Eval: Change stack-based TCO to loop-based TCO

src/eval-special-form.cpp

@@ -30,8 +30,8 @@ ValuePtr Eval::evalDef(const ValueVector& nodes, EnvironmentPtr env)
 	VALUE_CAST(symbol, Symbol, nodes.front());
 
 	// Eval second argument
-	m_ast_stack.push(*std::next(nodes.begin()));
-	m_env_stack.push(env);
+	m_ast = *std::next(nodes.begin());
+	m_env = env;
 	ValuePtr value = evalImpl();
 
 	// Dont overwrite symbols after an error
@@ -52,8 +52,8 @@ ValuePtr Eval::evalDefMacro(const ValueVector& nodes, EnvironmentPtr env)
 	VALUE_CAST(symbol, Symbol, nodes.front());
 
 	// Eval second argument
-	m_ast_stack.push(*std::next(nodes.begin()));
-	m_env_stack.push(env);
+	m_ast = *std::next(nodes.begin());
+	m_env = env;
 	ValuePtr value = evalImpl();
 	VALUE_CAST(lambda, Lambda, value);
 
@@ -118,14 +118,14 @@ void Eval::evalDo(const ValueVector& nodes, EnvironmentPtr env)
 
 	// Evaluate all nodes except the last
 	for (auto it = nodes.begin(); it != std::prev(nodes.end(), 1); ++it) {
-		m_ast_stack.push(*it);
-		m_env_stack.push(env);
+		m_ast = *it;
+		m_env = env;
 		evalImpl();
 	}
 
 	// Eval last node
-	m_ast_stack.push(nodes.back());
-	m_env_stack.push(env);
+	m_ast = nodes.back();
+	m_env = env;
 	return; // TCO
 }
 
@@ -138,18 +138,18 @@ void Eval::evalIf(const ValueVector& nodes, EnvironmentPtr env)
 	auto second_argument = *std::next(nodes.begin());
 	auto third_argument = (nodes.size() == 3) ? *std::next(std::next(nodes.begin())) : makePtr<Constant>(Constant::Nil);
 
-	m_ast_stack.push(first_argument);
-	m_env_stack.push(env);
+	m_ast = first_argument;
+	m_env = env;
 	auto first_evaluated = evalImpl();
 	if (!is<Constant>(first_evaluated.get())
 	    || std::static_pointer_cast<Constant>(first_evaluated)->state() == Constant::True) {
-		m_ast_stack.push(second_argument);
-		m_env_stack.push(env);
+		m_ast = second_argument;
+		m_env = env;
 		return; // TCO
 	}
 
-	m_ast_stack.push(third_argument);
-	m_env_stack.push(env);
+	m_ast = third_argument;
+	m_env = env;
 	return; // TCO
 }
 
@@ -173,16 +173,16 @@ void Eval::evalLet(const ValueVector& nodes, EnvironmentPtr env)
 		VALUE_CAST(elt, Symbol, (*it), void());
 
 		std::string key = elt->symbol();
-		m_ast_stack.push(*std::next(it));
-		m_env_stack.push(let_env);
+		m_ast = *std::next(it);
+		m_env = let_env;
 		ValuePtr value = evalImpl();
 		let_env->set(key, value);
 	}
 
 	// TODO: Remove limitation of 3 arguments
 	// Eval all arguments in this new env, return last sexp of the result
-	m_ast_stack.push(*std::next(nodes.begin()));
-	m_env_stack.push(let_env);
+	m_ast = *std::next(nodes.begin());
+	m_env = let_env;
 	return; // TCO
 }
 
@@ -277,8 +277,8 @@ void Eval::evalQuasiQuote(const ValueVector& nodes, EnvironmentPtr env)
 
 	auto result = evalQuasiQuoteImpl(nodes.front());
 
-	m_ast_stack.push(result);
-	m_env_stack.push(env);
+	m_ast = result;
+	m_env = env;
 	return; // TCO
 }
 
@@ -288,8 +288,8 @@ void Eval::evalTry(const ValueVector& nodes, EnvironmentPtr env)
 	CHECK_ARG_COUNT_AT_LEAST("try*", nodes.size(), 1, void());
 
 	// Try 'x'
-	m_ast_stack.push(nodes.front());
-	m_env_stack.push(env);
+	m_ast = nodes.front();
+	m_env = env;
 	auto result = evalImpl();
 
 	// Catch
@@ -317,13 +317,13 @@ void Eval::evalTry(const ValueVector& nodes, EnvironmentPtr env)
 		catch_env->set(catch_binding->symbol(), error);
 
 		// Evaluate 'z' using the new Environment
-		m_ast_stack.push(catch_nodes.back());
-		m_env_stack.push(catch_env);
+		m_ast = catch_nodes.back();
+		m_env = catch_env;
 		return; // TCO
 	}
 
-	m_ast_stack.push(result);
-	m_env_stack.push(env);
+	m_ast = result;
+	m_env = env;
 	return; // TCO
 }
 

src/eval.cpp

@@ -23,6 +23,7 @@ namespace blaze {
 Eval::Eval(ValuePtr ast, EnvironmentPtr env)
 	: m_ast(ast)
 	, m_env(env)
+	, m_outer_env(env)
 {
 }
 
@@ -30,11 +31,6 @@ Eval::Eval(ValuePtr ast, EnvironmentPtr env)
 
 void Eval::eval()
 {
-	m_ast_stack = std::stack<ValuePtr>();
-	m_env_stack = std::stack<EnvironmentPtr>();
-	m_ast_stack.push(m_ast);
-	m_env_stack.push(m_env);
-
 	m_ast = evalImpl();
 }
 
@@ -48,20 +44,13 @@ ValuePtr Eval::evalImpl()
 			return nullptr;
 		}
 
-		if (m_ast_stack.size() == 0) {
-			return nullptr;
-		}
+		ast = m_ast;
+		env = m_env;
 
-		if (m_env_stack.size() == 0) {
-			m_env_stack.push(m_env);
+		if (env == nullptr) {
+			env = m_outer_env;
 		}
 
-		ast = m_ast_stack.top();
-		env = m_env_stack.top();
-
-		m_ast_stack.pop();
-		m_env_stack.pop();
-
 		if (!is<List>(ast.get())) {
 			return evalAst(ast, env);
 		}
@@ -133,8 +122,8 @@ ValuePtr Eval::evalImpl()
 		if (is<Lambda>(evaluated_list->front().get())) {
 			auto lambda = std::static_pointer_cast<Lambda>(evaluated_list->front());
 
-			m_ast_stack.push(lambda->body());
-			m_env_stack.push(Environment::create(lambda, evaluated_list->rest()));
+			m_ast = lambda->body();
+			m_env = Environment::create(lambda, evaluated_list->rest());
 			continue; // TCO
 		}
 
@@ -165,8 +154,8 @@ ValuePtr Eval::evalAst(ValuePtr ast, EnvironmentPtr env)
 		auto evaluated_nodes = ValueVector(count);
 
 		for (size_t i = 0; i < count; ++i) {
-			m_ast_stack.push(nodes[i]);
-			m_env_stack.push(env);
+			m_ast = nodes[i];
+			m_env = env;
 			ValuePtr eval_node = evalImpl();
 			if (eval_node == nullptr) {
 				return nullptr;
@@ -184,8 +173,8 @@ ValuePtr Eval::evalAst(ValuePtr ast, EnvironmentPtr env)
 		const auto& elements = std::static_pointer_cast<HashMap>(ast)->elements();
 		Elements evaluated_elements;
 		for (const auto& element : elements) {
-			m_ast_stack.push(element.second);
-			m_env_stack.push(env);
+			m_ast = element.second;
+			m_env = env;
 			ValuePtr element_node = evalImpl();
 			if (element_node == nullptr) {
 				return nullptr;
@@ -231,11 +220,12 @@ ValuePtr Eval::macroExpand(ValuePtr ast, EnvironmentPtr env)
 {
 	while (isMacroCall(ast, env)) {
 		auto list = std::static_pointer_cast<List>(ast);
+
 		auto value = env->get(std::static_pointer_cast<Symbol>(list->front())->symbol());
 		auto lambda = std::static_pointer_cast<Lambda>(value);
 
-		m_ast_stack.push(lambda->body());
-		m_env_stack.push(Environment::create(lambda, list->rest()));
+		m_ast = lambda->body();
+		m_env = Environment::create(lambda, list->rest());
 		ast = evalImpl();
 	}
 

src/eval.h

@@ -48,9 +48,7 @@ private:
 
 	ValuePtr m_ast;
 	EnvironmentPtr m_env;
-
-	std::stack<ValuePtr> m_ast_stack;
-	std::stack<EnvironmentPtr> m_env_stack;
+	EnvironmentPtr m_outer_env;
 };
 
 } // namespace blaze

src/functions.cpp

@@ -518,7 +518,7 @@ ADD_FUNCTION(
 		return makePtr<List>(collection->rest());
 	});
 
-// (apply + 1 2 (list 3 4)) -> (+ 1 2 3 4)
+// (apply + 1 2 (list 3 4)) -> (+ 1 2 3 4) -> 10
 ADD_FUNCTION(
 	"apply",
 	{
@@ -984,8 +984,8 @@ ADD_FUNCTION(
 
 void installFunctions(EnvironmentPtr env)
 {
-	for (const auto& [name, lambda] : s_functions) {
-		env->set(name, makePtr<Function>(name, lambda));
+	for (const auto& [name, function] : s_functions) {
+		env->set(name, makePtr<Function>(name, function));
 	}
 }