Refactor Driver

 - Make parts more reusable
 - Introduce hook "sourcesRequired" that controls whether
   no sources on the command line give a help message.

Author: odersky

src/dotty/tools/dotc/Driver.scala

@@ -8,32 +8,40 @@ import scala.util.control.NonFatal
 
 abstract class Driver extends DotClass {
 
-  val prompt = "\ndotc>"
+  val prompt = "\ndotc> "
 
   protected def newCompiler(): Compiler
 
   protected def emptyReporter: Reporter = new StoreReporter
 
   protected def doCompile(compiler: Compiler, fileNames: List[String])(implicit ctx: Context): Reporter =
-    if (fileNames.nonEmpty) {
-      val run = compiler.newRun
-      run.compile(fileNames)
-      run.printSummary()
-    } else emptyReporter
+    if (fileNames.nonEmpty)
+      try {
+        val run = compiler.newRun
+        run.compile(fileNames)
+        run.printSummary()
+      }
+      catch {
+        case ex: FatalError  =>
+          ctx.error(ex.getMessage) // signals that we should fail compilation.
+          ctx.typerState.reporter
+      }
+    else emptyReporter
 
   protected def initCtx = (new ContextBase).initialCtx
 
-  def process(args: Array[String], rootCtx: Context): Reporter = {
+  protected def sourcesRequired = true
+
+  def setup(args: Array[String], rootCtx: Context): (List[String], Context) = {
     val summary = CompilerCommand.distill(args)(rootCtx)
     implicit val ctx: Context = initCtx.fresh.setSettings(summary.sstate)
-    val fileNames = CompilerCommand.checkUsage(summary)
-    try {
-      doCompile(newCompiler(), fileNames)
-    } catch {
-      case ex: FatalError  =>
-        ctx.error(ex.getMessage) // signals that we should fail compilation.
-        ctx.typerState.reporter
-    }
+    val fileNames = CompilerCommand.checkUsage(summary, sourcesRequired)
+    (fileNames, ctx)
+  }
+
+  def process(args: Array[String], rootCtx: Context): Reporter = {
+    val (fileNames, ctx) = setup(args, rootCtx)
+    doCompile(newCompiler(), fileNames)(ctx)
   }
 
   def main(args: Array[String]): Unit =

src/dotty/tools/dotc/config/CompilerCommand.scala

@@ -60,7 +60,7 @@ object CompilerCommand extends DotClass {
    *  are already applied in context.
    *  @return  The list of files to compile.
    */
-  def checkUsage(summary: ArgsSummary)(implicit ctx: Context): List[String] = {
+  def checkUsage(summary: ArgsSummary, sourcesRequired: Boolean)(implicit ctx: Context): List[String] = {
     val settings = ctx.settings
 
     /** Creates a help message for a subset of options based on cond */
@@ -121,8 +121,7 @@ object CompilerCommand extends DotClass {
       ctx.println(infoMessage)
       Nil
     } else {
-      if (summary.arguments.isEmpty && !settings.resident.value)
-        ctx.println(usageMessage)
+      if (sourcesRequired && summary.arguments.isEmpty) ctx.println(usageMessage)
       summary.arguments
     }
   }