diff --git a/examples/factorial.mq b/examples/factorial.mq
new file mode 100644
index 0000000..8f8c6c0
--- /dev/null
+++ b/examples/factorial.mq
@@ -0,0 +1,12 @@
+var for = [ start stop iteration |
+	if (start > stop)
+	[| nil ]
+	[| iteration start; for (start + 1) stop iteration ]
+];
+
+var factorial = [n | if (n <= 1)
+	[| 1]
+	[| n * (factorial (n-1)) ]
+];
+
+for 1 10 [i | say (factorial i)];
diff --git a/src/environment.cc b/src/environment.cc
index 6077cff..a91454e 100644
--- a/src/environment.cc
+++ b/src/environment.cc
@@ -1,5 +1,7 @@
 #include <musique.hh>
 
+#include <iostream>
+
 std::vector<Env> *Env::pool = nullptr;
 
 Env& Env::force_define(std::string name, Value new_value)
@@ -25,6 +27,7 @@ Value* Env::find(std::string const& name)
 
 usize Env::operator++() const
 {
+	std::cerr << "ENTER SCOPE" << std::endl;
 	auto const parent_id = this - pool->data();
 	auto const free = std::find_if(pool->begin(), pool->end(), [](Env const& env) { return env.parent_enviroment_id == Env::Unused; });
 	Env* next = free == pool->end()
@@ -37,6 +40,7 @@ usize Env::operator++() const
 
 usize Env::operator--()
 {
+	std::cerr << "LEAVE SCOPE" << std::endl;
 	if (this == pool->data())
 		return 0;
 	variables.clear();
diff --git a/src/interpreter.cc b/src/interpreter.cc
index c822f55..0e1e10d 100644
--- a/src/interpreter.cc
+++ b/src/interpreter.cc
@@ -2,14 +2,19 @@
 
 #include <iostream>
 
-static auto numeric_binary_operator(auto binop)
+static auto binary_operator(auto binop)
 {
-	return [binop = std::move(binop)](std::vector<Value> args) -> Result<Value> {
+	return [binop = std::move(binop)](Interpreter&, std::vector<Value> args) -> Result<Value> {
 		auto result = std::move(args.front());
 		for (auto &v : std::span(args).subspan(1)) {
 			assert(result.type == Value::Type::Number, "LHS should be a number");
 			assert(v.type == Value::Type::Number,      "RHS should be a number");
-			result.n = binop(std::move(result.n), v.n);
+			if constexpr (std::is_same_v<Number, std::invoke_result_t<decltype(binop), Number, Number>>) {
+				result.n = binop(std::move(result.n), std::move(v).n);
+			} else {
+				result.type = Value::Type::Bool;
+				result.b = binop(std::move(result.n), std::move(v).n);
+			}
 		}
 		return result;
 	};
@@ -34,25 +39,43 @@ Interpreter::Interpreter(std::ostream& out)
 	auto &global = env_pool.emplace_back();
 	global.parent_enviroment_id = 0;
 
-	global.force_define("typeof", [](std::vector<Value> args) -> Value {
+	global.force_define("typeof", [](Interpreter&, std::vector<Value> args) -> Value {
 		assert(args.size() == 1, "typeof expects only one argument");
 		return Value::symbol(std::string(type_name(args.front().type)));
 	});
 
-	global.force_define("say", [&out](std::vector<Value> args) -> Value {
-		for (auto it = args.begin(); it != args.end(); ++it) {
-			out << *it;
-			if (std::next(it) != args.end())
-				out << ' ';
+	global.force_define("if", [](Interpreter &i, std::vector<Value> args) -> Result<Value> {
+		assert(args.size() == 2 || args.size() == 3, "argument count does not add up - expected: if <condition> <then> [<else>]");
+		if (args.front().truthy()) {
+			return args[1](i, {});
+		} else if (args.size() == 3) {
+			return args[2](i, {});
+		} else {
+			return Value{};
 		}
-		out << '\n';
+	});
+
+	global.force_define("say", [](Interpreter &i, std::vector<Value> args) -> Value {
+		for (auto it = args.begin(); it != args.end(); ++it) {
+			i.out << *it;
+			if (std::next(it) != args.end())
+				i.out << ' ';
+		}
+		i.out << '\n';
 		return {};
 	});
 
-	operators["+"] = numeric_binary_operator(std::plus<>{});
-	operators["-"] = numeric_binary_operator(std::minus<>{});
-	operators["*"] = numeric_binary_operator(std::multiplies<>{});
-	operators["/"] = numeric_binary_operator(std::divides<>{});
+	operators["+"] = binary_operator(std::plus<>{});
+	operators["-"] = binary_operator(std::minus<>{});
+	operators["*"] = binary_operator(std::multiplies<>{});
+	operators["/"] = binary_operator(std::divides<>{});
+
+	operators["<"]  = binary_operator(std::less<>{});
+	operators[">"]  = binary_operator(std::greater<>{});
+	operators["<="] = binary_operator(std::less_equal<>{});
+	operators[">="] = binary_operator(std::greater_equal<>{});
+	operators["=="] = binary_operator(std::equal_to<>{});
+	operators["!="] = binary_operator(std::not_equal_to<>{});
 }
 
 Env& Interpreter::env()
@@ -73,7 +96,7 @@ Result<Value> Interpreter::eval(Ast &&ast)
 		case Token::Type::Symbol:
 			if (ast.token.source != "nil") {
 				auto const value = env().find(std::string(ast.token.source));
-				assert(value, "Missing variable error is not implemented yet");
+				assert(value, "Missing variable error is not implemented yet: variable: "s + std::string(ast.token.source));
 				return *value;
 			}
 			return Value{};
@@ -97,7 +120,7 @@ Result<Value> Interpreter::eval(Ast &&ast)
 				values.push_back(Try(eval(std::move(a))));
 			}
 
-			return op->second(std::move(values));
+			return op->second(*this, std::move(values));
 		}
 		break;
 
@@ -118,7 +141,7 @@ Result<Value> Interpreter::eval(Ast &&ast)
 			for (auto& a : std::span(ast.arguments).subspan(1)) {
 				values.push_back(Try(eval(std::move(a))));
 			}
-			return std::move(func)(std::move(values));
+			return std::move(func)(*this, std::move(values));
 		}
 
 	case Ast::Type::Variable_Declaration:
@@ -130,6 +153,19 @@ Result<Value> Interpreter::eval(Ast &&ast)
 			return Value{};
 		}
 
+	case Ast::Type::Lambda:
+		{
+			Lambda lambda;
+			auto parameters = std::span(ast.arguments.begin(), std::prev(ast.arguments.end()));
+			lambda.parameters.reserve(parameters.size());
+			for (auto &param : parameters) {
+				assert(param.type == Ast::Type::Literal && param.token.type == Token::Type::Symbol, "Not a name in parameter section of Ast::lambda");
+				lambda.parameters.push_back(std::string(std::move(param).token.source));
+			}
+			lambda.body = std::move(ast.arguments.back());
+			return Value::lambda(std::move(lambda));
+		}
+
 	default:
 		unimplemented();
 	}
diff --git a/src/musique.hh b/src/musique.hh
index 670d7a5..d96270f 100644
--- a/src/musique.hh
+++ b/src/musique.hh
@@ -19,6 +19,9 @@
 #undef assert
 #endif
 
+using namespace std::string_literals;
+using namespace std::string_view_literals;
+
 using u8  = std::uint8_t;
 using u16 = std::uint16_t;
 using u32 = std::uint32_t;
@@ -422,7 +425,7 @@ std::ostream& operator<<(std::ostream& os, Number const& num);
 struct Value;
 struct Interpreter;
 
-using Function = std::function<Result<Value>(std::vector<Value>)>;
+using Function = std::function<Result<Value>(Interpreter &i, std::vector<Value>)>;
 
 struct Lambda
 {
@@ -440,6 +443,7 @@ constexpr auto is_one_of = (std::is_same_v<T, XS> || ...);
 struct Value
 {
 	static Result<Value> from(Token t);
+	static Value boolean(bool b);
 	static Value number(Number n);
 	static Value symbol(std::string s);
 	static Value lambda(Function f);
@@ -447,6 +451,7 @@ struct Value
 	enum class Type
 	{
 		Nil,
+		Bool,
 		Number,
 		Symbol,
 		Lambda,
@@ -460,21 +465,22 @@ struct Value
 
 	template<typename Callable>
 	requires (!std::is_same_v<std::remove_cvref_t<Callable>, Value>)
-		&& std::invocable<Callable, std::vector<Value>>
-		&& is_one_of<std::invoke_result_t<Callable, std::vector<Value>>, Value, Result<Value>>
+		&& std::invocable<Callable, Interpreter&, std::vector<Value>>
+		&& is_one_of<std::invoke_result_t<Callable, Interpreter&, std::vector<Value>>, Value, Result<Value>>
 	inline Value(Callable &&callable)
 		: type{Type::Lambda}
 	{
-		if constexpr (std::is_same_v<Result<Value>, std::invoke_result_t<Callable, std::vector<Value>>>) {
+		if constexpr (std::is_same_v<Result<Value>, std::invoke_result_t<Callable, Interpreter&, std::vector<Value>>>) {
 			f = std::move(callable);
 		} else {
-			f = [fun = std::move(callable)](std::vector<Value> args) -> Result<Value> {
-				return fun(std::move(args));
+			f = [fun = std::move(callable)](Interpreter &i, std::vector<Value> args) -> Result<Value> {
+				return fun(i, std::move(args));
 			};
 		}
 	}
 
 	Type type = Type::Nil;
+	bool b{};
 	Number n{};
 	Function f{};
 
@@ -484,7 +490,9 @@ struct Value
 	//   std::string_view - not-owning string type
 	std::string s{};
 
-	Result<Value> operator()(std::vector<Value> args);
+	bool truthy() const;
+	bool falsy() const;
+	Result<Value> operator()(Interpreter &i, std::vector<Value> args);
 
 	bool operator==(Value const& other) const;
 };
diff --git a/src/value.cc b/src/value.cc
index bd492cb..75eea17 100644
--- a/src/value.cc
+++ b/src/value.cc
@@ -7,6 +7,10 @@ Result<Value> Value::from(Token t)
 		return Number::from(std::move(t)).map(Value::number);
 
 	case Token::Type::Symbol:
+		if (t.source == "true")
+			return Value::boolean(true);
+		if (t.source == "false")
+			return Value::boolean(false);
 		if (t.source == "nil")
 			return Value{};
 		return Value::symbol(std::string(t.source));
@@ -16,6 +20,14 @@ Result<Value> Value::from(Token t)
 	}
 }
 
+Value Value::boolean(bool b)
+{
+	Value v;
+	v.type = Value::Type::Bool;
+	v.b = b;
+	return v;
+}
+
 Value Value::number(Number n)
 {
 	Value v;
@@ -40,15 +52,32 @@ Value Value::lambda(Function f)
 	return v;
 }
 
-Result<Value> Value::operator()(std::vector<Value> args)
+Result<Value> Value::operator()(Interpreter &i, std::vector<Value> args)
 {
 	if (type == Type::Lambda) {
-		return f(std::move(args));
+		return f(i, std::move(args));
 	}
 	// TODO Fill location
 	return errors::not_callable(std::nullopt, type);
 }
 
+bool Value::truthy() const
+{
+	switch (type) {
+	case Type::Bool:   return b;
+	case Type::Nil:    return false;
+	case Type::Number: return n != Number(0);
+	case Type::Lambda:
+	case Type::Symbol: return true;
+	}
+	unreachable();
+}
+
+bool Value::falsy() const
+{
+	return not truthy();
+}
+
 bool Value::operator==(Value const& other) const
 {
 	if (type != other.type)
@@ -59,6 +88,7 @@ bool Value::operator==(Value const& other) const
 	case Type::Number: return n == other.n;
 	case Type::Symbol: return s == other.s;
 	case Type::Lambda: return false; // TODO Reconsider if functions are comparable
+	case Type::Bool:   return b == other.b;
 	}
 
 	unreachable();
@@ -76,6 +106,9 @@ std::ostream& operator<<(std::ostream& os, Value const& v)
 	case Value::Type::Symbol:
 		return os << v.s;
 
+	case Value::Type::Bool:
+		return os << (v.b ? "true" : "false");
+
 	case Value::Type::Lambda:
 		return os << "<lambda>";
 	}
@@ -85,6 +118,7 @@ std::ostream& operator<<(std::ostream& os, Value const& v)
 std::string_view type_name(Value::Type t)
 {
 	switch (t) {
+	case Value::Type::Bool:   return "bool";
 	case Value::Type::Lambda: return "lambda";
 	case Value::Type::Nil:    return "nil";
 	case Value::Type::Number: return "number";
@@ -92,3 +126,19 @@ std::string_view type_name(Value::Type t)
 	}
 	unreachable();
 }
+
+Result<Value> Lambda::operator()(Interpreter &i, std::vector<Value> arguments)
+{
+	i.current_env = ++i.env();
+	assert(parameters.size() == arguments.size(), "wrong number of arguments");
+
+	auto &env = i.env();
+	for (usize j = 0; j < parameters.size(); ++j) {
+		env.force_define(parameters[j], std::move(arguments[j]));
+	}
+
+	auto result = i.eval((Ast)body);
+
+	i.current_env = --i.env();
+	return result;
+}