diff --git a/mistral/tests/unit/test_expressions.py b/mistral/tests/unit/test_expressions.py
index 2f008ce2..fb80d3bb 100644
--- a/mistral/tests/unit/test_expressions.py
+++ b/mistral/tests/unit/test_expressions.py
@@ -72,6 +72,7 @@ class YaqlEvaluatorTest(base.BaseTest):
         self.assertEqual(item, {'name': 'ubuntu'})
 
     def test_function_length(self):
+        # Lists.
         self.assertEqual(3, expr.evaluate('$.length()', [1, 2, 3]))
         self.assertEqual(2, expr.evaluate('$.length()', ['one', 'two']))
         self.assertEqual(4, expr.evaluate(
@@ -79,6 +80,27 @@ class YaqlEvaluatorTest(base.BaseTest):
             {'array': ['1', '2', '3', '4']})
         )
 
+        # Strings.
+        self.assertEqual(3, expr.evaluate('$.length()', '123'))
+        self.assertEqual(2, expr.evaluate('$.length()', '12'))
+        self.assertEqual(
+            4,
+            expr.evaluate('$.string.length()', {'string': '1234'})
+        )
+
+        # Generators.
+        self.assertEqual(
+            2,
+            expr.evaluate(
+                "$[$.state = 'active'].length()",
+                [
+                    {'state': 'active'},
+                    {'state': 'active'},
+                    {'state': 'passive'}
+                ]
+            )
+        )
+
 
 class InlineYAQLEvaluatorTest(base.BaseTest):
     def setUp(self):
diff --git a/mistral/yaql_utils.py b/mistral/yaql_utils.py
index cd814640..804aeb2b 100644
--- a/mistral/yaql_utils.py
+++ b/mistral/yaql_utils.py
@@ -14,11 +14,14 @@
 #    See the License for the specific language governing permissions and
 #    limitations under the License.
 
+import collections
+import six
+import types
 import yaql
+from yaql import context as ctx
 
 
 def create_yaql_context():
-
     ctx = yaql.create_context()
 
     _register_functions(ctx)
@@ -27,11 +30,17 @@ def create_yaql_context():
 
 
 def _register_functions(yaql_ctx):
-    yaql_ctx.register_function(length, 'length')
-    yaql_ctx.register_function(length, 'size')
+    yaql_ctx.register_function(_generator_length, 'length')
+    yaql_ctx.register_function(_string_and_iterable_length, 'length')
 
 
 # Additional convenience YAQL functions.
 
-def length(a):
-    return len(a())
+@ctx.EvalArg('a', arg_type=(six.string_types, collections.Iterable))
+def _string_and_iterable_length(a):
+    return len(a)
+
+
+@ctx.EvalArg('a', arg_type=types.GeneratorType)
+def _generator_length(a):
+    return sum(1 for i in a)