docs: add f-string example

docs/examples/basics.md

@@ -92,4 +92,22 @@ This example demonstrates the use of logical operators `AND` and `OR` in Zynk.
             println("Both c and d are true.");
         }
     }
+    main();
+
+## Example 7: Formatted Strings (f-string)
+
+This example demonstrates how to use f-strings to create formatted strings in Zynk.
+
+    def main() {
+        var name: string = "Alice";
+        var age: int = 30;
+        var height: float = 5.9;
+
+        // Using f-string to create a formatted string
+        var message: string = f"Name: {name}, Age: {age}, Height: {height}";
+
+        // Printing the formatted string
+        println(message);
+        println(f"{name} will be {age + 1} years old next year.");
+    }
     main();

tests/test_evaluator.cpp

@@ -717,4 +717,16 @@ TEST(EvaluatorTest, EvaluateFStringWithStringInterpolation) {
     Evaluator evaluator;
     evaluator.evaluate(program.get());
     ASSERT_EQ(testing::internal::GetCapturedStdout(), "My name is Alice and I am 30 years old.\n");
+}
+
+TEST(EvaluatorTest, EvaluateFStringWithUndefinedVariable) {
+    const std::string code = "println(f\"{abc}\");";
+    Lexer lexer(code);
+    const std::vector<Token> tokens = lexer.tokenize();
+
+    Parser parser(tokens);
+    const auto program = parser.parse();
+
+    Evaluator evaluator;
+    ASSERT_THROW(evaluator.evaluate(program.get()), ZynkError);
 }