Adds aggregate function to MuranoPL (YAQL)

Aggregate is similar to python's reduce() built-in

Change-Id: If1e3ed20172c40344b771627ce20739348b80163

murano/engine/system/yaql_functions.py

@@ -312,6 +312,16 @@ def _take(collection, count):
     return itertools.islice(collection, count)
 
 
+@yaql.context.EvalArg('collection', collections.Iterable)
+def _aggregate(collection, selector):
+    return reduce(selector, collection)
+
+
+@yaql.context.EvalArg('collection', collections.Iterable)
+def _aggregate_with_seed(collection, selector, seed):
+    return reduce(selector, collection, seed())
+
+
 def register(context):
     context.register_function(
         lambda json, mappings: _transform_json(json(), mappings()), 'bind')
@@ -347,3 +357,5 @@ def register(context):
     context.register_function(_merge_with, 'mergeWith')
     context.register_function(_skip, 'skip')
     context.register_function(_take, 'take')
+    context.register_function(_aggregate, 'aggregate')
+    context.register_function(_aggregate_with_seed, 'aggregate')

murano/tests/unit/dsl/meta/TestEngineFunctions.yaml

@@ -253,3 +253,20 @@ Methods:
           Contract: $.int()
     Body:
       - Return: $list.skip($start).take($count)
+
+
+  testAggregate:
+    Arguments:
+      - list:
+          Contract: [$.int()]
+    Body:
+      - Return: $list.aggregate($1 + $2)
+
+  testAggregateWithInitializer:
+    Arguments:
+      - list:
+          Contract: [$.int()]
+      - initializer:
+          Contract: $.int()
+    Body:
+      - Return: $list.aggregate($1 + $2, $initializer)

murano/tests/unit/dsl/test_engine_yaql_functions.py

@@ -224,3 +224,15 @@ class TestEngineYaqlFunctions(test_case.DslTestCase):
                 [1, 2, 3, 4, 5, 6, 7, 8],
                 2, 3)
         )
+
+    def test_aggregate(self):
+        self.assertEqual(
+            10,
+            self._runner.testAggregate([1, 2, 3, 4])
+        )
+
+    def test_aggregate_with_initializer(self):
+        self.assertEqual(
+            21,
+            self._runner.testAggregateWithInitializer([1, 2, 3, 4], 11)
+        )