diff --git a/src/Pure/System/scala.scala b/src/Pure/System/scala.scala
--- a/src/Pure/System/scala.scala
+++ b/src/Pure/System/scala.scala
@@ -1,196 +1,198 @@
 /*  Title:      Pure/System/scala.scala
     Author:     Makarius
 
 Support for Scala at runtime.
 */
 
 package isabelle
 
 
 import java.lang.reflect.{Modifier, InvocationTargetException}
 import java.io.{File => JFile, StringWriter, PrintWriter}
 
 import scala.util.matching.Regex
 import scala.tools.nsc.{GenericRunnerSettings, ConsoleWriter, NewLinePrintWriter}
 import scala.tools.nsc.interpreter.IMain
 
 
 object Scala
 {
   /* compiler */
 
   object Compiler
   {
     def context(
       error: String => Unit = Exn.error,
       jar_dirs: List[JFile] = Nil): Context =
     {
       def find_jars(dir: JFile): List[String] =
         File.find_files(dir, file => file.getName.endsWith(".jar")).
           map(File.absolute_name)
 
       val class_path =
         for {
           prop <- List("scala.boot.class.path", "java.class.path")
           path = System.getProperty(prop, "") if path != "\"\""
           elem <- space_explode(JFile.pathSeparatorChar, path)
         } yield elem
 
       val settings = new GenericRunnerSettings(error)
       settings.classpath.value =
         (class_path ::: jar_dirs.flatMap(find_jars)).mkString(JFile.pathSeparator)
 
       new Context(settings)
     }
 
     def default_print_writer: PrintWriter =
       new NewLinePrintWriter(new ConsoleWriter, true)
 
     class Context private [Compiler](val settings: GenericRunnerSettings)
     {
+      override def toString: String = settings.toString
+
       def interpreter(
         print_writer: PrintWriter = default_print_writer,
         class_loader: ClassLoader = null): IMain =
       {
         new IMain(settings, print_writer)
         {
           override def parentClassLoader: ClassLoader =
             if (class_loader == null) super.parentClassLoader
             else class_loader
         }
       }
 
       def toplevel(source: String): List[String] =
       {
         val out = new StringWriter
         val interp = interpreter(new PrintWriter(out))
         val rep = new interp.ReadEvalPrint
         val ok = interp.withLabel("\u0001") { rep.compile(source) }
         out.close
 
         val Error = """(?s)^\S* error: (.*)$""".r
         val errors =
           space_explode('\u0001', Library.strip_ansi_color(out.toString)).
             collect({ case Error(msg) => "Scala error: " + Library.trim_line(msg) })
 
         if (!ok && errors.isEmpty) List("Error") else errors
       }
     }
   }
 
 
 
   /** invoke JVM method via Isabelle/Scala **/
 
   /* method reflection */
 
   private val Ext = new Regex("(.*)\\.([^.]*)")
   private val STRING = Class.forName("java.lang.String")
 
   private def get_method(name: String): String => String =
     name match {
       case Ext(class_name, method_name) =>
         val m =
           try { Class.forName(class_name).getMethod(method_name, STRING) }
           catch {
             case _: ClassNotFoundException | _: NoSuchMethodException =>
               error("No such method: " + quote(name))
           }
         if (!Modifier.isStatic(m.getModifiers)) error("Not at static method: " + m.toString)
         if (m.getReturnType != STRING) error("Bad method return type: " + m.toString)
 
         (arg: String) => {
           try { m.invoke(null, arg).asInstanceOf[String] }
           catch {
             case e: InvocationTargetException if e.getCause != null =>
               throw e.getCause
           }
         }
       case _ => error("Malformed method name: " + quote(name))
     }
 
 
   /* method invocation */
 
   object Tag extends Enumeration
   {
     val NULL, OK, ERROR, FAIL, INTERRUPT = Value
   }
 
   def method(name: String, arg: String): (Tag.Value, String) =
     Exn.capture { get_method(name) } match {
       case Exn.Res(f) =>
         Exn.capture { f(arg) } match {
           case Exn.Res(null) => (Tag.NULL, "")
           case Exn.Res(res) => (Tag.OK, res)
           case Exn.Exn(Exn.Interrupt()) => (Tag.INTERRUPT, "")
           case Exn.Exn(e) => (Tag.ERROR, Exn.message(e))
         }
       case Exn.Exn(e) => (Tag.FAIL, Exn.message(e))
     }
 }
 
 
 /* protocol handler */
 
 class Scala extends Session.Protocol_Handler
 {
   private var session: Session = null
   private var futures = Map.empty[String, Future[Unit]]
 
   override def init(init_session: Session): Unit =
     synchronized { session = init_session }
 
   override def exit(): Unit = synchronized
   {
     for ((id, future) <- futures) cancel(id, future)
     futures = Map.empty
   }
 
   private def fulfill(id: String, tag: Scala.Tag.Value, res: String): Unit =
     synchronized
     {
       if (futures.isDefinedAt(id)) {
         session.protocol_command("Scala.fulfill", id, tag.id.toString, res)
         futures -= id
       }
     }
 
   private def cancel(id: String, future: Future[Unit])
   {
     future.cancel
     fulfill(id, Scala.Tag.INTERRUPT, "")
   }
 
   private def invoke_scala(msg: Prover.Protocol_Output): Boolean = synchronized
   {
     msg.properties match {
       case Markup.Invoke_Scala(name, id) =>
         futures += (id ->
           Future.fork {
             val (tag, result) = Scala.method(name, msg.text)
             fulfill(id, tag, result)
           })
         true
       case _ => false
     }
   }
 
   private def cancel_scala(msg: Prover.Protocol_Output): Boolean = synchronized
   {
     msg.properties match {
       case Markup.Cancel_Scala(id) =>
         futures.get(id) match {
           case Some(future) => cancel(id, future)
           case None =>
         }
         true
       case _ => false
     }
   }
 
   val functions =
     List(
       Markup.Invoke_Scala.name -> invoke_scala,
       Markup.Cancel_Scala.name -> cancel_scala)
 }