diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index f1796ce1..5d3c5f9d 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -374,7 +374,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
         gc_mark_object(pic, (struct pic_object *)proc->u.i.cxt);
       }
     } else {
-      gc_mark_object(pic, (struct pic_object *)proc->u.func.name);
+      gc_mark_object(pic, (struct pic_object *)proc->u.f.name);
     }
     break;
   }
diff --git a/extlib/benz/include/picrin/proc.h b/extlib/benz/include/picrin/proc.h
index 17605590..eaac2ef7 100644
--- a/extlib/benz/include/picrin/proc.h
+++ b/extlib/benz/include/picrin/proc.h
@@ -9,12 +9,6 @@
 extern "C" {
 #endif
 
-/* native C function */
-struct pic_func {
-  pic_func_t f;
-  pic_sym *name;
-};
-
 struct pic_context {
   PIC_OBJECT_HEADER
   pic_value *regs;
@@ -25,9 +19,15 @@ struct pic_context {
 
 struct pic_proc {
   PIC_OBJECT_HEADER
-  char kind;
+  enum {
+    PIC_PROC_TAG_IREP,
+    PIC_PROC_TAG_FUNC
+  } tag;
   union {
-    struct pic_func func;
+    struct {
+      pic_func_t func;
+      pic_sym *name;
+    } f;
     struct {
       struct pic_irep *irep;
       struct pic_context *cxt;
@@ -35,11 +35,8 @@ struct pic_proc {
   } u;
 };
 
-#define PIC_PROC_KIND_FUNC 1
-#define PIC_PROC_KIND_IREP 2
-
-#define pic_proc_func_p(proc) ((proc)->kind == PIC_PROC_KIND_FUNC)
-#define pic_proc_irep_p(proc) ((proc)->kind == PIC_PROC_KIND_IREP)
+#define pic_proc_func_p(proc) ((proc)->tag == PIC_PROC_TAG_FUNC)
+#define pic_proc_irep_p(proc) ((proc)->tag == PIC_PROC_TAG_IREP)
 
 #define pic_proc_p(o) (pic_type(o) == PIC_TT_PROC)
 #define pic_proc_ptr(o) ((struct pic_proc *)pic_ptr(o))
diff --git a/extlib/benz/proc.c b/extlib/benz/proc.c
index 93432759..e8c80f7a 100644
--- a/extlib/benz/proc.c
+++ b/extlib/benz/proc.c
@@ -15,9 +15,9 @@ pic_make_proc(pic_state *pic, pic_func_t func, const char *name)
   sym = pic_intern_cstr(pic, name);
 
   proc = (struct pic_proc *)pic_obj_alloc(pic, sizeof(struct pic_proc), PIC_TT_PROC);
-  proc->kind = PIC_PROC_KIND_FUNC;
-  proc->u.func.f = func;
-  proc->u.func.name = sym;
+  proc->tag = PIC_PROC_TAG_FUNC;
+  proc->u.f.func = func;
+  proc->u.f.name = sym;
   return proc;
 }
 
@@ -27,7 +27,7 @@ pic_make_proc_irep(pic_state *pic, struct pic_irep *irep, struct pic_context *cx
   struct pic_proc *proc;
 
   proc = (struct pic_proc *)pic_obj_alloc(pic, sizeof(struct pic_proc), PIC_TT_PROC);
-  proc->kind = PIC_PROC_KIND_IREP;
+  proc->tag = PIC_PROC_TAG_IREP;
   proc->u.i.irep = irep;
   proc->u.i.cxt = cxt;
   return proc;
@@ -36,10 +36,10 @@ pic_make_proc_irep(pic_state *pic, struct pic_irep *irep, struct pic_context *cx
 pic_sym *
 pic_proc_name(struct pic_proc *proc)
 {
-  switch (proc->kind) {
-  case PIC_PROC_KIND_FUNC:
-    return proc->u.func.name;
-  case PIC_PROC_KIND_IREP:
+  switch (proc->tag) {
+  case PIC_PROC_TAG_FUNC:
+    return proc->u.f.name;
+  case PIC_PROC_TAG_IREP:
     return proc->u.i.irep->name;
   }
   PIC_UNREACHABLE();
diff --git a/extlib/benz/vm.c b/extlib/benz/vm.c
index 95ada7a5..baea1bfe 100644
--- a/extlib/benz/vm.c
+++ b/extlib/benz/vm.c
@@ -862,7 +862,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value args)
       if (pic_proc_func_p(pic_proc_ptr(x))) {
 
         /* invoke! */
-        v = proc->u.func.f(pic);
+        v = proc->u.f.func(pic);
         pic->sp[0] = v;
         pic->sp += pic->ci->retc;