CE-160 oap-template: A default value (??) can follow the function call

oap-formats/oap-template/README.md

@@ -10,6 +10,7 @@ A compile-time template engine for the OAP framework. Each unique template strin
   - [Delimiters](#delimiters)
     - [Custom expression delimiters](#custom-expression-delimiters)
   - [Whitespace trimming](#whitespace-trimming)
+  - [Literal value expressions](#literal-value-expressions)
   - [Field access](#field-access)
   - [Null safety](#null-safety)
   - [Default values (`??`)](#default-values-)
@@ -163,6 +164,24 @@ line1
 
 Renders as `line1content` when `flag` is `true` (the `\n` after `line1` is stripped).
 
+### Literal value expressions
+
+A template expression can contain a bare literal — no field access required:
+
+| Template | Output |
+|---|---|
+| `{{ 1 }}` | `1` |
+| `{{ 1.2 }}` | `1.2` |
+| `{{ -1.2 }}` | `-1.2` |
+| `{{ 'text' }}` | `text` |
+
+Useful for static values in otherwise-dynamic templates, or as branch values in if-then-else:
+
+```
+price={{ 0.0 }}, label={{ 'unknown' }}
+{{ if premium then user.discount else 0 end }}
+```
+
 ### Field access
 
 ```
@@ -616,6 +635,14 @@ All range forms accept an optional `{{% else %}}` branch. It renders when the co
 {{ obj ; toJson() }}
 ```
 
+A default value (`??`) can follow the function call:
+
+```
+{{ byteField ; toString() ?? 'n/a' }}
+```
+
+This renders `'n/a'` when `byteField` is null.
+
 ### Cast types
 
 `${ <java.lang.Double>field ?? 0.0 }` — forces the expression result to be interpreted as the given type. Useful when the field is typed as `Object` (e.g., in `Map<String, Object>`) but the actual runtime type is known.
@@ -651,6 +678,7 @@ Registered automatically from `META-INF/oap-template-macros.list` on the classpa
 | `toLowerCase()` | `(String src)` | Converts to lower case; null-safe |
 | `format(pattern)` | `(DateTime dt, String pattern)` | Formats a Joda `DateTime`; predefined patterns: `SIMPLE`, `MILLIS`, `SIMPLE_CLEAN`, `DATE` |
 | `toJson()` | `(Object obj)` | Serialises the value to JSON |
+| `toString()` | `(Byte\|Short\|Integer\|Long\|Float\|Double src)` | Converts a numeric wrapper to its string representation; returns `null` for `null` input (so `?? default` applies) |
 | `default(fallback)` | `(Object in, Object fallback)` | Returns fallback if in is null or empty |
 
 ---

oap-formats/oap-template/src/main/antlr4/TemplateGrammarExpression.g4

@@ -32,7 +32,7 @@ import oap.util.Lists;
 }
 
 expression returns [Expression ret]
-    : (BLOCK_COMMENT)? (CAST_TYPE)? (ifCode | pipeCode | withCode | exprsCode) defaultValue? function? (IF ifCondition)? {
+    : (BLOCK_COMMENT)? (CAST_TYPE)? (ifCode | pipeCode | withCode | exprsCode) function? defaultValue? (IF ifCondition)? {
         $ret = new Expression(
           $BLOCK_COMMENT.text,
           $CAST_TYPE.text != null ? $CAST_TYPE.text.substring( 1, $CAST_TYPE.text.length() - 1 ) : null,
@@ -106,6 +106,24 @@ exprsCode returns [ArrayList<Exprs> ret = new ArrayList<>()]
     | exprs {
         $ret.add( $exprs.ret );
       }
+    | SSTRING {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( sdStringToString( $SSTRING.text ) ) ); $ret.add( e );
+      }
+    | DSTRING {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( sdStringToString( $DSTRING.text ) ) ); $ret.add( e );
+      }
+    | DECDIGITS {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( new NumericLiteral( $DECDIGITS.text ) ) ); $ret.add( e );
+      }
+    | MINUS DECDIGITS {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( new NumericLiteral( "-" + $DECDIGITS.text ) ) ); $ret.add( e );
+      }
+    | FLOAT {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( new NumericLiteral( $FLOAT.text ) ) ); $ret.add( e );
+      }
+    | MINUS FLOAT {
+        Exprs e = new Exprs(); e.concatenation = new Concatenation( List.of( new NumericLiteral( "-" + $FLOAT.text ) ) ); $ret.add( e );
+      }
     ;
 
 ifCondition returns [ConditionExpr ret]