diff --git a/src/base.h b/src/base.h
index 7f748d3..f06ebe7 100644
--- a/src/base.h
+++ b/src/base.h
@@ -271,7 +271,7 @@ DEFOBJTYPE(Vector, VECTOR, VECTORP, {
         REGISTER_GLOBAL_SYMBOL(cname);                                     \
         ((LispSymbol *) Q##cname)->function = BUILTIN_FUNCTION_OBJ(cname); \
         ((LispFunction *) ((LispSymbol *) Q##cname)->function)             \
-            ->flags.no_eval_args = true;                                   \
+            ->impl.native.no_eval_args = true;                             \
     }
 
 DECLARE_SYMBOL(nil);
diff --git a/src/function.c b/src/function.c
index 95a79b9..868f679 100644
--- a/src/function.c
+++ b/src/function.c
@@ -206,10 +206,10 @@ LispVal *make_builtin_function(LispVal *name, LispVal *(*cfunc)(void),
                                LispVal *docstr) {
     LispFunction *obj = lisp_alloc_object(sizeof(LispFunction), TYPE_FUNCTION);
     obj->name = name;
-    obj->flags.type = FUNCTION_NATIVE;
-    obj->flags.no_eval_args = false;
+    obj->type = FUNCTION_NATIVE;
     obj->docstr = docstr;
-    obj->impl.native.zero = cfunc;
+    obj->impl.native.no_eval_args = false;
+    obj->impl.native.addr.zero = cfunc;
     ReadStream stream;
     read_stream_init(&stream, lisp_args, args_len);
     LispVal *args_form = read(&stream);
@@ -331,7 +331,7 @@ process_complex_native_args(LispFunction *fobj, LispVal *args,
 static ALWAYS_INLINE LispVal *call_native(LispVal *orig_func,
                                           LispFunction *fobj, LispVal *args) {
     push_stack_frame(orig_func, fobj, args);
-    if (!fobj->flags.no_eval_args) {
+    if (!fobj->impl.native.no_eval_args) {
         args = evaluate_function_arguments(args);
     }
     set_stack_evaluated_args(args);
@@ -356,24 +356,25 @@ static ALWAYS_INLINE LispVal *call_native(LispVal *orig_func,
     LispVal *retval;
     switch (count) {
     case 0:
-        retval = fobj->impl.native.zero();
+        retval = fobj->impl.native.addr.zero();
         break;
     case 1:
-        retval = fobj->impl.native.one(arg_arr[0]);
+        retval = fobj->impl.native.addr.one(arg_arr[0]);
         break;
     case 2:
-        retval = fobj->impl.native.two(arg_arr[0], arg_arr[1]);
+        retval = fobj->impl.native.addr.two(arg_arr[0], arg_arr[1]);
         break;
     case 3:
-        retval = fobj->impl.native.three(arg_arr[0], arg_arr[1], arg_arr[2]);
+        retval =
+            fobj->impl.native.addr.three(arg_arr[0], arg_arr[1], arg_arr[2]);
         break;
     case 4:
-        retval = fobj->impl.native.four(arg_arr[0], arg_arr[1], arg_arr[2],
-                                        arg_arr[3]);
+        retval = fobj->impl.native.addr.four(arg_arr[0], arg_arr[1], arg_arr[2],
+                                             arg_arr[3]);
         break;
     case 5:
-        retval = fobj->impl.native.five(arg_arr[0], arg_arr[1], arg_arr[2],
-                                        arg_arr[3], arg_arr[4]);
+        retval = fobj->impl.native.addr.five(arg_arr[0], arg_arr[1], arg_arr[2],
+                                             arg_arr[3], arg_arr[4]);
         break;
     default:
         abort();
@@ -485,13 +486,12 @@ DEFUN(funcall, "funcall", (LispVal * func, LispVal *args), "(func &rest args)",
     }
     // include symbol here for the error message
     CHECK_TYPE(fobj, TYPE_FUNCTION, TYPE_SYMBOL);
-    switch (fobj->flags.type) {
+    switch (fobj->type) {
     case FUNCTION_NATIVE:
         return call_native(func, fobj, args);
     case FUNCTION_INTERP:
         return call_interpreted(func, fobj, args);
     default:
-        // TODO implement
         abort();
     }
 }
@@ -531,8 +531,7 @@ DEFSPECIAL(lambda, "lambda", (LispVal * args, LispVal *body),
     LispFunction *fobj = lisp_alloc_object(sizeof(LispFunction), TYPE_FUNCTION);
     fobj->name = Qnil;
     fobj->args = llpr.lambda_list;
-    fobj->flags.type = FUNCTION_INTERP;
-    fobj->flags.no_eval_args = false;
+    fobj->type = FUNCTION_INTERP;
     if (STRINGP(XCAR(body))) {
         fobj->docstr = XCAR(body);
         if (CONSP(XCDR(body))) {
diff --git a/src/function.h b/src/function.h
index ab70971..d68c11c 100644
--- a/src/function.h
+++ b/src/function.h
@@ -21,13 +21,16 @@ struct LambdaList {
 };
 
 #define MAX_NATIVE_FUNCTION_ARGS 5
-union native_function {
-    LispVal *(*zero)(void);
-    LispVal *(*one)(LispVal *);
-    LispVal *(*two)(LispVal *, LispVal *);
-    LispVal *(*three)(LispVal *, LispVal *, LispVal *);
-    LispVal *(*four)(LispVal *, LispVal *, LispVal *, LispVal *);
-    LispVal *(*five)(LispVal *, LispVal *, LispVal *, LispVal *, LispVal *);
+struct native_function {
+    bool no_eval_args;
+    union {
+        LispVal *(*zero)(void);
+        LispVal *(*one)(LispVal *);
+        LispVal *(*two)(LispVal *, LispVal *);
+        LispVal *(*three)(LispVal *, LispVal *, LispVal *);
+        LispVal *(*four)(LispVal *, LispVal *, LispVal *, LispVal *);
+        LispVal *(*five)(LispVal *, LispVal *, LispVal *, LispVal *, LispVal *);
+    } addr;
 };
 
 struct interp_function {
@@ -40,18 +43,13 @@ typedef enum {
     FUNCTION_INTERP,
 } LispFunctionType;
 
-struct function_flags {
-    LispFunctionType type : 2;
-    unsigned int no_eval_args : 1;
-};
-
 DEFOBJTYPE(Function, FUNCTION, FUNCTIONP, {
     LispVal *name; // symbol (or nil for a lambda)
-    struct function_flags flags;
+    LispFunctionType type;
     struct LambdaList args;
     LispVal *docstr;
     union {
-        union native_function native;
+        struct native_function native;
         struct interp_function interp;
     } impl;
 });