Add ability to refer to subfields of objects via dot notation in string formatting (#1011)

* added ability to refer to subfields of objects via dot notation in string formatting, e.g. "%(a.b)s" % {a:{b:"hello world"}}

Author: fedya-at-db

stdlib/std.jsonnet

@@ -770,11 +770,24 @@ limitations under the License.
               error 'Cannot use * precision with object.'
             else
               code.prec;
-          local val =
-            if std.objectHasAll(obj, f) then
-              obj[f]
+          local get_nested_val(obj, f, start=0, end=0) =
+            if !std.isObject(obj) then
+              error 'Subfield ' + f[:start] + ' didn\'t yield an object.'
+            else if end >= std.length(f) then
+              if std.objectHasAll(obj, f[start:]) then
+                obj[f[start:]]
+              else
+                error 'No such field: ' + f
+            else if f[end] == "." then
+              local key = f[start:end];
+              if std.objectHasAll(obj, key) then
+                get_nested_val(obj[key], f, end+1, end+1) tailstrict
+              else
+                error 'No such partial field: ' + f[:end]
             else
-              error 'No such field: ' + f;
+              get_nested_val(obj, f, start, end+1) tailstrict
+          ;
+          local val = get_nested_val(obj, f);
           local s =
             if code.ctype == '%' then
               '%'

test_suite/error.format.invalid_object_subfield.jsonnet

@@ -0,0 +1,2 @@
+"%(a.b.c)s" % {a: {c: "hello"}}
+

test_suite/error.format.invalid_object_subfield.jsonnet.golden

@@ -0,0 +1,22 @@
+RUNTIME ERROR: No such partial field: a.b
+	std.jsonnet:<stdlib_position_redacted>	function <get_nested_val>
+	std.jsonnet:<stdlib_position_redacted>	function <get_nested_val>
+	std.jsonnet:<stdlib_position_redacted>	thunk <val>
+	std.jsonnet:<stdlib_position_redacted>	thunk <val>
+	std.jsonnet:<stdlib_position_redacted>	thunk <a>
+	std.jsonnet:<stdlib_position_redacted>	
+	std.jsonnet:<stdlib_position_redacted>	thunk <a>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	std.jsonnet:<stdlib_position_redacted>	function <format_code>
+	...
+	std.jsonnet:<stdlib_position_redacted>	function <aux>
+	std.jsonnet:<stdlib_position_redacted>	function <padding>
+	std.jsonnet:<stdlib_position_redacted>	function <pad_left>
+	std.jsonnet:<stdlib_position_redacted>	thunk <s_padded>
+	std.jsonnet:<stdlib_position_redacted>	thunk <v>
+	std.jsonnet:<stdlib_position_redacted>	function <format_codes_obj>
+	std.jsonnet:<stdlib_position_redacted>	function <format_codes_obj>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	error.format.invalid_object_subfield.jsonnet:1:1-32	

test_suite/format.jsonnet

@@ -312,5 +312,6 @@ local text = |||
 
 std.assertEqual(std.length(text), 1244) &&
 
+std.assertEqual(std.format('%(a.b.c.d)s %(b.c)s', {a:{b:{c:{d:'hello'}}}, b:{c:'goodbye'}}), 'hello goodbye') &&
 
 true