improve error messages for cycles

util/collection/Dag.scala

@@ -22,20 +22,31 @@ object Dag
 		val finished = asSet(new java.util.LinkedHashSet[T])
 
 		def visitAll(nodes: Iterable[T]) = nodes foreach visit
-		def visit(dag : T){
-			if (!discovered(dag)) {
-				discovered(dag) = true; 
-				visitAll(dependencies(dag));
-				finished += dag;
+		def visit(node : T){
+			if (!discovered(node)) {
+				discovered(node) = true; 
+				try { visitAll(dependencies(node)); } catch { case c: Cyclic => throw node :: c }
+				finished += node;
 			}
-			else if(!finished(dag))
-				throw new Cyclic(dag)
+			else if(!finished(node))
+				throw new Cyclic(node)
 		}
 
 		visitAll(nodes);
 	
 		finished.toList;
 	}
-	final class Cyclic(val value: Any) extends Exception("Cyclic reference involving " + value)
+	final class Cyclic(val value: Any, val all: List[Any], val complete: Boolean)
+		extends Exception( "Cyclic reference involving " + (if(complete) all.mkString(", ") else value) )
+	{
+		def this(value: Any) = this(value, value :: Nil, false)
+		def ::(a: Any): Cyclic = 
+			if(complete)
+				this
+			else if(a == value)
+				new Cyclic(value, all, true)
+			else
+				new Cyclic(value, a :: all, false)
+	}
 }