diff --git a/README.md b/README.md
index cf46890..03fe7ec 100644
--- a/README.md
+++ b/README.md
@@ -201,7 +201,7 @@ Environment env = Environment();
  * - callback function. Implemented with std::function, you can for example use lambdas.
  */
 env.add_callback("double", 1, [&env](Parsed::Arguments args, json data) {
-	const int number = env.renderer.eval_expression<int>(args[0], data); // Adapt the type and index of the argument
+	int number = env.get_argument<int>(args, 0, data); // Adapt the type and index of the argument
 	return 2 * number;
 });
 
diff --git a/src/inja.hpp b/src/inja.hpp
index 025f6a7..c5a4713 100644
--- a/src/inja.hpp
+++ b/src/inja.hpp
@@ -193,7 +193,6 @@ inline MatchType<T> match(const std::string& input, std::map<T, Regex> regexes)
 			return match;
 		}
 	}
-	// throw std::runtime_error("Could not match input: " + input);
 	return match;
 }
 
@@ -447,7 +446,7 @@ public:
 			case Parsed::Function::Default: {
 				try {
 					return eval_expression(element.args[0], data);
-				} catch (std::exception e) {
+				} catch (std::exception& exception) {
 					return eval_expression(element.args[1], data);
 				}
 			}
@@ -842,17 +841,22 @@ public:
 		return parser.load_file(input_path + filename);
 	}
 
-	void add_callback(std::string name, int number_arguments, std::function<json(Parsed::Arguments, json)> callback) {
-		parser.regex_map_callbacks[name] = Parser::function_regex(name, number_arguments);
-		renderer.map_callbacks[name] = callback;
-	}
-
 	json load_json(const std::string& filename) {
 		std::ifstream file(input_path + filename);
 		json j;
 		file >> j;
 		return j;
 	}
+
+	void add_callback(std::string name, int number_arguments, std::function<json(Parsed::Arguments, json)> callback) {
+		parser.regex_map_callbacks[name] = Parser::function_regex(name, number_arguments);
+		renderer.map_callbacks[name] = callback;
+	}
+
+	template<typename T>
+	T get_argument(Parsed::Arguments args, int index, json data) {
+		return renderer.eval_expression<T>(args[index], data);
+	}
 };
 
 
diff --git a/test/src/unit-renderer.cpp b/test/src/unit-renderer.cpp
index d8d483f..4d42896 100644
--- a/test/src/unit-renderer.cpp
+++ b/test/src/unit-renderer.cpp
@@ -150,12 +150,12 @@ TEST_CASE("callbacks") {
 	data["age"] = 28;
 
 	env.add_callback("double", 1, [&env](inja::Parsed::Arguments args, json data) {
-		const int number = env.renderer.eval_expression<int>(args[0], data);
+		int number = env.get_argument<double>(args, 0, data);
 		return 2 * number;
 	});
 
 	env.add_callback("half", 1, [&env](inja::Parsed::Arguments args, json data) {
-		const int number = env.renderer.eval_expression<int>(args[0], data);
+		int number = env.get_argument<double>(args, 0, data);
 		return number / 2;
 	});
 
@@ -165,8 +165,8 @@ TEST_CASE("callbacks") {
 	});
 
 	env.add_callback("multiply", 2, [&env](inja::Parsed::Arguments args, json data) {
-		const double number1 = env.renderer.eval_expression<double>(args[0], data);
-		const double number2 = env.renderer.eval_expression<double>(args[1], data);
+		double number1 = env.get_argument<double>(args, 0, data);
+		double number2 = env.get_argument<double>(args, 1, data);
 		return number1 * number2;
 	});