportHandler(stat)
+//    case DocDef(_, documented) => chooseHandler(documented)
+      case stat => new GenericHandler(stat)
+    }
+
+    private val handlers: List[StatementHandler] = trees.map(chooseHandler)
+
+    /** all (public) names defined by these statements */
+    private val boundNames = ListSet(handlers.flatMap(_.boundNames): _*).toList
+
+    /** list of names used by this expression */
+    private val usedNames: List[Name] = handlers.flatMap(_.usedNames)
+
+    private val (importsPreamble, importsTrailer, accessPath) =
+      importsCode(usedNames.toSet)
+
+    /** Code to access a variable with the specified name */
+    private def fullPath(vname: String): String = s"$objectName$accessPath.`$vname`"
+
+    /** Code to access a variable with the specified name */
+    private def fullPath(vname: Name): String = fullPath(vname.toString)
+
+    /** the line of code to compute */
+    private def toCompute = line
+
+    /** generate the source code for the object that computes this request
+     *  TODO Reformulate in a functional way
+     */
+    private def objectSourceCode: String =
+      stringFrom { code =>
+        // header for the wrapper object
+        code.println("object " + objectName + " {")
+        code.print(importsPreamble)
+        code.println(indentCode(toCompute))
+        handlers.foreach(_.extraCodeToEvaluate(this,code))
+        code.println(importsTrailer)
+        //end the wrapper object
+        code.println(";}")
+      }
+
+    /** Types of variables defined by this request.  They are computed
+        after compilation of the main object */
+    private var typeOf: Map[Name, String] = _
+
+    /** generate source code for the object that retrieves the result
+        from objectSourceCode */
+    private def resultObjectSourceCode: String =
+      stringFrom(code => {
+        code.println("object " + resultObjectName)
+        code.println("{ val result: String = {")
+        code.println(objectName + accessPath + ";")  // evaluate the object, to make sure its constructor is run
+        code.print("(\"\"")  // print an initial empty string, so later code can
+                            // uniformly be: + morestuff
+        handlers.foreach(_.resultExtractionCode(this, code))
+        code.println("\n)}")
+        code.println(";}")
+      })
+
+
+    /** Compile the object file.  Returns whether the compilation succeeded.
+     *  If all goes well, the "types" map is computed. */
+    def compile(): Boolean = {
+      val compileCtx = compileSources(
+        List(new SourceFile("<console>", objectSourceCode.toCharArray)))
+      !compileCtx.reporter.hasErrors && {
+        this.typeOf = findTypes(compileCtx)
+        val resultCtx = compileSources(
+          List(new SourceFile("<console>", resultObjectSourceCode.toCharArray)))
+        !resultCtx.reporter.hasErrors
+      }
+    }
+
+    /** Dig the types of all bound variables out of the compiler run.
+     *  TODO: Change the interface so that we typecheck, and then transform
+     *  directly. Treating the compiler as less of a blackbox will require
+     *  much less magic here.
+     */
+    private def findTypes(implicit ctx: Context): Map[Name, String] = {
+      def valAndVarNames = handlers.flatMap(_.valAndVarNames)
+      def defNames = handlers.flatMap(_.defNames)
+
+      def getTypes(names: List[Name], nameMap: Name => Name): Map[Name, String] = {
+        /** the outermost wrapper object */
+        val outerResObjSym: Symbol =
+          defn.EmptyPackageClass.info.decl(objectName.toTermName).symbol
+
+        /** the innermost object inside the wrapper, found by
+          * following accessPath into the outer one. */
+        val resObjSym =
+          (accessPath.split("\\.")).foldLeft(outerResObjSym) { (sym,str) =>
+            if (str.isEmpty) sym
+            else
+              ctx.atPhase(ctx.typerPhase.next) { implicit ctx =>
+                sym.info.member(str.toTermName).symbol
+              }
+          }
+
+        names.foldLeft(Map.empty[Name,String]) { (map, name) =>
+          val rawType =
+            ctx.atPhase(ctx.typerPhase.next) { implicit ctx =>
+              resObjSym.info.member(name).info
+            }
+
+          // the types are all =>T; remove the =>
+          val cleanedType = rawType.widenExpr
+
+          map + (name ->
+            ctx.atPhase(ctx.typerPhase.next) { implicit ctx =>
+              cleanedType.show
+            })
+        }
+      }
+
+      val names1 = getTypes(valAndVarNames, n => n.toTermName.fieldName)
+      val names2 = getTypes(defNames, identity)
+      names1 ++ names2
+    }
+
+    /** load and run the code using reflection.
+     *  @return  A pair consisting of the run's result as a string, and
+     *           a boolean indicating whether the run succeeded without throwing
+     *           an exception.
+     */
+    def loadAndRun(): (String, Boolean) = {
+      val interpreterResultObject: Class[_] =
+        Class.forName(resultObjectName, true, classLoader)
+      val resultValMethod: java.lang.reflect.Method =
+        interpreterResultObject.getMethod("result")
+      try {
+        (resultValMethod.invoke(interpreterResultObject).toString, true)
+      } catch {
+        case NonFatal(ex) =>
+          def cause(ex: Throwable): Throwable =
+            if (ex.getCause eq null) ex else cause(ex.getCause)
+          val orig = cause(ex)
+          (stringFrom(str => orig.printStackTrace(str)), false)
+      }
+    }
+
+    /** Compute imports that allow definitions from previous
+     *  requests to be visible in a new request.  Returns
+     *  three pieces of related code as strings:
+     *
+     *  1. A _preamble_: An initial code fragment that should go before
+     *  the code of the new request.
+     *
+     *  2. A _trailer_: A code fragment that should go after the code
+     *  of the new request.
+     *
+     *  3. An _access path_ which can be traversed to access
+     *  any bindings inside code wrapped by #1 and #2 .
+     *
+     *  The argument is a set of Names that need to be imported.
+     *
+     *  Limitations: This method is not as precise as it could be.
+     *  (1) It does not process wildcard imports to see what exactly
+     *  they import.
+     *  (2) If it imports any names from a request, it imports all
+     *  of them, which is not really necessary.
+     *  (3) It imports multiple same-named implicits, but only the
+     *  last one imported is actually usable.
+     */
+    private def importsCode(wanted: Set[Name]): (String, String, String) = {
+      /** Narrow down the list of requests from which imports
+       *  should be taken.  Removes requests which cannot contribute
+       *  useful imports for the specified set of wanted names.
+       */
+      def reqsToUse: List[(Request, StatementInfo)] = {
+        /** Loop through a list of StatementHandlers and select
+         *  which ones to keep.  'wanted' is the set of
+         *  names that need to be imported.
+         */
+        def select(reqs: List[(Request, StatementInfo)], wanted: Set[Name]): List[(Request, StatementInfo)] = {
+          reqs match {
+            case Nil => Nil
+
+            case (req, handler) :: rest =>
+              val keepit =
+                (handler.definesImplicit ||
+                  handler.importsWildcard ||
+                  handler.importedNames.exists(wanted.contains(_)) ||
+                  handler.boundNames.exists(wanted.contains(_)))
+
+              val newWanted =
+                if (keepit) {
+                  (wanted
+                    ++ handler.usedNames
+                    -- handler.boundNames
+                    -- handler.importedNames)
+                } else {
+                  wanted
+                }
+
+              val restToKeep = select(rest, newWanted)
+
+              if (keepit)
+                (req, handler) :: restToKeep
+              else
+                restToKeep
+          }
+        }
+
+        val rhpairs = for {
+          req <- prevRequests.toList.reverse
+          handler <- req.handlers
+        } yield (req, handler)
+
+        select(rhpairs, wanted).reverse
+      }
+
+      val preamble = new StringBuffer
+      val trailingBraces = new StringBuffer
+      val accessPath = new StringBuffer
+      val impname = INTERPRETER_IMPORT_WRAPPER
+      val currentImps = mutable.Set[Name]()
+
+      // add code for a new object to hold some imports
+      def addWrapper(): Unit = {
+        preamble.append("object " + impname + "{\n")
+        trailingBraces.append("}\n")
+        accessPath.append("." + impname)
+        currentImps.clear
+      }
+
+      addWrapper()
+
+      // loop through previous requests, adding imports
+      // for each one
+      for ((req, handler) <- reqsToUse) {
+        // If the user entered an import, then just use it
+
+        // add an import wrapping level if the import might
+        // conflict with some other import
+        if (handler.importsWildcard ||
+          currentImps.exists(handler.importedNames.contains))
+          if (!currentImps.isEmpty)
+            addWrapper()
+
+        if (handler.statement.isInstanceOf[Import])
+          preamble.append(handler.statement.toString + ";\n")
+
+        // give wildcard imports a import wrapper all to their own
+        if (handler.importsWildcard)
+          addWrapper()
+        else
+          currentImps ++= handler.importedNames
+
+        // For other requests, import each bound variable.
+        // import them explicitly instead of with _, so that
+        // ambiguity errors will not be generated. Also, quote
+        // the name of the variable, so that we don't need to
+        // handle quoting keywords separately.
+        for (imv <- handler.boundNames) {
+          if (currentImps.contains(imv))
+            addWrapper()
+          preamble.append("import ")
+          preamble.append(req.objectName + req.accessPath + ".`" + imv + "`;\n")
+          currentImps += imv
+        }
+      }
+
+      addWrapper() // Add one extra wrapper, to prevent warnings
+      // in the frequent case of redefining
+      // the value bound in the last interpreter
+      // request.
+
+      (preamble.toString, trailingBraces.toString, accessPath.toString)
+    }
+
+    // ------ Handlers ------------------------------------------
+
+    /** Class to handle one statement among all the statements included
+     *  in a single interpreter request.
+     */
+    private sealed abstract class StatementHandler(val statement: Tree) extends StatementInfo {
+      val usedNames: List[Name] = {
+        val ivt = new UntypedTreeAccumulator[mutable.Set[Name]] {
+          override def apply(ns: mutable.Set[Name], tree: Tree)(implicit ctx: Context) =
+            tree match {
+              case Ident(name) => ns += name
+              case _ => foldOver(ns, tree)
+            }
+        }
+        ivt.foldOver(HashSet(), statement).toList
+      }
+      val boundNames: List[Name] = Nil
+      def valAndVarNames: List[Name] = Nil
+      def defNames: List[Name] = Nil
+      val importsWildcard = false
+      val importedNames: Seq[Name] = Nil
+      val definesImplicit = statement match {
+        case tree: MemberDef => tree.mods.is(Flags.Implicit)
+        case _ => false
+      }
+
+      def extraCodeToEvaluate(req: Request, code: PrintWriter) = {}
+      def resultExtractionCode(req: Request, code: PrintWriter) = {}
+    }
+
+    private class GenericHandler(statement: Tree) extends StatementHandler(statement)
+
+    private class ValHandler(statement: ValDef) extends StatementHandler(statement) {
+      override val boundNames = List(statement.name)
+      override def valAndVarNames = boundNames
+
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        val vname = statement.name
+        if (!statement.mods.is(Flags.AccessFlags) &&
+          !(isGeneratedVarName(vname.toString) &&
+            req.typeOf(vname.encode) == "Unit")) {
+          val prettyName = vname.decode
+          code.print(" + \"" + prettyName + ": " +
+            string2code(req.typeOf(vname)) +
+            " = \" + " +
+            " (if(" +
+            req.fullPath(vname) +
+            ".asInstanceOf[AnyRef] != null) " +
+            " ((if(" +
+            req.fullPath(vname) +
+            ".toString().contains('\\n')) " +
+            " \"\\n\" else \"\") + " +
+            req.fullPath(vname) + ".toString() + \"\\n\") else \"null\\n\") ")
+        }
+      }
+    }
+
+    private class DefHandler(defDef: DefDef) extends StatementHandler(defDef) {
+      override val boundNames = List(defDef.name)
+      override def defNames = boundNames
+
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        if (!defDef.mods.is(Flags.AccessFlags))
+          code.print("+\"" + string2code(defDef.name.toString) + ": " +
+            string2code(req.typeOf(defDef.name)) + "\\n\"")
+      }
+    }
+
+    private class AssignHandler(statement: Assign) extends StatementHandler(statement) {
+      val lhs = statement.lhs.asInstanceOf[Ident] // an unfortunate limitation
+
+      val helperName = newInternalVarName().toTermName
+      override val valAndVarNames = List(helperName)
+
+      override def extraCodeToEvaluate(req: Request, code: PrintWriter): Unit = {
+        code.println("val " + helperName + " = " + statement.lhs + ";")
+      }
+
+      /** Print out lhs instead of the generated varName */
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        code.print(" + \"" + lhs + ": " +
+          string2code(req.typeOf(helperName.encode)) +
+          " = \" + " +
+          string2code(req.fullPath(helperName))
+          + " + \"\\n\"")
+      }
+    }
+
+    private class ModuleHandler(module: ModuleDef) extends StatementHandler(module) {
+      override val boundNames = List(module.name)
+
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        code.println(" + \"defined module " +
+          string2code(module.name.toString)
+          + "\\n\"")
+      }
+    }
+
+    private class ClassHandler(classdef: TypeDef)
+      extends StatementHandler(classdef) {
+      override val boundNames =
+        List(classdef.name) :::
+          (if (classdef.mods.is(Flags.Case))
+            List(classdef.name.toTermName)
+          else
+            Nil)
+
+      // TODO: MemberDef.keyword does not include "trait";
+      // otherwise it could be used here
+      def keyword: String =
+        if (classdef.mods.is(Flags.Trait)) "trait" else "class"
+
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        code.print(
+          " + \"defined " +
+            keyword +
+            " " +
+            string2code(classdef.name.toString) +
+            "\\n\"")
+      }
+    }
+
+    private class TypeAliasHandler(typeDef: TypeDef)
+      extends StatementHandler(typeDef) {
+      override val boundNames =
+        if (!typeDef.mods.is(Flags.AccessFlags) && !typeDef.rhs.isInstanceOf[TypeBoundsTree])
+          List(typeDef.name)
+        else
+          Nil
+
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        code.println(" + \"defined type alias " +
+          string2code(typeDef.name.toString) + "\\n\"")
+      }
+    }
+
+    private class ImportHandler(imp: Import) extends StatementHandler(imp) {
+      override def resultExtractionCode(req: Request, code: PrintWriter): Unit = {
+        code.println("+ \"" + imp.toString + "\\n\"")
+      }
+
+      def isWildcardSelector(tree: Tree) = tree match {
+        case Ident(nme.USCOREkw) => true
+        case _ => false
+      }
+
+      /** Whether this import includes a wildcard import */
+      override val importsWildcard = imp.selectors.exists(isWildcardSelector)
+
+      /** The individual names imported by this statement */
+      override val importedNames: Seq[Name] =
+        imp.selectors.filterNot(isWildcardSelector).flatMap {
+          case sel: RefTree => List(sel.name.toTypeName, sel.name.toTermName)
+          case _ => Nil
+        }
+    }
+
+  } // end Request
+
+  // ------- String handling ----------------------------------
+
+  /** next line number to use */
+  private var nextLineNo = 0
+
+  /** allocate a fresh line name */
+  private def newLineName = {
+    val num = nextLineNo
+    nextLineNo += 1
+    INTERPRETER_LINE_PREFIX + num
+  }
+
+  /** next result variable number to use */
+  private var nextVarNameNo = 0
+
+  /** allocate a fresh variable name */
+  private def newVarName = {
+    val num = nextVarNameNo
+    nextVarNameNo += 1
+    INTERPRETER_VAR_PREFIX + num
+  }
+
+  /** next internal variable number to use */
+  private var nextInternalVarNo = 0
+
+  /** allocate a fresh internal variable name */
+  private def newInternalVarName() = {
+    val num = nextVarNameNo
+    nextVarNameNo += 1
+    INTERPRETER_SYNTHVAR_PREFIX + num
+  }
+
+  /** Check if a name looks like it was generated by newVarName */
+  private def isGeneratedVarName(name: String): Boolean =
+    name.startsWith(INTERPRETER_VAR_PREFIX) && {
+      val suffix = name.drop(INTERPRETER_VAR_PREFIX.length)
+      suffix.forall(_.isDigit)
+    }
+
+  /** generate a string using a routine that wants to write on a stream */
+  private def stringFrom(writer: PrintWriter => Unit): String = {
+    val stringWriter = new StringWriter()
+    val stream = new NewLinePrintWriter(stringWriter)
+    writer(stream)
+    stream.close
+    stringWriter.toString
+  }
+
+  /** Truncate a string if it is longer than settings.maxPrintString */
+  private def truncPrintString(str: String)(implicit ctx: Context): String = {
+    val maxpr = ctx.settings.XreplLineWidth.value
+
+    if (maxpr <= 0)
+      return str
+
+    if (str.length <= maxpr)
+      return str
+
+    val trailer = "..."
+    if (maxpr >= trailer.length+1)
+      return str.substring(0, maxpr-3) + trailer
+
+    str.substring(0, maxpr)
+  }
+
+  /** Clean up a string for output */
+  private def clean(str: String)(implicit ctx: Context) =
+    truncPrintString(stripWrapperGunk(str))
+
+  /** Indent some code by the width of the scala> prompt.
+   *  This way, compiler error messages read better.
+   */
+  def indentCode(code: String) = {
+    val spaces = "       "
+
+    stringFrom(str =>
+      for (line <- code.lines) {
+        str.print(spaces)
+        str.print(line + "\n")
+        str.flush()
+      })
+  }
+}
+
+/** Utility methods for the Interpreter. */
+object CompilingInterpreter {
+  val INTERPRETER_WRAPPER_SUFFIX = "$object"
+  val INTERPRETER_LINE_PREFIX = "line"
+  val INTERPRETER_VAR_PREFIX = "res"
+  val INTERPRETER_IMPORT_WRAPPER = "$iw"
+  val INTERPRETER_SYNTHVAR_PREFIX = "synthvar$"
+
+  /** Delete a directory tree recursively.  Use with care!
+   */
+  private[repl] def deleteRecursively(path: File): Unit = {
+    path match  {
+      case _ if !path.exists =>
+        ()
+      case _ if path.isDirectory =>
+        for (p <- path.listFiles)
+          deleteRecursively(p)
+        path.delete
+      case _ =>
+        path.delete
+    }
+  }
+
+  /** Heuristically strip interpreter wrapper prefixes
+   *  from an interpreter output string.
+   */
+  def stripWrapperGunk(str: String): String = {
+    val wrapregex = "(line[0-9]+\\$object[$.])?(\\$iw[$.])*"
+    str.replaceAll(wrapregex, "")
+  }
+
+  /** Convert a string into code that can recreate the string.
+   *  This requires replacing all special characters by escape
+   *  codes. It does not add the surrounding " marks.  */
+  def string2code(str: String): String = {
+    /** Convert a character to a backslash-u escape */
+    def char2uescape(c: Char): String = {
+      var rest = c.toInt
+      val buf = new StringBuilder
+      for (i <- 1 to 4) {
+	      buf ++= (rest % 16).toHexString
+	      rest = rest / 16
+      }
+      "\\" + "u" + buf.toString.reverse
+    }
+    val res = new StringBuilder
+    for (c <- str) {
+      if ("'\"\\" contains c) {
+	      res += '\\'
+	      res += c
+      } else if (!c.isControl) {
+	      res += c
+      } else {
+	      res ++= char2uescape(c)
+      }
+    }
+    res.toString
+  }
+}
diff --git a/src/dotty/tools/dotc/repl/ConsoleWriter.scala b/src/dotty/tools/dotc/repl/ConsoleWriter.scala
new file mode 100644
index 000000000000..9387f366a25f
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/ConsoleWriter.scala
@@ -0,0 +1,21 @@
+package dotty.tools
+package dotc
+package repl
+import java.io.Writer
+
+/** A Writer that writes onto the Scala Console.
+ *
+ *  @author  Lex Spoon
+ *  @version 1.0
+ */
+class ConsoleWriter extends Writer {
+  def close = flush
+
+  def flush = Console.flush
+
+  def write(cbuf: Array[Char], off: Int, len: Int): Unit =
+    if (len > 0)
+      write(new String(cbuf, off, len))
+
+  override def write(str: String): Unit = Console.print(str)
+}
diff --git a/src/dotty/tools/dotc/repl/InteractiveReader.scala b/src/dotty/tools/dotc/repl/InteractiveReader.scala
new file mode 100644
index 000000000000..96c55ebd0c8c
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/InteractiveReader.scala
@@ -0,0 +1,31 @@
+package dotty.tools
+package dotc
+package repl
+
+/** Reads lines from an input stream */
+trait InteractiveReader {
+  def readLine(prompt: String): String
+  val interactive: Boolean
+}
+
+/** TODO Enable jline support.
+ *  The current Scala REPL know how to do this flexibly.
+ */
+object InteractiveReader {
+  /** Create an interactive reader.  Uses JLine if the
+   *  library is available, but otherwise uses a
+   *  SimpleReader. */
+  def createDefault(): InteractiveReader = new SimpleReader()
+  /*
+  {
+    try {
+      new JLineReader
+    } catch {
+      case e =>
+        //out.println("jline is not available: " + e) //debug
+	      new SimpleReader()
+    }
+  }
+*/
+
+}
diff --git a/src/dotty/tools/dotc/repl/Interpreter.scala b/src/dotty/tools/dotc/repl/Interpreter.scala
new file mode 100644
index 000000000000..ea587a097a27
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/Interpreter.scala
@@ -0,0 +1,36 @@
+package dotty.tools
+package dotc
+package repl
+
+import core.Contexts.Context
+
+/** This object defines the type of interpreter results */
+object Interpreter {
+
+  /** A result from interpreting one line of input. */
+  abstract sealed class Result
+
+  /** The line was interpreted successfully. */
+  case object Success extends Result
+
+  /** The line was erroneous in some way. */
+  case object Error extends Result
+
+  /** The input was incomplete.  The caller should request more input.
+   */
+  case object Incomplete extends Result
+}
+
+/** The exported functionality of the interpreter */
+trait Interpreter {
+  import Interpreter._
+
+  /** Interpret one line of input.  All feedback, including parse errors
+   *  and evaluation results, are printed via the context's reporter.
+   *  reporter.  Values defined are available for future interpreted strings.
+   */
+  def interpret(line: String)(implicit ctx: Context): Result
+
+  /** Suppress output during evaluation of `operation`. */
+  def beQuietDuring[T](operation: => T): T
+}
diff --git a/src/dotty/tools/dotc/repl/InterpreterLoop.scala b/src/dotty/tools/dotc/repl/InterpreterLoop.scala
new file mode 100644
index 000000000000..eedec3c8282d
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/InterpreterLoop.scala
@@ -0,0 +1,228 @@
+package dotty.tools
+package dotc
+package repl
+
+import java.io.{BufferedReader, File, FileReader, PrintWriter}
+import java.io.IOException
+import java.lang.{ClassLoader, System}
+import scala.concurrent.{Future, Await}
+import scala.concurrent.duration.Duration
+import reporting.Reporter
+import core._
+import Contexts._
+import annotation.tailrec
+import scala.concurrent.ExecutionContext.Implicits.global
+
+/** The interactive shell.  It provides a read-eval-print loop around
+ *  the Interpreter class.
+ *  After instantiation, clients should call the `run` method.
+ *
+ *  @author Moez A. Abdel-Gawad
+ *  @author  Lex Spoon
+ *  @author Martin Odersky
+ */
+class InterpreterLoop(
+    compiler: Compiler,
+    private var in: InteractiveReader,
+    out: PrintWriter)(implicit ctx: Context) {
+
+  val interpreter = compiler.asInstanceOf[Interpreter]
+
+  /** The context class loader at the time this object was created */
+  protected val originalClassLoader =
+    Thread.currentThread.getContextClassLoader
+
+  /** A reverse list of commands to replay if the user
+    * requests a :replay */
+  var replayCommandsRev: List[String] = Nil
+
+  /** A list of commands to replay if the user requests a :replay */
+  def replayCommands = replayCommandsRev.reverse
+
+  /** Record a command for replay should the user request a :replay */
+  def addReplay(cmd: String) =
+    replayCommandsRev = cmd :: replayCommandsRev
+
+  /** Close the interpreter */
+  def closeInterpreter()(implicit ctx: Context): Unit = {
+    ctx.reporter.flush()
+    Thread.currentThread.setContextClassLoader(originalClassLoader)
+  }
+
+  /** print a friendly help message */
+  def printHelp(): Unit = {
+    printWelcome()
+    out.println("Type :load followed by a filename to load a Scala file.")
+    out.println("Type :replay to reset execution and replay all previous commands.")
+    out.println("Type :quit to exit the interpreter.")
+  }
+
+  /** Print a welcome message */
+  def printWelcome(): Unit = {
+    out.println(s"Welcome to Scala$version " + " (" +
+                System.getProperty("java.vm.name") + ", Java " + System.getProperty("java.version") + ")." )
+    out.println("Type in expressions to have them evaluated.")
+    out.println("Type :help for more information.")
+    out.flush()
+  }
+
+  /** Prompt to print when awaiting input */
+  val prompt             = "scala> "
+  val continuationPrompt = "     | "
+
+  val version = ".next (pre-alpha)"
+
+  /** The first interpreted command always takes a couple of seconds
+   *  due to classloading. To bridge the gap, we warm up the interpreter
+   *  by letting it interpret a dummy line while waiting for the first
+   *  line of input to be entered.
+   */
+  def firstLine(): String = {
+    val futLine = Future(in.readLine(prompt))
+    interpreter.beQuietDuring(
+      interpreter.interpret("val theAnswerToLifeInTheUniverseAndEverything = 21 * 2"))
+    Await.result(futLine, Duration.Inf)
+  }
+
+  /** The main read-eval-print loop for the interpreter.  It calls
+   *  `command()` for each line of input.
+   */
+  @tailrec final def repl(line: String = in.readLine(prompt)): Unit =
+    if (line != null) {
+      val (keepGoing, finalLineOpt) = command(line)
+      if (keepGoing) {
+        finalLineOpt.foreach(addReplay)
+        out.flush()
+        repl()
+      }
+    }
+
+  /** interpret all lines from a specified file */
+  def interpretAllFrom(filename: String): Unit = {
+    val fileIn = try {
+      new FileReader(filename)
+    } catch {
+      case _: IOException =>
+        out.println("Error opening file: " + filename)
+        return
+    }
+    val oldIn = in
+    val oldReplay = replayCommandsRev
+    try {
+      val inFile = new BufferedReader(fileIn)
+      in = new SimpleReader(inFile, out, false)
+      out.println("Loading " + filename + "...")
+      out.flush
+      repl()
+    } finally {
+      in = oldIn
+      replayCommandsRev = oldReplay
+      fileIn.close
+    }
+  }
+
+  /** create a new interpreter and replay all commands so far */
+  def replay(): Unit = {
+    for (cmd <- replayCommands) {
+      out.println("Replaying: " + cmd)
+      out.flush()  // because maybe cmd will have its own output
+      command(cmd)
+      out.println
+    }
+  }
+
+  /** Run one command submitted by the user.  Three values are returned:
+    * (1) whether to keep running, (2) the line to record for replay,
+    * if any. */
+  def command(line: String): (Boolean, Option[String]) = {
+    def withFile(command: String)(action: String => Unit): Unit = {
+      val spaceIdx = command.indexOf(' ')
+      if (spaceIdx <= 0) {
+        out.println("That command requires a filename to be specified.")
+        return
+      }
+      val filename = command.substring(spaceIdx).trim
+      if (!new File(filename).exists) {
+        out.println("That file does not exist")
+        return
+      }
+      action(filename)
+    }
+
+    val helpRegexp    = ":h(e(l(p)?)?)?"
+    val quitRegexp    = ":q(u(i(t)?)?)?"
+    val loadRegexp    = ":l(o(a(d)?)?)?.*"
+    val replayRegexp  = ":r(e(p(l(a(y)?)?)?)?)?.*"
+
+    var shouldReplay: Option[String] = None
+
+    if (line.matches(helpRegexp))
+      printHelp()
+    else if (line.matches(quitRegexp))
+      return (false, None)
+    else if (line.matches(loadRegexp)) {
+      withFile(line)(f => {
+        interpretAllFrom(f)
+        shouldReplay = Some(line)
+      })
+    }
+    else if (line matches replayRegexp)
+      replay()
+    else if (line startsWith ":")
+      out.println("Unknown command.  Type :help for help.")
+    else
+      shouldReplay = interpretStartingWith(line)
+
+    (true, shouldReplay)
+  }
+
+  /** Interpret expressions starting with the first line.
+    * Read lines until a complete compilation unit is available
+    * or until a syntax error has been seen.  If a full unit is
+    * read, go ahead and interpret it.  Return the full string
+    * to be recorded for replay, if any.
+    */
+  def interpretStartingWith(code: String): Option[String] = {
+    interpreter.interpret(code) match {
+      case Interpreter.Success => Some(code)
+      case Interpreter.Error => None
+      case Interpreter.Incomplete =>
+        if (in.interactive && code.endsWith("\n\n")) {
+          out.println("You typed two blank lines.  Starting a new command.")
+          None
+        } else {
+          val nextLine = in.readLine(continuationPrompt)
+          if (nextLine == null)
+            None  // end of file
+          else
+            interpretStartingWith(code + "\n" + nextLine)
+        }
+    }
+  }
+/*
+  def loadFiles(settings: Settings) {
+    settings match {
+      case settings: GenericRunnerSettings =>
+        for (filename <- settings.loadfiles.value) {
+          val cmd = ":load " + filename
+          command(cmd)
+          replayCommandsRev = cmd :: replayCommandsRev
+          out.println()
+        }
+      case _ =>
+    }
+  }
+*/
+  def run(): Reporter = {
+    // loadFiles(settings)
+    try {
+      if (!ctx.reporter.hasErrors) { // if there are already errors, no sense to continue
+        printWelcome()
+        repl(firstLine())
+      }
+    } finally {
+      closeInterpreter()
+    }
+    ctx.reporter
+  }
+}
diff --git a/src/dotty/tools/dotc/repl/Main.scala b/src/dotty/tools/dotc/repl/Main.scala
new file mode 100644
index 000000000000..b2b92299eeea
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/Main.scala
@@ -0,0 +1,29 @@
+package dotty.tools
+package dotc
+package repl
+
+/* This REPL was adapted from an old (2008-ish) version of the Scala
+ * REPL. The original version from which the adaptation was done is found in:
+ *
+ *      https://github.com/odersky/legacy-svn-scala/tree/spoon
+ *
+ * The reason this version was picked instead of a more current one is that
+ * the older version is much smaller, therefore easier to port. It is also
+ * considerably less intertwined with nsc than later versions.
+ *
+ * There are a number of TODOs:
+ *
+ *  - re-enable jline support (urgent, easy, see TODO in InteractiveReader.scala)
+ *  - figure out why we can launch REPL only with `java`, not with `scala`.
+ *  - make a doti command (urgent, easy)
+ *  - create or port REPL tests (urgent, intermediate)
+ *  - copy improvements of current Scala REPL wrt to this version
+ *    (somewhat urgent, intermediate)
+ *  - re-enable bindSettings (not urgent, easy, see TODO in InterpreterLoop.scala)
+ *  - make string generation more functional (not urgent, easy)
+ *  - better handling of ^C (not urgent, intermediate)
+ *  - syntax highlighting (not urgent, intermediate)
+ *  - integrate with presentation compiler for command completion (not urgent, hard)
+ */
+/** The main entry point of the REPL */
+object Main extends REPL
\ No newline at end of file
diff --git a/src/dotty/tools/dotc/repl/NewLinePrintWriter.scala b/src/dotty/tools/dotc/repl/NewLinePrintWriter.scala
new file mode 100644
index 000000000000..8e36a0ae47a0
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/NewLinePrintWriter.scala
@@ -0,0 +1,11 @@
+package dotty.tools
+package dotc
+package repl
+import java.io.{Writer, PrintWriter}
+
+class NewLinePrintWriter(out: Writer, autoFlush: Boolean)
+extends PrintWriter(out, autoFlush) {
+  def this(out: Writer) = this(out, false)
+  override def println(): Unit = { print("\n"); flush() }
+}
+
diff --git a/src/dotty/tools/dotc/repl/REPL.scala b/src/dotty/tools/dotc/repl/REPL.scala
new file mode 100644
index 000000000000..2d6a3c742227
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/REPL.scala
@@ -0,0 +1,49 @@
+package dotty.tools
+package dotc
+package repl
+
+import core.Contexts.Context
+import reporting.Reporter
+import java.io.{BufferedReader, File, FileReader, PrintWriter}
+
+/** A compiler which stays resident between runs.
+ *  Usage:
+ *
+ *  > scala dotty.tools.dotc.Resident <options> <initial files>
+ *
+ *  dotc> "more options and files to compile"
+ *
+ *  ...
+ *
+ *  dotc> :reset  // reset all options to the ones passed on the command line
+ *
+ *  ...
+ *
+ *  dotc> :q     // quit
+ */
+class REPL extends Driver {
+
+  /** The default input reader */
+  def input(implicit ctx: Context): InteractiveReader = {
+    val emacsShell = System.getProperty("env.emacs", "") != ""
+    //println("emacsShell="+emacsShell) //debug
+    if (ctx.settings.Xnojline.value || emacsShell) new SimpleReader()
+    else InteractiveReader.createDefault()
+  }
+
+  /** The defult output writer */
+  def output: PrintWriter = new NewLinePrintWriter(new ConsoleWriter, true)
+
+  override def newCompiler(implicit ctx: Context): Compiler =
+    new repl.CompilingInterpreter(output, ctx)
+
+  override def sourcesRequired = false
+
+  override def doCompile(compiler: Compiler, fileNames: List[String])(implicit ctx: Context): Reporter = {
+    if (fileNames.isEmpty)
+      new InterpreterLoop(compiler, input, output).run()
+    else
+      ctx.error(s"don't now what to do with $fileNames%, %")
+    ctx.reporter
+  }
+}
diff --git a/src/dotty/tools/dotc/repl/SimpleReader.scala b/src/dotty/tools/dotc/repl/SimpleReader.scala
new file mode 100644
index 000000000000..9fd563382155
--- /dev/null
+++ b/src/dotty/tools/dotc/repl/SimpleReader.scala
@@ -0,0 +1,23 @@
+package dotty.tools
+package dotc
+package repl
+
+import java.io.{BufferedReader, PrintWriter}
+
+
+/** Reads using standard JDK API */
+class SimpleReader(
+  in: BufferedReader,
+  out: PrintWriter,
+  val interactive: Boolean)
+extends InteractiveReader {
+  def this() = this(Console.in, new PrintWriter(Console.out), true)
+
+  def readLine(prompt: String) = {
+    if (interactive) {
+      out.print(prompt)
+      out.flush()
+    }
+    in.readLine()
+  }
+}
diff --git a/tests/run/t920.scala b/tests/run/t920.scala
new file mode 100644
index 000000000000..d42388c4abcf
--- /dev/null
+++ b/tests/run/t920.scala
@@ -0,0 +1,24 @@
+object Test {
+  trait A;
+  trait Foo0 { def foo : A; }
+  trait Baz extends Foo0;
+  trait B extends A {
+    def initialize = {
+      trait Foo extends Test.Foo0 {
+        def foo : B.this.type = B.this;
+      }
+      class baz extends Baz with Foo {
+        override def toString = "baz"
+      }
+      Console.println(new baz);
+    }
+  }
+  object bb extends B;
+  def main(args : Array[String]) : Unit = {
+    bb.initialize;
+  }
+}
+
+
+
+