Make type check error messages a bit more helpful

src/main/java/de/prob/voparser/VOTypeChecker.java

@@ -94,11 +94,10 @@ public class VOTypeChecker extends DepthFirstAdapter {
 	private PersistentHashSet<AnimatorState> visitIdentifierNode(AIdentifierVo node, PersistentHashSet<AnimatorState> animatorState) {
 		final String id = node.getIdentifierLiteral().getText();
 		if (!voParser.getTasks().containsKey(id)) {
-			throw new VOTypeCheckException("Scope error in VO");
+			throw new VOTypeCheckException("Unknown validation task identifier: " + id);
 		}
 		VTType type = voParser.getTasks().get(id);
 		PersistentHashSet<AnimatorState> newAnimatorState = animatorState;
-		boolean valid = true;
 		switch (type) {
 			case RELOAD:
 				newAnimatorState = newAnimatorState.cons(AnimatorState.TRACE);
@@ -112,23 +111,26 @@ public class VOTypeChecker extends DepthFirstAdapter {
 				newAnimatorState = newAnimatorState.disjoin(AnimatorState.STATE_SPACE);
 				break;
 			case TRACE:
-				valid = newAnimatorState.contains(AnimatorState.TRACE);
+				if (!newAnimatorState.contains(AnimatorState.TRACE)) {
+					throw new VOTypeCheckException("Validation task " + id + " requires a trace");
+				}
 				break;
 			case STATE_SPACE:
-				valid = newAnimatorState.contains(AnimatorState.STATE_SPACE);
+				if (!newAnimatorState.contains(AnimatorState.STATE_SPACE)) {
+					throw new VOTypeCheckException("Validation task " + id + " requires a state space");
+				}
 				newAnimatorState = newAnimatorState.disjoin(AnimatorState.TRACE);
 				newAnimatorState = newAnimatorState.disjoin(AnimatorState.STATE_SPACE);
 				break;
 			case EXPLORE:
-				valid = newAnimatorState.contains(AnimatorState.TRACE);
+				if (!newAnimatorState.contains(AnimatorState.TRACE)) {
+					throw new VOTypeCheckException("Validation task " + id + " requires a trace");
+				}
 				newAnimatorState = newAnimatorState.disjoin(AnimatorState.TRACE);
 				break;
 			default:
 				break;
 		}
-		if(!valid) {
-			throw new VOTypeCheckException("Type error in VO");
-		}
 		return newAnimatorState;
 	}
 }