diff --git a/extlib/benz/eval.c b/extlib/benz/eval.c
index eaf6a72a..248b67f9 100644
--- a/extlib/benz/eval.c
+++ b/extlib/benz/eval.c
@@ -147,13 +147,13 @@ define_var(pic_state *pic, analyze_scope *scope, pic_sym *sym)
   int ret;
 
   if (search_scope(pic, scope, sym)) {
-    if (scope->depth > 0 || pic_weak_has(pic, pic->globals, sym)) {
+    if (scope->depth > 0 || pic_weak_has(pic, pic->globals, pic_obj_value(sym))) {
       pic_warnf(pic, "redefining variable: ~s", pic_obj_value(sym));
     }
     return;
   }
 
-  pic_weak_set(pic, pic->globals, sym, pic_invalid_value());
+  pic_weak_set(pic, pic->globals, pic_obj_value(sym), pic_invalid_value());
 
   kh_put(a, &scope->locals, sym, &ret);
 }
diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index 8baa2239..b903f048 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -471,14 +471,10 @@ gc_mark_phase(pic_state *pic)
   M(sADD); M(sSUB); M(sMUL); M(sDIV); M(sEQ); M(sLT); M(sLE); M(sGT); M(sGE); M(sNOT);
 
   /* global variables */
-  if (pic->globals) {
-    gc_mark_object(pic, (struct pic_object *)pic->globals);
-  }
+  gc_mark(pic, pic->globals);
 
   /* macro objects */
-  if (pic->macros) {
-    gc_mark_object(pic, (struct pic_object *)pic->macros);
-  }
+  gc_mark(pic, pic->macros);
 
   /* error object */
   gc_mark(pic, pic->err);
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 20934ef8..2b092037 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -242,11 +242,11 @@ int pic_dict_size(pic_state *, pic_value dict);
 bool pic_dict_next(pic_state *, pic_value dict, int *iter, pic_sym **key, pic_value *val);
 
 /* ephemeron */
-struct pic_weak *pic_make_weak(pic_state *);
-pic_value pic_weak_ref(pic_state *, struct pic_weak *, void *);
-void pic_weak_set(pic_state *, struct pic_weak *, void *, pic_value);
-void pic_weak_del(pic_state *, struct pic_weak *, void *);
-bool pic_weak_has(pic_state *, struct pic_weak *, void *);
+pic_value pic_make_weak(pic_state *);
+pic_value pic_weak_ref(pic_state *, pic_value weak, pic_value key);
+void pic_weak_set(pic_state *, pic_value weak, pic_value key, pic_value val);
+void pic_weak_del(pic_state *, pic_value weak, pic_value key);
+bool pic_weak_has(pic_state *, pic_value weak, pic_value key);
 
 /* symbol */
 pic_sym *pic_intern(pic_state *, pic_value str);
diff --git a/extlib/benz/include/picrin/object.h b/extlib/benz/include/picrin/object.h
index 22953098..cbe3e264 100644
--- a/extlib/benz/include/picrin/object.h
+++ b/extlib/benz/include/picrin/object.h
@@ -11,7 +11,7 @@ extern "C" {
 
 KHASH_DECLARE(env, pic_id *, pic_sym *)
 KHASH_DECLARE(dict, pic_sym *, pic_value)
-KHASH_DECLARE(weak, void *, pic_value)
+KHASH_DECLARE(weak, struct pic_object *, pic_value)
 
 struct pic_id {
   union {
@@ -125,11 +125,11 @@ struct pic_port {
 #define pic_pair_ptr(pic, o) ((struct pic_pair *)pic_obj_ptr(o))
 #define pic_vec_ptr(pic, o) ((struct pic_vector *)pic_obj_ptr(o))
 #define pic_dict_ptr(pic, o) ((struct pic_dict *)pic_obj_ptr(o))
+#define pic_weak_ptr(pic, o) ((struct pic_weak *)pic_obj_ptr(o))
 #define pic_data_ptr(pic, o) ((struct pic_data *)pic_obj_ptr(o))
 #define pic_proc_ptr(pic, o) ((struct pic_proc *)pic_obj_ptr(o))
 #define pic_sym_ptr(v) ((pic_sym *)pic_obj_ptr(v))
 #define pic_id_ptr(v) ((pic_id *)pic_obj_ptr(v))
-#define pic_weak_ptr(v) ((struct pic_weak *)pic_obj_ptr(v))
 #define pic_context_ptr(o) ((struct pic_context *)pic_obj_ptr(o))
 #define pic_rec_ptr(v) ((struct pic_record *)pic_obj_ptr(v))
 #define pic_error_ptr(v) ((struct pic_error *)pic_obj_ptr(v))
diff --git a/extlib/benz/include/picrin/state.h b/extlib/benz/include/picrin/state.h
index c5590788..30220021 100644
--- a/extlib/benz/include/picrin/state.h
+++ b/extlib/benz/include/picrin/state.h
@@ -79,8 +79,8 @@ struct pic_state {
 
   khash_t(oblist) oblist;       /* string to symbol */
   int ucnt;
-  struct pic_weak *globals;
-  struct pic_weak *macros;
+  pic_value globals;            /* weak */
+  pic_value macros;             /* weak */
   khash_t(ltable) ltable;
   struct pic_list ireps;        /* chain */
 
diff --git a/extlib/benz/macro.c b/extlib/benz/macro.c
index 436df18f..486362a1 100644
--- a/extlib/benz/macro.c
+++ b/extlib/benz/macro.c
@@ -109,26 +109,26 @@ pic_find_identifier(pic_state *pic, pic_id *id, struct pic_env *env)
 static void
 define_macro(pic_state *pic, pic_sym *uid, pic_value mac)
 {
-  if (pic_weak_has(pic, pic->macros, uid)) {
+  if (pic_weak_has(pic, pic->macros, pic_obj_value(uid))) {
     pic_warnf(pic, "redefining syntax variable: ~s", pic_obj_value(uid));
   }
-  pic_weak_set(pic, pic->macros, uid, mac);
+  pic_weak_set(pic, pic->macros, pic_obj_value(uid), mac);
 }
 
 static pic_value
 find_macro(pic_state *pic, pic_sym *uid)
 {
-  if (! pic_weak_has(pic, pic->macros, uid)) {
+  if (! pic_weak_has(pic, pic->macros, pic_obj_value(uid))) {
     return pic_false_value(pic);
   }
-  return pic_weak_ref(pic, pic->macros, uid);
+  return pic_weak_ref(pic, pic->macros, pic_obj_value(uid));
 }
 
 static void
 shadow_macro(pic_state *pic, pic_sym *uid)
 {
-  if (pic_weak_has(pic, pic->macros, uid)) {
-    pic_weak_del(pic, pic->macros, uid);
+  if (pic_weak_has(pic, pic->macros, pic_obj_value(uid))) {
+    pic_weak_del(pic, pic->macros, pic_obj_value(uid));
   }
 }
 
diff --git a/extlib/benz/proc.c b/extlib/benz/proc.c
index aa7a4ee0..69d7d34d 100644
--- a/extlib/benz/proc.c
+++ b/extlib/benz/proc.c
@@ -182,16 +182,16 @@ pic_get_args(pic_state *pic, const char *format, ...)
 static pic_value
 vm_gref(pic_state *pic, pic_sym *uid)
 {
-  if (! pic_weak_has(pic, pic->globals, uid)) {
+  if (! pic_weak_has(pic, pic->globals, pic_obj_value(uid))) {
     pic_errorf(pic, "uninitialized global variable: %s", pic_str(pic, pic_sym_name(pic, uid)));
   }
-  return pic_weak_ref(pic, pic->globals, uid);
+  return pic_weak_ref(pic, pic->globals, pic_obj_value(uid));
 }
 
 static void
 vm_gset(pic_state *pic, pic_sym *uid, pic_value value)
 {
-  pic_weak_set(pic, pic->globals, uid, value);
+  pic_weak_set(pic, pic->globals, pic_obj_value(uid), value);
 }
 
 static void
@@ -896,7 +896,7 @@ pic_define(pic_state *pic, const char *lib, const char *name, pic_value val)
   if ((uid = pic_find_identifier(pic, (pic_id *)sym, env)) == NULL) {
     uid = pic_add_identifier(pic, (pic_id *)sym, env);
   } else {
-    if (pic_weak_has(pic, pic->globals, uid)) {
+    if (pic_weak_has(pic, pic->globals, pic_obj_value(uid))) {
       pic_warnf(pic, "redefining variable: ~s", pic_obj_value(uid));
     }
   }
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 77763c6a..60ed6d9a 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -259,10 +259,10 @@ pic_open(pic_allocf allocf, void *userdata)
   pic->ucnt = 0;
 
   /* global variables */
-  pic->globals = NULL;
+  pic->globals = pic_make_weak(pic);
 
   /* macros */
-  pic->macros = NULL;
+  pic->macros = pic_make_weak(pic);
 
   /* features */
   pic->features = pic_nil_value(pic);
@@ -354,7 +354,7 @@ pic_open(pic_allocf allocf, void *userdata)
   pic_reader_init(pic);
 
   /* parameter table */
-  pic->ptable = pic_cons(pic, pic_obj_value(pic_make_weak(pic)), pic->ptable);
+  pic->ptable = pic_cons(pic, pic_make_weak(pic), pic->ptable);
 
   /* standard libraries */
   pic_make_library(pic, "picrin.user");
@@ -394,8 +394,8 @@ pic_close(pic_state *pic)
   pic->xp = pic->xpbase;
   pic->arena_idx = 0;
   pic->err = pic_invalid_value();
-  pic->globals = NULL;
-  pic->macros = NULL;
+  pic->globals = pic_invalid_value();
+  pic->macros = pic_invalid_value();
   pic->features = pic_nil_value(pic);
 
   /* free all libraries */
diff --git a/extlib/benz/var.c b/extlib/benz/var.c
index 04a07e5e..6f59c0c7 100644
--- a/extlib/benz/var.c
+++ b/extlib/benz/var.c
@@ -8,13 +8,11 @@
 static pic_value
 var_get(pic_state *pic, pic_value var)
 {
-  pic_value elem, it;
-  struct pic_weak *weak;
+  pic_value weak, it;
 
-  pic_for_each (elem, pic->ptable, it) {
-    weak = pic_weak_ptr(elem);
-    if (pic_weak_has(pic, weak, pic_obj_ptr(var))) {
-      return pic_weak_ref(pic, weak, pic_obj_ptr(var));
+  pic_for_each (weak, pic->ptable, it) {
+    if (pic_weak_has(pic, weak, var)) {
+      return pic_weak_ref(pic, weak, var);
     }
   }
   pic_panic(pic, "logic flaw");
@@ -23,11 +21,11 @@ var_get(pic_state *pic, pic_value var)
 static pic_value
 var_set(pic_state *pic, pic_value var, pic_value val)
 {
-  struct pic_weak *weak;
+  pic_value weak;
 
-  weak = pic_weak_ptr(pic_car(pic, pic->ptable));
+  weak = pic_car(pic, pic->ptable);
 
-  pic_weak_set(pic, weak, pic_obj_ptr(var), val);
+  pic_weak_set(pic, weak, var, val);
 
   return pic_undef_value(pic);
 }
@@ -82,7 +80,7 @@ pic_var_with_parameter(pic_state *pic)
 
   pic_get_args(pic, "l", &body);
 
-  pic->ptable = pic_cons(pic, pic_obj_value(pic_make_weak(pic)), pic->ptable);
+  pic->ptable = pic_cons(pic, pic_make_weak(pic), pic->ptable);
 
   val = pic_call(pic, body, 0);
 
diff --git a/extlib/benz/weak.c b/extlib/benz/weak.c
index 69effdcc..1ba978b1 100644
--- a/extlib/benz/weak.c
+++ b/extlib/benz/weak.c
@@ -5,9 +5,9 @@
 #include "picrin.h"
 #include "picrin/object.h"
 
-KHASH_DEFINE(weak, void *, pic_value, kh_ptr_hash_func, kh_ptr_hash_equal)
+KHASH_DEFINE(weak, struct pic_object *, pic_value, kh_ptr_hash_func, kh_ptr_hash_equal)
 
-struct pic_weak *
+pic_value
 pic_make_weak(pic_state *pic)
 {
   struct pic_weak *weak;
@@ -16,97 +16,59 @@ pic_make_weak(pic_state *pic)
   weak->prev = NULL;
   kh_init(weak, &weak->hash);
 
-  return weak;
+  return pic_obj_value(weak);
 }
 
 pic_value
-pic_weak_ref(pic_state *pic, struct pic_weak *weak, void *key)
+pic_weak_ref(pic_state *pic, pic_value weak, pic_value key)
 {
-  khash_t(weak) *h = &weak->hash;
+  khash_t(weak) *h = &pic_weak_ptr(pic, weak)->hash;
   khiter_t it;
 
-  it = kh_get(weak, h, key);
+  it = kh_get(weak, h, pic_obj_ptr(key));
   if (it == kh_end(h)) {
-    pic_errorf(pic, "element not found for a key: ~s", pic_obj_value(key));
+    pic_errorf(pic, "element not found for a key: ~s", key);
   }
   return kh_val(h, it);
 }
 
-void *
-pic_weak_rev_ref(pic_state *pic, struct pic_weak *weak, pic_value val)
-{
-  khash_t(weak) *h = &weak->hash;
-
-  if (h->n_buckets) {
-    khint_t i = 0;
-    while ((i < h->n_buckets) && (ac_iseither(h->flags, i) || !pic_eq_p(pic, h->vals[i], val))) {
-      i += 1;
-    }
-    if (i < h->n_buckets) return kh_key(h, i);
-  }
-  pic_errorf(pic, "key not found for an element: ~s", val);
-  return NULL;
-}
-
 void
-pic_weak_set(pic_state PIC_UNUSED(*pic), struct pic_weak *weak, void *key, pic_value val)
+pic_weak_set(pic_state *pic, pic_value weak, pic_value key, pic_value val)
 {
-  khash_t(weak) *h = &weak->hash;
+  khash_t(weak) *h = &pic_weak_ptr(pic, weak)->hash;
   int ret;
   khiter_t it;
 
-  it = kh_put(weak, h, key, &ret);
+  it = kh_put(weak, h, pic_obj_ptr(key), &ret);
   kh_val(h, it) = val;
 }
 
 bool
-pic_weak_has(pic_state PIC_UNUSED(*pic), struct pic_weak *weak, void *key)
+pic_weak_has(pic_state *pic, pic_value weak, pic_value key)
 {
-  return kh_get(weak, &weak->hash, key) != kh_end(&weak->hash);
+  khash_t(weak) *h = &pic_weak_ptr(pic, weak)->hash;
+
+  return kh_get(weak, h, pic_obj_ptr(key)) != kh_end(h);
 }
 
 void
-pic_weak_del(pic_state *pic, struct pic_weak *weak, void *key)
+pic_weak_del(pic_state *pic, pic_value weak, pic_value key)
 {
-  khash_t(weak) *h = &weak->hash;
+  khash_t(weak) *h = &pic_weak_ptr(pic, weak)->hash;
   khiter_t it;
 
-  it = kh_get(weak, h, key);
+  it = kh_get(weak, h, pic_obj_ptr(key));
   if (it == kh_end(h)) {
-    pic_errorf(pic, "no slot named ~s found in ephemeron", pic_obj_value(key));
+    pic_errorf(pic, "no slot named ~s found in ephemeron", key);
   }
   kh_del(weak, h, it);
 }
 
 
-static pic_value
-weak_get(pic_state *pic, struct pic_weak *weak, void *key)
-{
-  if (! pic_weak_has(pic, weak, key)) {
-    return pic_false_value(pic);
-  }
-  return pic_cons(pic, pic_obj_value(key), pic_weak_ref(pic, weak, key));
-}
-
-static pic_value
-weak_set(pic_state *pic, struct pic_weak *weak, void *key, pic_value val)
-{
-  if (pic_undef_p(pic, val)) {
-    if (pic_weak_has(pic, weak, key)) {
-      pic_weak_del(pic, weak, key);
-    }
-  } else {
-    pic_weak_set(pic, weak, key, val);
-  }
-
-  return pic_undef_value(pic);
-}
-
 static pic_value
 weak_call(pic_state *pic)
 {
-  struct pic_weak *weak;
-  pic_value key, val;
+  pic_value key, val, weak;
   int n;
 
   n = pic_get_args(pic, "o|o", &key, &val);
@@ -115,12 +77,22 @@ weak_call(pic_state *pic)
     pic_errorf(pic, "attempted to set a non-object key '~s' in an ephemeron", key);
   }
 
-  weak = pic_weak_ptr(pic_closure_ref(pic, 0));
+  weak = pic_closure_ref(pic, 0);
 
   if (n == 1) {
-    return weak_get(pic, weak, pic_obj_ptr(key));
+    if (! pic_weak_has(pic, weak, key)) {
+      return pic_false_value(pic);
+    }
+    return pic_cons(pic, key, pic_weak_ref(pic, weak, key));
   } else {
-    return weak_set(pic, weak, pic_obj_ptr(key), val);
+    if (pic_undef_p(pic, val)) {
+      if (pic_weak_has(pic, weak, key)) {
+        pic_weak_del(pic, weak, key);
+      }
+    } else {
+      pic_weak_set(pic, weak, key, val);
+    }
+    return pic_undef_value(pic);
   }
 }
 
@@ -129,7 +101,7 @@ pic_weak_make_ephemeron(pic_state *pic)
 {
   pic_get_args(pic, "");
 
-  return pic_lambda(pic, weak_call, 1, pic_obj_value(pic_make_weak(pic)));
+  return pic_lambda(pic, weak_call, 1, pic_make_weak(pic));
 }
 
 void