diff --git a/source/eval/ast_eval.cpp b/source/eval/ast_eval.cpp
index d06fa08b..ae24e7d7 100644
--- a/source/eval/ast_eval.cpp
+++ b/source/eval/ast_eval.cpp
@@ -322,10 +322,7 @@ auto unary_expression::eval(environment* env) const -> const object*
             }
             return make<integer_object>(-evaluated_value->as<integer_object>()->value);
         case exclamation:
-            if (!evaluated_value->is(object::object_type::boolean)) {
-                return native_bool_to_object(/*val=*/false);
-            }
-            return native_bool_to_object(!evaluated_value->as<boolean_object>()->value);
+            return native_bool_to_object(!evaluated_value->is_truthy());
         default:
             return make_error("unknown operator: {}{}", op, evaluated_value->type());
     }
@@ -386,6 +383,7 @@ const builtin_function_expression builtin_puts {"puts",
                                                     fmt::print("\n");
                                                     return native_null();
                                                 }};
+
 const builtin_function_expression builtin_first {
     "first",
     {"arr"},
@@ -412,6 +410,7 @@ const builtin_function_expression builtin_first {
         }
         return make_error("argument of type {} to first() is not supported", maybe_string_or_array->type());
     }};
+
 const builtin_function_expression builtin_last {
     "last",
     {"arr"},
@@ -438,6 +437,7 @@ const builtin_function_expression builtin_last {
         }
         return make_error("argument of type {} to last() is not supported", maybe_string_or_array->type());
     }};
+
 const builtin_function_expression builtin_rest {
     "rest",
     {"arr"},
@@ -466,6 +466,7 @@ const builtin_function_expression builtin_rest {
         }
         return make_error("argument of type {} to rest() is not supported", maybe_string_or_array->type());
     }};
+
 const builtin_function_expression builtin_push {
     "push",
     {"arr", "val"},
@@ -694,12 +695,16 @@ TEST_CASE("bangOperator")
         et {"!true", false},
         et {"!false", true},
         et {"!false", true},
+        et {"!0", true},
         et {"!5", false},
         et {R"(!"a")", false},
+        et {R"(!"")", true},
         et {"!!true", true},
         et {"!!false", false},
         et {"!!5", true},
         et {R"(!!"a")", true},
+        et {R"(![])", true},
+        et {R"(!{})", true},
     };
     for (const auto& [input, expected] : tests) {
         const auto evaluated = run(input);
diff --git a/source/eval/environment.cpp b/source/eval/environment.cpp
index 03e56ab8..d71d17a9 100644
--- a/source/eval/environment.cpp
+++ b/source/eval/environment.cpp
@@ -4,18 +4,15 @@
 
 #include "object.hpp"
 
-environment::environment(environment* parent_env)
-    : m_parent(parent_env)
+environment::environment(environment* outer_env)
+    : outer(outer_env)
 {
-    if (m_parent == this) {
-        abort();
-    }
 }
 
 auto environment::get(const std::string& name) const -> const object*
 {
-    for (const auto* ptr = this; ptr != nullptr; ptr = ptr->m_parent) {
-        if (const auto itr = ptr->m_store.find(name); itr != ptr->m_store.end()) {
+    for (const auto* ptr = this; ptr != nullptr; ptr = ptr->outer) {
+        if (const auto itr = ptr->store.find(name); itr != ptr->store.end()) {
             return itr->second;
         }
     }
@@ -24,12 +21,12 @@ auto environment::get(const std::string& name) const -> const object*
 
 auto environment::set(const std::string& name, const object* val) -> void
 {
-    m_store[name] = val;
+    store[name] = val;
 }
 
 auto environment::debug() const -> void
 {
-    for (const auto& [k, v] : m_store) {
+    for (const auto& [k, v] : store) {
         fmt::print("[{}] = {}\n", k, v->inspect());
     }
 }
diff --git a/source/eval/environment.hpp b/source/eval/environment.hpp
index df85d773..2e8d2d11 100644
--- a/source/eval/environment.hpp
+++ b/source/eval/environment.hpp
@@ -6,13 +6,12 @@ struct object;
 
 struct environment
 {
-    explicit environment(environment* parent_env = nullptr);
+    explicit environment(environment* outer_env = nullptr);
 
     auto debug() const -> void;
     auto get(const std::string& name) const -> const object*;
     auto set(const std::string& name, const object* val) -> void;
 
-  private:
-    std::unordered_map<std::string, const object*> m_store;
-    environment* m_parent {};
+    std::unordered_map<std::string, const object*> store;
+    environment* outer {};
 };
diff --git a/source/eval/object.cpp b/source/eval/object.cpp
index 1ce96d78..39c5d64c 100644
--- a/source/eval/object.cpp
+++ b/source/eval/object.cpp
@@ -117,6 +117,14 @@ const boolean_object true_obj {/*val=*/true};
 const null_object null_obj;
 }  // namespace
 
+auto native_bool_to_object(bool val) -> const object*
+{
+    if (val) {
+        return &true_obj;
+    }
+    return &false_obj;
+}
+
 auto native_true() -> const object*
 {
     return &true_obj;
diff --git a/source/eval/object.hpp b/source/eval/object.hpp
index a09348d8..d0825349 100644
--- a/source/eval/object.hpp
+++ b/source/eval/object.hpp
@@ -85,6 +85,8 @@ struct integer_object : hashable_object
     {
     }
 
+    [[nodiscard]] auto is_truthy() const -> bool override { return value != 0; }
+
     [[nodiscard]] auto type() const -> object_type override { return object_type::integer; }
 
     [[nodiscard]] auto inspect() const -> std::string override { return std::to_string(value); }
@@ -124,14 +126,7 @@ struct boolean_object : hashable_object
 
 auto native_true() -> const object*;
 auto native_false() -> const object*;
-
-inline auto native_bool_to_object(bool val) -> const object*
-{
-    if (val) {
-        return native_true();
-    }
-    return native_false();
-}
+auto native_bool_to_object(bool val) -> const object*;
 
 struct string_object : hashable_object
 {
@@ -140,6 +135,8 @@ struct string_object : hashable_object
     {
     }
 
+    [[nodiscard]] auto is_truthy() const -> bool override { return !value.empty(); }
+
     [[nodiscard]] auto type() const -> object_type override { return object_type::string; }
 
     [[nodiscard]] auto inspect() const -> std::string override { return fmt::format(R"("{}")", value); }
@@ -201,6 +198,8 @@ struct array_object : object
     {
     }
 
+    [[nodiscard]] auto is_truthy() const -> bool override { return !elements.empty(); }
+
     [[nodiscard]] auto type() const -> object_type override { return object_type::array; }
 
     [[nodiscard]] auto inspect() const -> std::string override;
@@ -230,6 +229,8 @@ struct hash_object : object
     {
     }
 
+    [[nodiscard]] auto is_truthy() const -> bool override { return !pairs.empty(); }
+
     [[nodiscard]] auto type() const -> object_type override { return object_type::hash; }
 
     [[nodiscard]] auto inspect() const -> std::string override;
diff --git a/source/vm/vm.cpp b/source/vm/vm.cpp
index 8881a587..1a39c88c 100644
--- a/source/vm/vm.cpp
+++ b/source/vm/vm.cpp
@@ -281,15 +281,7 @@ auto vm::exec_bang() -> void
 {
     using enum object::object_type;
     const auto* operand = pop();
-    if (operand->is(boolean)) {
-        push(native_bool_to_object(!operand->as<boolean_object>()->value));
-        return;
-    }
-    if (operand->is(null)) {
-        push(native_true());
-        return;
-    }
-    push(native_false());
+    push(native_bool_to_object(!operand->is_truthy()));
 }
 
 auto vm::exec_minus() -> void
@@ -647,8 +639,12 @@ TEST_CASE("booleanExpressions")
         vt<bool> {R"(("a" > "b") == false)", true},
         vt<bool> {R"(!true)", false},
         vt<bool> {R"(!false)", true},
+        vt<bool> {R"(!!"")", false},
         vt<bool> {R"(!"a")", false},
         vt<bool> {R"(!!"a")", true},
+        vt<bool> {R"(!0)", true},
+        vt<bool> {R"(![])", true},
+        vt<bool> {R"(!{})", true},
         vt<bool> {R"(!5)", false},
         vt<bool> {R"(!!true)", true},
         vt<bool> {R"(!!false)", false},