YAML: simple error when constructing undefined anchors

extra/yaml/yaml-tests.factor

@@ -95,6 +95,9 @@ CONSTANT: test-anchors-obj {
 ${ test-anchors-obj } [ $ test-anchors yaml> ] unit-test
 ${ test-anchors-obj } [ $ test-anchors-obj >yaml yaml> ] unit-test
 
+! Missing anchors
+[ "*foo" yaml> ] [ "No previous anchor" = ] must-fail-with
+
 ! Lifted from pyyaml
 ! http://pyyaml.org/browser/pyyaml/trunk/tests/data
 

extra/yaml/yaml.factor

@@ -16,7 +16,8 @@ SYMBOL: anchors
 : ?register-anchor ( obj event -- obj )
     dupd anchor>> [ anchors get set-at ] [ drop ] if* ;
 : deref-anchor ( event -- obj )
-    data>> alias>> anchor>> anchors get at ;
+    data>> alias>> anchor>> anchors get at*
+    [ "No previous anchor" throw ] unless ;
 
 : event>scalar ( event -- obj )
     data>> scalar>>