support string concatenation with +

README.md

@@ -108,7 +108,7 @@ From <https://github.com/google/cel-spec/blob/master/doc/langdef.md#functions>.
 | _+_ | (int, int) -> int | arithmetic | ✅ |
 |  | (uint, uint) -> uint | arithmetic | ✅ |
 |  | (double, double) -> double | arithmetic |  |
-|  | (string, string) -> string | String concatenation. |  |
+|  | (string, string) -> string | String concatenation. | ✅ |
 |  | (bytes, bytes) -> bytes | bytes concatenation |  |
 |  | (list(A), list(A)) -> list(A) | List concatenation. |  |
 |  | (google.protobuf.Timestamp, google.protobuf.Duration) -> google.protobuf.Timestamp | arithmetic | ❌ |

lib/src/common/types/string.dart

@@ -1,5 +1,6 @@
 import 'package:cel/src/common/types/bool.dart';
 import 'package:cel/src/common/types/traits/matcher.dart';
+import 'package:cel/src/common/types/traits/math.dart';
 import 'package:cel/src/common/types/traits/traits.dart';
 
 import '../overloads/overloads.dart';
@@ -16,7 +17,7 @@ final stringType = Type_('string', {
   Traits.SizerType
 });
 
-class StringValue extends Value implements Receiver, Matcher {
+class StringValue extends Value implements Receiver, Matcher, Adder {
   StringValue(this.value);
 
   @override
@@ -34,6 +35,11 @@ class StringValue extends Value implements Receiver, Matcher {
   BooleanValue match(Value pattern) {
     return BooleanValue(RegExp(pattern.value).hasMatch(value));
   }
+
+  @override
+  add(Value other) {
+    return StringValue(value + other.value);
+  }
 }
 
 // class String_ extends String implements Receiver {

test/cel_test.dart

@@ -219,6 +219,12 @@ void main() {
           final p = environment.makeProgram(ast);
           expect(p.evaluate({}), 2.6);
         });
+        test('string', () {
+          final environment = Environment.standard();
+          final ast = environment.compile('"ab" + "cd"');
+          final p = environment.makeProgram(ast);
+          expect(p.evaluate({}), 'abcd');
+        });
       });
       test('-', () {
         final environment = Environment.standard();