diff --git a/Makefile b/Makefile
index cca822d..d448178 100644
--- a/Makefile
+++ b/Makefile
@@ -1,14 +1,17 @@
 MAKEFLAGS="-j $(grep -c ^processor /proc/cpuinfo)"
-CXXFLAGS:=$(CXXFLAGS) -std=c++20 -Wall -Wextra -Werror=switch -Werror=unused-result
+CXXFLAGS:=$(CXXFLAGS) -std=c++20 -Wall -Wextra -Werror=switch -Werror=return-type -Werror=unused-result
 CPPFLAGS:=$(CPPFLAGS) -Ilib/expected/ -Ilib/ut/ -Isrc/
 
-Obj=bin/errors.o        \
-		bin/lexer.o         \
-		bin/location.o      \
-		bin/parser.o        \
-		bin/unicode.o       \
-		bin/number.o        \
-		bin/unicode_tables.o
+Obj=                     \
+		bin/errors.o         \
+		bin/interpreter.o    \
+		bin/lexer.o          \
+		bin/location.o       \
+		bin/number.o         \
+		bin/parser.o         \
+		bin/unicode.o        \
+		bin/unicode_tables.o \
+		bin/value.o
 
 all: bin/musique bin/unit-tests
 
diff --git a/examples/arithmetic.mq b/examples/arithmetic.mq
new file mode 100644
index 0000000..ad36d64
--- /dev/null
+++ b/examples/arithmetic.mq
@@ -0,0 +1,3 @@
+say (1 + 3);
+say (3 * 10);
+say 42
diff --git a/src/errors.cc b/src/errors.cc
index 3b478ad..0f90965 100644
--- a/src/errors.cc
+++ b/src/errors.cc
@@ -69,8 +69,9 @@ std::ostream& operator<<(std::ostream& os, Error const& err)
 	case errors::Unrecognized_Character:
 		return err.message.empty() ? os << "unrecognized character\n" : os << err.message;
 
+	case errors::Function_Not_Defined:
 	case errors::Unexpected_Token_Type:
-		return os << err.message;
+		return os << err.message << '\n';
 
 	case errors::Unexpected_Empty_Source:
 		return os << "unexpected end of input\n";
@@ -144,6 +145,14 @@ Error errors::failed_numeric_parsing(Location location, std::errc errc, std::str
 	return err;
 }
 
+Error errors::function_not_defined(Value const& value)
+{
+	Error err;
+	err.type = Function_Not_Defined;
+	err.message = "Function '" + value.s + "' has not been defined yet";
+	return err;
+}
+
 void errors::all_tokens_were_not_parsed(std::span<Token> tokens)
 {
 	error_heading(std::cerr, std::nullopt, Error_Level::Bug);
diff --git a/src/interpreter.cc b/src/interpreter.cc
new file mode 100644
index 0000000..e61317a
--- /dev/null
+++ b/src/interpreter.cc
@@ -0,0 +1,96 @@
+#include <musique.hh>
+
+#include <iostream>
+
+Interpreter::Interpreter()
+	: Interpreter(std::cout)
+{
+}
+
+Interpreter::Interpreter(std::ostream& out)
+	: out(out)
+{
+	functions["typeof"] = [](std::vector<Value> args) -> Value {
+		assert(args.size() == 1, "typeof expects only one argument");
+		switch (args.front().type) {
+		case Value::Type::Nil:    return Value::symbol("nil");
+		case Value::Type::Number: return Value::symbol("number");
+		case Value::Type::Symbol: return Value::symbol("symbol");
+		}
+		unreachable();
+	};
+
+	functions["say"] = [&](std::vector<Value> args) -> Value {
+		for (auto it = args.begin(); it != args.end(); ++it) {
+			out << *it;
+			if (std::next(it) != args.end())
+				out << ' ';
+		}
+		out << '\n';
+		return {};
+	};
+}
+
+Result<Value> Interpreter::eval(Ast &&ast)
+{
+	switch (ast.type) {
+	case Ast::Type::Literal:
+		return Value::from(ast.token);
+
+	case Ast::Type::Binary:
+		{
+			std::vector<Value> values;
+			values.reserve(ast.arguments.size());
+			for (auto& a : ast.arguments) {
+				values.push_back(Try(eval(std::move(a))));
+			}
+
+			Value result = values.front();
+			if (ast.token.source == "+") {
+				for (auto &v : std::span(values).subspan(1)) {
+					assert(result.type == Value::Type::Number, "LHS of + should be a number");
+					assert(v.type == Value::Type::Number,      "RHS of + should be a number");
+					result.n += v.n;
+				}
+				return result;
+			} else if (ast.token.source == "*") {
+				for (auto &v : std::span(values).subspan(1)) {
+					assert(result.type == Value::Type::Number, "LHS of * should be a number");
+					assert(v.type == Value::Type::Number,      "RHS of * should be a number");
+					result.n *= v.n;
+				}
+				return result;
+			}
+
+			unimplemented();
+		}
+		break;
+
+	case Ast::Type::Sequence:
+		{
+			Value v;
+			for (auto &a : ast.arguments)
+				v = Try(eval(std::move(a)));
+			return v;
+		}
+
+	case Ast::Type::Call:
+		{
+			Value func = Try(eval(std::move(ast.arguments.front())));
+			assert(func.type == Value::Type::Symbol, "Currently only symbols can be called");
+			if (auto it = functions.find(func.s); it != functions.end()) {
+				std::vector<Value> values;
+				values.reserve(ast.arguments.size());
+				for (auto& a : std::span(ast.arguments).subspan(1)) {
+					values.push_back(Try(eval(std::move(a))));
+				}
+				return it->second(std::move(values));
+			} else {
+				return errors::function_not_defined(func);
+			}
+		}
+
+	default:
+		unimplemented();
+	}
+}
diff --git a/src/main.cc b/src/main.cc
index 989a0b9..f14941d 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -7,6 +7,8 @@
 
 namespace fs = std::filesystem;
 
+static bool ast_only_mode = false;
+
 static std::string_view pop(std::span<char const*> &span)
 {
 	auto element = span.front();
@@ -23,15 +25,23 @@ void usage()
 		"  where options are:\n"
 		"    -c CODE\n"
 		"    --run CODE\n"
-		"      executes given code\n";
+		"      executes given code\n"
+		"\n"
+		"    --ast\n"
+		"      prints ast for given code\n";
 	std::exit(1);
 }
 
 static Result<void> run(std::string_view source, std::string_view filename)
 {
 	auto ast = Try(Parser::parse(source, filename));
-	std::cout << "successfully parsed: " << source << " \n";
-	dump(ast);
+
+	if (ast_only_mode) {
+		dump(ast);
+		return {};
+	}
+	Interpreter interpreter;
+	std::cout << Try(interpreter.eval(std::move(ast))) << std::endl;
 	return {};
 }
 
@@ -44,12 +54,13 @@ static Result<void> Main(std::span<char const*> args)
 		usage();
 	}
 
+	bool runned_something = false;
 	std::vector<std::string_view> files;
 
 	while (not args.empty()) {
 		std::string_view arg = pop(args);
 
-		if (!arg.starts_with('-')) {
+		if (arg == "-" || !arg.starts_with('-')) {
 			files.push_back(std::move(arg));
 			continue;
 		}
@@ -62,6 +73,12 @@ static Result<void> Main(std::span<char const*> args)
 
 			auto const source = pop(args);
 			Try(run(source, "arguments"));
+			runned_something = true;
+			continue;
+		}
+
+		if (arg == "--ast") {
+			ast_only_mode = true;
 			continue;
 		}
 
@@ -69,13 +86,18 @@ static Result<void> Main(std::span<char const*> args)
 		std::exit(1);
 	}
 
-	for (auto const& path : files) {
-		if (not fs::exists(path)) {
-			std::cerr << "musique: error: couldn't open file: " << path << std::endl;
-			std::exit(1);
-		}
+	if (!runned_something && files.empty()) {
+		usage();
+	}
 
-		{
+	for (auto const& path : files) {
+		if (path == "-") {
+			eternal_sources.emplace_back(std::istreambuf_iterator<char>(std::cin), std::istreambuf_iterator<char>());
+		} else {
+			if (not fs::exists(path)) {
+				std::cerr << "musique: error: couldn't open file: " << path << std::endl;
+				std::exit(1);
+			}
 			std::ifstream source_file{fs::path(path)};
 			eternal_sources.emplace_back(std::istreambuf_iterator<char>(source_file), std::istreambuf_iterator<char>());
 		}
diff --git a/src/musique.hh b/src/musique.hh
index bfe3b2a..d457323 100644
--- a/src/musique.hh
+++ b/src/musique.hh
@@ -44,6 +44,8 @@ namespace errors
 		Unexpected_Token_Type,
 		Unexpected_Empty_Source,
 		Failed_Numeric_Parsing,
+
+		Function_Not_Defined
 	};
 }
 
@@ -380,6 +382,49 @@ struct Number
 
 std::ostream& operator<<(std::ostream& os, Number const& num);
 
+// TODO Add location
+struct Value
+{
+	static Result<Value> from(Token t);
+	static Value number(Number n);
+	static Value symbol(std::string s);
+
+	enum class Type
+	{
+		Nil,
+		Number,
+		Symbol,
+	};
+
+	Type type = Type::Nil;
+	Number n{};
+
+	// TODO Most strings should not be allocated by Value, but reference to string allocated previously
+	// Wrapper for std::string is needed that will allocate only when needed, middle ground between:
+	//   std::string      - always owning string type
+	//   std::string_view - not-owning string type
+	std::string s{};
+
+	bool operator==(Value const& other) const;
+};
+
+std::ostream& operator<<(std::ostream& os, Value const& v);
+
+using Function = std::function<Value(std::vector<Value>)>;
+
+struct Interpreter
+{
+	std::ostream &out;
+	std::unordered_map<std::string, Function> functions;
+
+	Interpreter();
+	Interpreter(std::ostream& out);
+	Interpreter(Interpreter const&) = delete;
+	Interpreter(Interpreter &&) = default;
+
+	Result<Value> eval(Ast &&ast);
+};
+
 namespace errors
 {
 	Error unrecognized_character(u32 invalid_character);
@@ -391,6 +436,8 @@ namespace errors
 
 	Error failed_numeric_parsing(Location location, std::errc errc, std::string_view source);
 
+	Error function_not_defined(Value const& v);
+
 	[[noreturn]]
 	void all_tokens_were_not_parsed(std::span<Token>);
 }
diff --git a/src/parser.cc b/src/parser.cc
index 26b0c43..16a770c 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -2,7 +2,19 @@
 #include <iostream>
 
 static Ast wrap_if_several(std::vector<Ast> &&ast, Ast(*wrapper)(std::vector<Ast>));
-static Result<std::vector<Ast>> parse_one_or_more(Parser &p, Result<Ast> (Parser::*parser)(), std::optional<Token::Type> separator = std::nullopt);
+
+enum class At_Least : bool
+{
+	Zero,
+	One
+};
+
+static Result<std::vector<Ast>> parse_many(
+	Parser &p,
+	Result<Ast> (Parser::*parser)(),
+	std::optional<Token::Type> separator,
+	At_Least at_least);
+
 
 Result<Ast> Parser::parse(std::string_view source, std::string_view filename)
 {
@@ -29,7 +41,7 @@ Result<Ast> Parser::parse(std::string_view source, std::string_view filename)
 
 Result<Ast> Parser::parse_sequence()
 {
-	auto seq = Try(parse_one_or_more(*this, &Parser::parse_expression, Token::Type::Expression_Separator));
+	auto seq = Try(parse_many(*this, &Parser::parse_expression, Token::Type::Expression_Separator, At_Least::Zero));
 	return wrap_if_several(std::move(seq), Ast::sequence);
 }
 
@@ -40,7 +52,7 @@ Result<Ast> Parser::parse_expression()
 
 Result<Ast> Parser::parse_infix_expression()
 {
-	auto atomics = Try(parse_one_or_more(*this, &Parser::parse_atomic_expression));
+	auto atomics = Try(parse_many(*this, &Parser::parse_atomic_expression, std::nullopt, At_Least::One));
 	auto lhs = wrap_if_several(std::move(atomics), Ast::call);
 
 	if (expect(Token::Type::Operator)) {
@@ -71,7 +83,7 @@ Result<Ast> Parser::parse_atomic_expression()
 			auto start = token_id;
 			std::vector<Ast> parameters;
 
-			if (auto p = parse_one_or_more(*this, &Parser::parse_identifier); p && expect(Token::Type::Variable_Separator)) {
+			if (auto p = parse_many(*this, &Parser::parse_identifier, std::nullopt, At_Least::One); p && expect(Token::Type::Variable_Separator)) {
 				consume();
 				parameters = std::move(p).value();
 			} else {
@@ -108,10 +120,20 @@ Result<Ast> Parser::parse_identifier()
 	return lit;
 }
 
-Result<std::vector<Ast>> parse_one_or_more(Parser &p, Result<Ast> (Parser::*parser)(), std::optional<Token::Type> separator)
+
+
+static Result<std::vector<Ast>> parse_many(
+	Parser &p,
+	Result<Ast> (Parser::*parser)(),
+	std::optional<Token::Type> separator,
+	At_Least at_least)
 {
 	std::vector<Ast> trees;
 	Result<Ast> expr;
+
+	if (at_least == At_Least::Zero && p.token_id >= p.tokens.size())
+		return {};
+
 	while ((expr = (p.*parser)()).has_value()) {
 		trees.push_back(std::move(expr).value());
 		if (separator) {
@@ -197,8 +219,8 @@ Ast Ast::sequence(std::vector<Ast> expressions)
 	ast.type = Type::Sequence;
 	if (!expressions.empty()) {
 		ast.location = expressions.front().location;
+		ast.arguments = std::move(expressions);
 	}
-	ast.arguments = std::move(expressions);
 	return ast;
 }
 
@@ -280,8 +302,7 @@ struct Indent
 
 std::ostream& operator<<(std::ostream& os, Indent n)
 {
-	while (n.count-- > 0)
-		os.put(' ');
+	std::fill_n(std::ostreambuf_iterator<char>(os), n.count, ' ');
 	return os;
 }
 
diff --git a/src/tests/interpreter.cc b/src/tests/interpreter.cc
new file mode 100644
index 0000000..9ac3c49
--- /dev/null
+++ b/src/tests/interpreter.cc
@@ -0,0 +1,61 @@
+#include <boost/ut.hpp>
+#include <musique.hh>
+
+using namespace boost::ut;
+using namespace std::string_view_literals;
+
+static auto capture_errors(auto lambda, reflection::source_location sl = reflection::source_location::current())
+{
+	return [=] {
+		auto result = lambda();
+		if (not result.has_value()) {
+			expect(false, sl) << "failed test with error: " << result.error();
+		}
+	};
+}
+
+void evaluates_to(Value value, std::string_view source_code, reflection::source_location sl = reflection::source_location::current())
+{
+	capture_errors([=]() -> Result<void> {
+		Interpreter interpreter;
+		auto result = Try(interpreter.eval(Try(Parser::parse(source_code, "test"))));
+		expect(eq(result, value));
+		return {};
+	}, sl)();
+}
+
+void produces_output(std::string_view source_code, std::string_view expected_output, reflection::source_location sl = reflection::source_location::current())
+{
+	capture_errors([=]() -> Result<void> {
+		std::stringstream ss;
+		Interpreter interpreter(ss);
+		Try(interpreter.eval(Try(Parser::parse(source_code, "test"))));
+		expect(eq(ss.str(), expected_output)) << "different evaluation output";
+		return {};
+	}, sl)();
+}
+
+suite intepreter_test = [] {
+	"Interpreter"_test = [] {
+		should("evaluate literals") = [] {
+			evaluates_to(Value{}, "nil");
+			evaluates_to(Value::number(Number(10)), "10");
+			evaluates_to(Value::symbol("notexistingsymbol"), "notexistingsymbol");
+		};
+
+		should("evaluate arithmetic") = [] {
+			evaluates_to(Value::number(Number(10)), "5 + 3 + 2");
+			evaluates_to(Value::number(Number(25)), "5 * (3 + 2)");
+		};
+
+		should("call builtin functions") = [] {
+			evaluates_to(Value::symbol("nil"),    "typeof nil");
+			evaluates_to(Value::symbol("symbol"), "typeof foo");
+			evaluates_to(Value::symbol("number"), "typeof 100");
+
+			produces_output("say 5", "5\n");
+			produces_output("say 1; say 2; say 3", "1\n2\n3\n");
+			produces_output("say 1 2 3", "1 2 3\n");
+		};
+	};
+};
diff --git a/src/tests/parser.cc b/src/tests/parser.cc
index 0117597..1417f41 100644
--- a/src/tests/parser.cc
+++ b/src/tests/parser.cc
@@ -14,6 +14,10 @@ void expect_ast(
 }
 
 suite parser_test = [] {
+	"Empty file parsing"_test = [] {
+		expect_ast("", Ast::sequence({}));
+	};
+
 	"Literal parsing"_test = [] {
 		expect_ast("1", Ast::literal(Token { Token::Type::Numeric, "1", {} }));
 	};
diff --git a/src/value.cc b/src/value.cc
new file mode 100644
index 0000000..6e137c6
--- /dev/null
+++ b/src/value.cc
@@ -0,0 +1,62 @@
+#include <musique.hh>
+
+Result<Value> Value::from(Token t)
+{
+	switch (t.type) {
+	case Token::Type::Numeric:
+		return Number::from(std::move(t)).map(Value::number);
+
+	case Token::Type::Symbol:
+		if (t.source == "nil")
+			return Value{};
+		return Value::symbol(std::string(t.source));
+
+	default:
+		unimplemented();
+	}
+}
+
+Value Value::number(Number n)
+{
+	Value v;
+	v.type = Type::Number;
+	v.n = n;
+	return v;
+}
+
+Value Value::symbol(std::string s)
+{
+	Value v;
+	v.type = Type::Symbol;
+	v.s = s;
+	return v;
+}
+
+bool Value::operator==(Value const& other) const
+{
+	if (type != other.type)
+		return false;
+
+	switch (type) {
+	case Type::Nil:    return true;
+	case Type::Number: return n == other.n;
+	case Type::Symbol: return s == other.s;
+	}
+
+	unreachable();
+}
+
+std::ostream& operator<<(std::ostream& os, Value const& v)
+{
+	switch (v.type) {
+	case Value::Type::Nil:
+		return os << "nil";
+
+	case Value::Type::Number:
+		return os << v.n;
+
+	case Value::Type::Symbol:
+		return os << v.s;
+	}
+	unreachable();
+}