diff --git a/examples/quickstart/quickstart.cpp b/examples/quickstart/quickstart.cpp
index 01b9f3c..0e2a60c 100644
--- a/examples/quickstart/quickstart.cpp
+++ b/examples/quickstart/quickstart.cpp
@@ -1,4 +1,5 @@
 #include <kalcy/kalcy.hpp>
+#include <cassert>
 #include <format>
 #include <iomanip>
 #include <iostream>
@@ -6,11 +7,11 @@
 
 namespace {
 auto run(std::string_view const text, bool verbose) -> bool {
-	auto kalcy = kalcy::Kalcy{};
-	auto ast = std::string{};
 	try {
-		std::cout << kalcy.evaluate(text, &ast) << "\n";
-		if (verbose) { std::cout << "expression\t: " << text << "\nAST\t\t: " << ast << "\n"; }
+		auto expr = kalcy::Parser{text}.parse();
+		assert(expr != nullptr);
+		std::cout << kalcy::evaluate(*expr) << "\n";
+		if (verbose) { std::cout << std::format("expression\t: {}\nAST\t\t: {}\n", text, kalcy::to_string(*expr)); }
 		return true;
 	} catch (kalcy::Error const& err) {
 		std::cerr << err.what() << "\n";
diff --git a/kalcy/CMakeLists.txt b/kalcy/CMakeLists.txt
index 85c0770..077bd2e 100644
--- a/kalcy/CMakeLists.txt
+++ b/kalcy/CMakeLists.txt
@@ -33,7 +33,6 @@ target_sources(${PROJECT_NAME} PRIVATE
   src/error.cpp
   src/eval.cpp
   src/expr.cpp
-  src/kalcy.cpp
   src/parser.cpp
   src/scanner.cpp
 )
diff --git a/kalcy/include/kalcy/env.hpp b/kalcy/include/kalcy/env.hpp
index 625376f..ac32fed 100644
--- a/kalcy/include/kalcy/env.hpp
+++ b/kalcy/include/kalcy/env.hpp
@@ -21,6 +21,17 @@ class Env {
 	///
 	struct Mismatch {};
 
+	///
+	/// \brief Get a default instance (singleton).
+	/// \returns const reference to a static default instance.
+	///
+	static auto get_default() -> Env const&;
+
+	///
+	/// \brief Populate environment with builtins.
+	///
+	explicit Env();
+
 	///
 	/// \brief Define a function.
 	///
diff --git a/kalcy/include/kalcy/eval.hpp b/kalcy/include/kalcy/eval.hpp
index 0e102ad..7e17c99 100644
--- a/kalcy/include/kalcy/eval.hpp
+++ b/kalcy/include/kalcy/eval.hpp
@@ -20,4 +20,6 @@ struct Eval {
 	///
 	[[nodiscard]] auto operator()(Expr const& expr) const noexcept(false) -> double;
 };
+
+inline auto evaluate(Expr const& expr, Env const& env = Env::get_default()) { return Eval{env}(expr); }
 } // namespace kalcy
diff --git a/kalcy/include/kalcy/kalcy.hpp b/kalcy/include/kalcy/kalcy.hpp
index 50ca304..16fed93 100644
--- a/kalcy/include/kalcy/kalcy.hpp
+++ b/kalcy/include/kalcy/kalcy.hpp
@@ -2,31 +2,5 @@
 #include <kalcy/build_version.hpp>
 #include <kalcy/env.hpp>
 #include <kalcy/error.hpp>
-
-namespace kalcy {
-///
-/// \brief Expression evaluator with an owned environment.
-///
-class Kalcy {
-  public:
-	///
-	/// \brief Populate environment with builtins.
-	///
-	explicit Kalcy();
-
-	///
-	/// \brief Evaluate an expression.
-	/// \param expression expression to evaluate.
-	/// \param out_ast (optional) out parameter to build stringified AST to.
-	/// \throws sub-type of Error.
-	///
-	[[nodiscard]] auto evaluate(std::string_view expression, std::string* out_ast = {}) const noexcept(false) -> double;
-
-	///
-	/// \brief Execution environment.
-	///
-	/// The environment is read-only during evaluation of expressions.
-	///
-	Env environment{};
-};
-} // namespace kalcy
+#include <kalcy/eval.hpp>
+#include <kalcy/parser.hpp>
diff --git a/kalcy/src/env.cpp b/kalcy/src/env.cpp
index 18d28e2..94ce5a0 100644
--- a/kalcy/src/env.cpp
+++ b/kalcy/src/env.cpp
@@ -1,8 +1,23 @@
 #include <kalcy/env.hpp>
 #include <kalcy/error.hpp>
 #include <cassert>
+#include <cmath>
+#include <numbers>
 
 namespace kalcy {
+auto Env::get_default() -> Env const& {
+	static auto const ret{Env{}};
+	return ret;
+}
+
+Env::Env() {
+	define("pi", std::numbers::pi_v<double>);
+	define("sqrt", [](std::span<double const> args) {
+		if (args.size() != 1) { throw Mismatch{}; }
+		return std::sqrt(args.front());
+	});
+}
+
 void Env::define(std::string_view name, Func func) {
 	if (name.empty() || !func) { return; }
 	m_table.insert_or_assign(name, std::move(func));
diff --git a/kalcy/src/error.cpp b/kalcy/src/error.cpp
index 1942fbb..41df862 100644
--- a/kalcy/src/error.cpp
+++ b/kalcy/src/error.cpp
@@ -19,7 +19,7 @@ ParseError::ParseError(Token const token, Token::Type const expected)
 UndefinedSymbol::UndefinedSymbol(Token const token) : Error(token, std::format("undefined symbol: '{}'", token.lexeme)) {}
 
 ArgsMismatch::ArgsMismatch(Token const token, std::size_t argument_count)
-	: Error(token, std::format("{} does not take {} arguments", token.lexeme, argument_count)), argument_count(argument_count) {}
+	: Error(token, std::format("{} does not take {} argument(s)", token.lexeme, argument_count)), argument_count(argument_count) {}
 
 InvalidOperaor::InvalidOperaor(Token const token) : Error(token, std::format("invalid operator: '{}'", token.lexeme)) {}
 } // namespace kalcy
diff --git a/kalcy/src/kalcy.cpp b/kalcy/src/kalcy.cpp
deleted file mode 100644
index 7ff323c..0000000
--- a/kalcy/src/kalcy.cpp
+++ /dev/null
@@ -1,24 +0,0 @@
-#include <kalcy/error.hpp>
-#include <kalcy/eval.hpp>
-#include <kalcy/kalcy.hpp>
-#include <kalcy/parser.hpp>
-#include <cmath>
-#include <numbers>
-
-namespace kalcy {
-Kalcy::Kalcy() {
-	environment.define("pi", std::numbers::pi_v<double>);
-	environment.define("sqrt", [](std::span<double const> args) {
-		if (args.size() != 1) { throw Env::Mismatch{}; }
-		return std::sqrt(args.front());
-	});
-}
-
-auto Kalcy::evaluate(std::string_view const expression, std::string* out_ast) const noexcept(false) -> double {
-	if (expression.empty()) { return {}; }
-	auto expr = Parser{expression}.parse();
-	if (!expr) { return {}; }
-	if (out_ast != nullptr) { *out_ast = to_string(*expr); }
-	return Eval{environment}(*expr);
-}
-} // namespace kalcy