Render the empty tag expression as an empty string (#222)

While the empty tag expression, results in a True expression. This
True expression should be rendered as the empty string to make it
possible to parse the string representation of a parsed expression.

Author: mpkorstanje

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 ### Fixed
+- Render the empty tag expression as an empty string ([#222](https://github.com/cucumber/tag-expressions/pull/222))
 - Improve error message for missing operands ([#221](https://github.com/cucumber/tag-expressions/pull/221))
 
 ## [8.0.0] - 2025-10-14

go/parser.go

@@ -284,5 +284,5 @@ func (t *trueExpr) Evaluate(variables []string) bool {
 }
 
 func (t *trueExpr) ToString() string {
-	return "true"
+	return ""
 }

java/src/main/java/io/cucumber/tagexpressions/TagExpressionParser.java

@@ -270,7 +270,7 @@ public boolean evaluate(List<String> variables) {
 
         @Override
         public String toString() {
-            return "true";
+            return "";
         }
     }
 }

javascript/src/index.ts

@@ -238,6 +238,6 @@ class True implements Node {
   }
 
   public toString() {
-    return 'true'
+    return ''
   }
 }

perl/lib/Cucumber/TagExpressions/Node.pm

@@ -249,7 +249,7 @@ An instance of one of the other node class types.
     sub stringify {
         my ( $self ) = @_;
 
-        return 'true' if not defined $self->sub_expression;
+        return '' if not defined $self->sub_expression;
         return $self->sub_expression->stringify;
     }
 }

php/src/Expression/TrueExpression.php

@@ -16,6 +16,6 @@ public function evaluate(array $variables): bool
 
     public function __toString(): string
     {
-        return 'true';
+        return '';
     }
 }

python/src/cucumber_tag_expressions/model.py

@@ -222,7 +222,7 @@ def evaluate(self, values):
         return True
 
     def __str__(self):
-        return "true"
+        return ""
 
     def __repr__(self):
         return "True_()"

ruby/lib/cucumber/tag_expressions/expressions.rb

@@ -79,7 +79,7 @@ def evaluate(_variables)
       end
 
       def to_s
-        'true'
+        ''
       end
     end
   end

testdata/evaluations.yml

@@ -1,3 +1,11 @@
+- expression: ''
+  tests:
+    - variables: []
+      result: true
+    - variables: ['x']
+      result: true
+    - variables: ['y']
+      result: true
 - expression: 'not x'
   tests:
     - variables: ['x']

testdata/parsing.yml

@@ -1,5 +1,5 @@
 - expression: ''
-  formatted: 'true'
+  formatted: ''
 - expression: 'a and b'
   formatted: '( a and b )'
 - expression: 'a or b'