From 2c9a19acf54fd89c348b6ca66f53e28bf42b368e Mon Sep 17 00:00:00 2001
From: Yuichi Nishiwaki <yuichi.nishiwaki@gmail.com>
Date: Tue, 9 Jun 2015 16:02:23 +0900
Subject: [PATCH 1/4] introduce pic_invalid_value

---
 extlib/benz/debug.c                |  2 +-
 extlib/benz/error.c                |  2 +-
 extlib/benz/gc.c                   |  2 ++
 extlib/benz/include/picrin/value.h | 21 +++++++++++++++++++--
 extlib/benz/read.c                 | 18 +++++++++---------
 extlib/benz/state.c                |  4 ++--
 6 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/extlib/benz/debug.c b/extlib/benz/debug.c
index 2525a656..d61e9380 100644
--- a/extlib/benz/debug.c
+++ b/extlib/benz/debug.c
@@ -35,7 +35,7 @@ pic_get_backtrace(pic_state *pic)
 void
 pic_print_backtrace(pic_state *pic, xFILE *file)
 {
-  assert(! pic_undef_p(pic->err));
+  assert(! pic_invalid_p(pic->err));
 
   if (! pic_error_p(pic->err)) {
     xfprintf(file, "raise: ");
diff --git a/extlib/benz/error.c b/extlib/benz/error.c
index 15e34087..6fa5309d 100644
--- a/extlib/benz/error.c
+++ b/extlib/benz/error.c
@@ -52,7 +52,7 @@ pic_errmsg(pic_state *pic)
 {
   pic_str *str;
 
-  assert(! pic_undef_p(pic->err));
+  assert(! pic_invalid_p(pic->err));
 
   if (! pic_error_p(pic->err)) {
     str = pic_format(pic, "~s", pic->err);
diff --git a/extlib/benz/gc.c b/extlib/benz/gc.c
index f7056d94..65bf219d 100644
--- a/extlib/benz/gc.c
+++ b/extlib/benz/gc.c
@@ -482,6 +482,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
   case PIC_TT_CHAR:
   case PIC_TT_EOF:
   case PIC_TT_UNDEF:
+  case PIC_TT_INVALID:
     pic_panic(pic, "logic flaw");
   }
 }
@@ -694,6 +695,7 @@ gc_finalize_object(pic_state *pic, struct pic_object *obj)
   case PIC_TT_CHAR:
   case PIC_TT_EOF:
   case PIC_TT_UNDEF:
+  case PIC_TT_INVALID:
     pic_panic(pic, "logic flaw");
   }
 }
diff --git a/extlib/benz/include/picrin/value.h b/extlib/benz/include/picrin/value.h
index 02b06293..9790926b 100644
--- a/extlib/benz/include/picrin/value.h
+++ b/extlib/benz/include/picrin/value.h
@@ -10,8 +10,8 @@ extern "C" {
 #endif
 
 /**
- * `undef` values never seen from user-end: that is,
- *  it's used only for repsenting internal special state
+ * `invalid` value will never be seen from user-end:
+ *  it is only used for repsenting internal special state
  */
 
 enum pic_vtype {
@@ -19,6 +19,7 @@ enum pic_vtype {
   PIC_VTYPE_TRUE,
   PIC_VTYPE_FALSE,
   PIC_VTYPE_UNDEF,
+  PIC_VTYPE_INVALID,
 #if PIC_ENABLE_FLOAT
   PIC_VTYPE_FLOAT,
 #endif
@@ -146,6 +147,7 @@ enum pic_tt {
   PIC_TT_CHAR,
   PIC_TT_EOF,
   PIC_TT_UNDEF,
+  PIC_TT_INVALID,
   /* heap */
   PIC_TT_SYMBOL,
   PIC_TT_PAIR,
@@ -196,6 +198,7 @@ typedef struct pic_blob pic_blob;
 #define pic_true_p(v) (pic_vtype(v) == PIC_VTYPE_TRUE)
 #define pic_false_p(v) (pic_vtype(v) == PIC_VTYPE_FALSE)
 #define pic_undef_p(v) (pic_vtype(v) == PIC_VTYPE_UNDEF)
+#define pic_invalid_p(v) (pic_vtype(v) == PIC_VTYPE_INVALID)
 #define pic_float_p(v) (pic_vtype(v) == PIC_VTYPE_FLOAT)
 #define pic_int_p(v) (pic_vtype(v) == PIC_VTYPE_INT)
 #define pic_char_p(v) (pic_vtype(v) == PIC_VTYPE_CHAR)
@@ -231,6 +234,7 @@ PIC_INLINE pic_value pic_true_value();
 PIC_INLINE pic_value pic_false_value();
 PIC_INLINE pic_value pic_bool_value(bool);
 PIC_INLINE pic_value pic_undef_value();
+PIC_INLINE pic_value pic_invalid_value();
 PIC_INLINE pic_value pic_obj_value(void *);
 #if PIC_ENABLE_FLOAT
 PIC_INLINE pic_value pic_float_value(double);
@@ -255,6 +259,8 @@ pic_type(pic_value v)
     return PIC_TT_BOOL;
   case PIC_VTYPE_UNDEF:
     return PIC_TT_UNDEF;
+  case PIC_VTYPE_INVALID:
+    return PIC_TT_INVALID;
 #if PIC_ENABLE_FLOAT
   case PIC_VTYPE_FLOAT:
     return PIC_TT_FLOAT;
@@ -294,6 +300,8 @@ pic_type_repr(enum pic_tt tt)
     return "eof";
   case PIC_TT_UNDEF:
     return "undef";
+  case PIC_TT_INVALID:
+    return "invalid";
   case PIC_TT_PAIR:
     return "pair";
   case PIC_TT_STRING:
@@ -500,6 +508,15 @@ pic_undef_value()
   return v;
 }
 
+PIC_INLINE pic_value
+pic_invalid_value()
+{
+  pic_value v;
+
+  pic_init_value(v, PIC_VTYPE_INVALID);
+  return v;
+}
+
 PIC_INLINE pic_value
 pic_none_value()
 {
diff --git a/extlib/benz/read.c b/extlib/benz/read.c
index 50498ac2..dc7a28a5 100644
--- a/extlib/benz/read.c
+++ b/extlib/benz/read.c
@@ -92,7 +92,7 @@ read_comment(pic_state PIC_UNUSED(*pic), struct pic_port *port, int c)
     c = next(port);
   } while (! (c == EOF || c == '\n'));
 
-  return pic_undef_value();
+  return pic_invalid_value();
 }
 
 static pic_value
@@ -114,7 +114,7 @@ read_block_comment(pic_state PIC_UNUSED(*pic), struct pic_port *port, int PIC_UN
     }
   }
 
-  return pic_undef_value();
+  return pic_invalid_value();
 }
 
 static pic_value
@@ -122,7 +122,7 @@ read_datum_comment(pic_state *pic, struct pic_port *port, int PIC_UNUSED(c))
 {
   read(pic, port, next(port));
 
-  return pic_undef_value();
+  return pic_invalid_value();
 }
 
 static pic_value
@@ -132,13 +132,13 @@ read_directive(pic_state *pic, struct pic_port *port, int c)
   case 'n':
     if (expect(port, "no-fold-case")) {
       pic->reader->typecase = PIC_CASE_DEFAULT;
-      return pic_undef_value();
+      return pic_invalid_value();
     }
     break;
   case 'f':
     if (expect(port, "fold-case")) {
       pic->reader->typecase = PIC_CASE_FOLD;
-      return pic_undef_value();
+      return pic_invalid_value();
     }
     break;
   }
@@ -578,7 +578,7 @@ read_pair(pic_state *pic, struct pic_port *port, int c)
 
   closing:
     if ((c = skip(port, ' ')) != tCLOSE) {
-      if (pic_undef_p(read_nullable(pic, port, c))) {
+      if (pic_invalid_p(read_nullable(pic, port, c))) {
         goto closing;
       }
       read_error(pic, "unmatched parenthesis");
@@ -588,7 +588,7 @@ read_pair(pic_state *pic, struct pic_port *port, int c)
   else {
     car = read_nullable(pic, port, c);
 
-    if (pic_undef_p(car)) {
+    if (pic_invalid_p(car)) {
       goto retry;
     }
 
@@ -742,7 +742,7 @@ read(pic_state *pic, struct pic_port *port, int c)
  retry:
   val = read_nullable(pic, port, c);
 
-  if (pic_undef_p(val)) {
+  if (pic_invalid_p(val)) {
     c = next(port);
     goto retry;
   }
@@ -840,7 +840,7 @@ pic_read(pic_state *pic, struct pic_port *port)
 
   val = read_nullable(pic, port, c);
 
-  if (pic_undef_p(val)) {
+  if (pic_invalid_p(val)) {
     c = next(port);
     goto retry;
   }
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 75b2df9b..aa60da35 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -223,7 +223,7 @@ pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
   pic->lib = NULL;
 
   /* raised error object */
-  pic->err = pic_undef_value();
+  pic->err = pic_invalid_value();
 
   /* standard ports */
   pic->xSTDIN = NULL;
@@ -402,7 +402,7 @@ pic_close(pic_state *pic)
   pic->ci = pic->cibase;
   pic->xp = pic->xpbase;
   pic->arena_idx = 0;
-  pic->err = pic_undef_value();
+  pic->err = pic_invalid_value();
   pic->globals = NULL;
   pic->macros = NULL;
   xh_clear(&pic->syms);

From 084d39cd9ec851d6dd8708dfc3b45a5351b00189 Mon Sep 17 00:00:00 2001
From: Yuichi Nishiwaki <yuichi.nishiwaki@gmail.com>
Date: Tue, 9 Jun 2015 16:19:57 +0900
Subject: [PATCH 2/4] let undefined be public API

---
 extlib/benz/read.c  | 17 ++++++++++++++++-
 extlib/benz/state.c |  2 ++
 extlib/benz/undef.c | 21 +++++++++++++++++++++
 extlib/benz/write.c |  2 +-
 4 files changed, 40 insertions(+), 2 deletions(-)
 create mode 100644 extlib/benz/undef.c

diff --git a/extlib/benz/read.c b/extlib/benz/read.c
index dc7a28a5..a43b28de 100644
--- a/extlib/benz/read.c
+++ b/extlib/benz/read.c
@@ -560,6 +560,21 @@ read_blob(pic_state *pic, struct pic_port *port, int c)
   return pic_obj_value(blob);
 }
 
+static pic_value
+read_undef_or_blob(pic_state *pic, struct pic_port *port, int c)
+{
+  if ((c = peek(port)) == 'n') {
+    if (! expect(port, "ndefined")) {
+      read_error(pic, "unexpected character while reading #undefined");
+    }
+    return pic_undef_value();
+  }
+  if (! isdigit(c)) {
+    read_error(pic, "expect #undefined or #u8(...), but illegal character given");
+  }
+  return read_blob(pic, port, 'u');
+}
+
 static pic_value
 read_pair(pic_state *pic, struct pic_port *port, int c)
 {
@@ -786,7 +801,7 @@ reader_table_init(struct pic_reader *reader)
   reader->dispatch['f'] = read_false;
   reader->dispatch['\\'] = read_char;
   reader->dispatch['('] = read_vector;
-  reader->dispatch['u'] = read_blob;
+  reader->dispatch['u'] = read_undef_or_blob;
   reader->dispatch['.'] = read_eval;
 
   /* read labels */
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index aa60da35..a555dc34 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -10,6 +10,7 @@ pic_add_feature(pic_state *pic, const char *feature)
   pic_push(pic, pic_obj_value(pic_intern_cstr(pic, feature)), pic->features);
 }
 
+void pic_init_undef(pic_state *);
 void pic_init_bool(pic_state *);
 void pic_init_pair(pic_state *);
 void pic_init_port(pic_state *);
@@ -107,6 +108,7 @@ pic_init_core(pic_state *pic)
     pic_define_syntactic_keyword(pic, pic->lib->env, pic->sBEGIN, pic->rBEGIN);
     pic_define_syntactic_keyword(pic, pic->lib->env, pic->sDEFINE_SYNTAX, pic->rDEFINE_SYNTAX);
 
+    pic_init_undef(pic); DONE;
     pic_init_bool(pic); DONE;
     pic_init_pair(pic); DONE;
     pic_init_port(pic); DONE;
diff --git a/extlib/benz/undef.c b/extlib/benz/undef.c
new file mode 100644
index 00000000..9e709c0e
--- /dev/null
+++ b/extlib/benz/undef.c
@@ -0,0 +1,21 @@
+/**
+ * See Copyright Notice in picrin.h
+ */
+
+#include "picrin.h"
+
+static pic_value
+pic_undef_undefined_p(pic_state *pic)
+{
+  pic_value v;
+
+  pic_get_args(pic, "o", &v);
+
+  return pic_undef_p(v) ? pic_true_value() : pic_false_value();
+}
+
+void
+pic_init_undef(pic_state *pic)
+{
+  pic_defun(pic, "undefined?", pic_undef_undefined_p);
+}
diff --git a/extlib/benz/write.c b/extlib/benz/write.c
index d2c2a807..32aa3e8c 100644
--- a/extlib/benz/write.c
+++ b/extlib/benz/write.c
@@ -220,7 +220,7 @@ write_core(struct writer_control *p, pic_value obj)
 
   switch (pic_type(obj)) {
   case PIC_TT_UNDEF:
-    xfprintf(file, "#<undef>");
+    xfprintf(file, "#undefined");
     break;
   case PIC_TT_NIL:
     xfprintf(file, "()");

From 0cafbfdb0c6b4798b859097674c53b3d7570f1b7 Mon Sep 17 00:00:00 2001
From: Yuichi Nishiwaki <yuichi.nishiwaki@gmail.com>
Date: Tue, 9 Jun 2015 16:21:15 +0900
Subject: [PATCH 3/4] update picrin/base.scm

---
 piclib/picrin/base.scm | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/piclib/picrin/base.scm b/piclib/picrin/base.scm
index baf00023..2aa6a42b 100644
--- a/piclib/picrin/base.scm
+++ b/piclib/picrin/base.scm
@@ -37,6 +37,8 @@
           eqv?
           equal?)
 
+  (export undefined?)
+
   (export boolean?
           boolean=?
           not)

From 5185a73cd2fddd99cb9d3dd725d1b43e2d95c28f Mon Sep 17 00:00:00 2001
From: Yuichi Nishiwaki <yuichi.nishiwaki@gmail.com>
Date: Tue, 9 Jun 2015 16:34:45 +0900
Subject: [PATCH 4/4] replace none with undef

---
 contrib/05.r7rs/src/file.c           |  2 +-
 contrib/05.r7rs/src/load.c           |  2 +-
 contrib/05.r7rs/src/mutable-string.c |  6 +++---
 extlib/benz/blob.c                   |  6 +++---
 extlib/benz/codegen.c                | 20 +++++++-------------
 extlib/benz/cont.c                   |  4 ++--
 extlib/benz/dict.c                   |  8 ++++----
 extlib/benz/include/picrin/config.h  |  7 -------
 extlib/benz/include/picrin/irep.h    |  4 ++++
 extlib/benz/include/picrin/value.h   | 11 -----------
 extlib/benz/lib.c                    | 10 +++++-----
 extlib/benz/macro.c                  |  4 ++--
 extlib/benz/pair.c                   | 10 +++++-----
 extlib/benz/port.c                   | 14 +++++++-------
 extlib/benz/read.c                   |  2 +-
 extlib/benz/record.c                 |  2 +-
 extlib/benz/string.c                 |  2 +-
 extlib/benz/var.c                    |  2 +-
 extlib/benz/vector.c                 | 12 ++++++------
 extlib/benz/vm.c                     | 10 +++++++---
 extlib/benz/write.c                  |  8 ++++----
 21 files changed, 65 insertions(+), 81 deletions(-)

diff --git a/contrib/05.r7rs/src/file.c b/contrib/05.r7rs/src/file.c
index 4147bd84..ce9cb1b2 100644
--- a/contrib/05.r7rs/src/file.c
+++ b/contrib/05.r7rs/src/file.c
@@ -104,7 +104,7 @@ pic_file_delete(pic_state *pic)
   if (remove(fname) != 0) {
     file_error(pic, "file cannot be deleted");
   }
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/contrib/05.r7rs/src/load.c b/contrib/05.r7rs/src/load.c
index 5efb71c7..c887a1b2 100644
--- a/contrib/05.r7rs/src/load.c
+++ b/contrib/05.r7rs/src/load.c
@@ -35,7 +35,7 @@ pic_load_load(pic_state *pic)
 
   pic_load(pic, fn);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/contrib/05.r7rs/src/mutable-string.c b/contrib/05.r7rs/src/mutable-string.c
index 6937b0f6..85db9be0 100644
--- a/contrib/05.r7rs/src/mutable-string.c
+++ b/contrib/05.r7rs/src/mutable-string.c
@@ -30,7 +30,7 @@ pic_str_string_set(pic_state *pic)
   pic_get_args(pic, "sic", &str, &k, &c);
 
   pic_str_set(pic, str, k, c);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -54,7 +54,7 @@ pic_str_string_copy_ip(pic_state *pic)
   while (start < end) {
     pic_str_set(pic, to, at++, pic_str_ref(pic, from, start++));
   }
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -76,7 +76,7 @@ pic_str_string_fill_ip(pic_state *pic)
   while (start < end) {
     pic_str_set(pic, str, start++, c);
   }
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/extlib/benz/blob.c b/extlib/benz/blob.c
index 59791ee9..c2775ea3 100644
--- a/extlib/benz/blob.c
+++ b/extlib/benz/blob.c
@@ -105,7 +105,7 @@ pic_blob_bytevector_u8_set(pic_state *pic)
     pic_errorf(pic, "byte out of range");
 
   bv->data[k] = (unsigned char)v;
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -130,14 +130,14 @@ pic_blob_bytevector_copy_i(pic_state *pic)
     while (start < end) {
       to->data[--at] = from->data[--end];
     }
-    return pic_none_value();
+    return pic_undef_value();
   }
 
   while (start < end) {
     to->data[at++] = from->data[start++];
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/codegen.c b/extlib/benz/codegen.c
index ff53f12b..79d4126c 100644
--- a/extlib/benz/codegen.c
+++ b/extlib/benz/codegen.c
@@ -4,12 +4,6 @@
 
 #include "picrin.h"
 
-#if PIC_NONE_IS_FALSE
-# define OP_PUSHNONE OP_PUSHFALSE
-#else
-# error enable PIC_NONE_IS_FALSE
-#endif
-
 typedef xvect_t(pic_sym *) xvect;
 
 #define xv_push_sym(v, x) xv_push(pic_sym *, (v), (x))
@@ -428,7 +422,7 @@ analyze_if(analyze_state *state, pic_value obj, bool tailpos)
   pic_state *pic = state->pic;
   pic_value cond, if_true, if_false;
 
-  if_false = pic_none_value();
+  if_false = pic_undef_value();
   switch (pic_length(pic, obj)) {
   default:
     pic_errorf(pic, "syntax error");
@@ -456,7 +450,7 @@ analyze_begin(analyze_state *state, pic_value obj, bool tailpos)
 
   switch (pic_length(pic, obj)) {
   case 1:
-    return analyze(state, pic_none_value(), tailpos);
+    return analyze(state, pic_undef_value(), tailpos);
   case 2:
     return analyze(state, pic_list_ref(pic, obj, 1), tailpos);
   default:
@@ -965,7 +959,7 @@ create_activation(codegen_state *state)
       emit_i(state, OP_LREF, (int)n);
     } else {
       /* otherwise, just extend the stack */
-      emit_n(state, OP_PUSHNONE);
+      emit_n(state, OP_PUSHUNDEF);
     }
   }
 
@@ -1158,7 +1152,7 @@ codegen(codegen_state *state, pic_value obj)
     type = pic_sym_ptr(pic_list_ref(pic, var, 0));
     if (type == pic->sGREF) {
       emit_i(state, OP_GSET, index_symbol(state, pic_sym_ptr(pic_list_ref(pic, var, 1))));
-      emit_n(state, OP_PUSHNONE);
+      emit_n(state, OP_PUSHUNDEF);
       return;
     }
     else if (type == pic->sCREF) {
@@ -1168,7 +1162,7 @@ codegen(codegen_state *state, pic_value obj)
       depth = pic_int(pic_list_ref(pic, var, 1));
       name  = pic_sym_ptr(pic_list_ref(pic, var, 2));
       emit_r(state, OP_CSET, depth, index_capture(state, name, depth));
-      emit_n(state, OP_PUSHNONE);
+      emit_n(state, OP_PUSHUNDEF);
       return;
     }
     else if (type == pic->sLREF) {
@@ -1178,11 +1172,11 @@ codegen(codegen_state *state, pic_value obj)
       name = pic_sym_ptr(pic_list_ref(pic, var, 1));
       if ((i = index_capture(state, name, 0)) != -1) {
         emit_i(state, OP_LSET, i + (int)xv_size(cxt->args) + (int)xv_size(cxt->locals) + 1);
-        emit_n(state, OP_PUSHNONE);
+        emit_n(state, OP_PUSHUNDEF);
         return;
       }
       emit_i(state, OP_LSET, index_local(state, name));
-      emit_n(state, OP_PUSHNONE);
+      emit_n(state, OP_PUSHUNDEF);
       return;
     }
   }
diff --git a/extlib/benz/cont.c b/extlib/benz/cont.c
index 72a9b2cb..4b213f52 100644
--- a/extlib/benz/cont.c
+++ b/extlib/benz/cont.c
@@ -197,7 +197,7 @@ pic_values_by_array(pic_state *pic, size_t argc, pic_value *argv)
   }
   pic->ci->retc = (int)argc;
 
-  return argc == 0 ? pic_none_value() : pic->sp[0];
+  return argc == 0 ? pic_undef_value() : pic->sp[0];
 }
 
 pic_value
@@ -212,7 +212,7 @@ pic_values_by_list(pic_state *pic, pic_value list)
   }
   pic->ci->retc = i;
 
-  return pic_nil_p(list) ? pic_none_value() : pic->sp[0];
+  return pic_nil_p(list) ? pic_undef_value() : pic->sp[0];
 }
 
 size_t
diff --git a/extlib/benz/dict.c b/extlib/benz/dict.c
index b802fe06..8a3d0ce7 100644
--- a/extlib/benz/dict.c
+++ b/extlib/benz/dict.c
@@ -107,7 +107,7 @@ pic_dict_dictionary_ref(pic_state *pic)
   if (pic_dict_has(pic, dict, key)) {
     return pic_values2(pic, pic_dict_ref(pic, dict, key), pic_true_value());
   } else {
-    return pic_values2(pic, pic_none_value(), pic_false_value());
+    return pic_values2(pic, pic_undef_value(), pic_false_value());
   }
 }
 
@@ -122,7 +122,7 @@ pic_dict_dictionary_set(pic_state *pic)
 
   pic_dict_set(pic, dict, key, val);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -135,7 +135,7 @@ pic_dict_dictionary_del(pic_state *pic)
 
   pic_dict_del(pic, dict, key);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -238,7 +238,7 @@ pic_dict_dictionary_for_each(pic_state *pic)
 
   pic_free(pic, it);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/include/picrin/config.h b/extlib/benz/include/picrin/config.h
index 4b98679d..b30bc398 100644
--- a/extlib/benz/include/picrin/config.h
+++ b/extlib/benz/include/picrin/config.h
@@ -17,9 +17,6 @@
 /** no dependency on libc */
 /* #define PIC_ENABLE_LIBC 1 */
 
-/** treat false value as none */
-/* #define PIC_NONE_IS_FALSE 1 */
-
 /** custom setjmp/longjmp */
 /* #define PIC_JMPBUF jmp_buf */
 /* #define PIC_SETJMP(pic, buf) setjmp(buf) */
@@ -96,10 +93,6 @@
 # error cannot disable float support when nan boxing is on
 #endif
 
-#ifndef PIC_NONE_IS_FALSE
-# define PIC_NONE_IS_FALSE 1
-#endif
-
 #ifndef PIC_JMPBUF
 # include <setjmp.h>
 # define PIC_JMPBUF jmp_buf
diff --git a/extlib/benz/include/picrin/irep.h b/extlib/benz/include/picrin/irep.h
index 213d0456..319d1b31 100644
--- a/extlib/benz/include/picrin/irep.h
+++ b/extlib/benz/include/picrin/irep.h
@@ -12,6 +12,7 @@ extern "C" {
 enum pic_opcode {
   OP_NOP,
   OP_POP,
+  OP_PUSHUNDEF,
   OP_PUSHNIL,
   OP_PUSHTRUE,
   OP_PUSHFALSE,
@@ -93,6 +94,9 @@ pic_dump_code(pic_code c)
   case OP_POP:
     puts("OP_POP");
     break;
+  case OP_PUSHUNDEF:
+    puts("OP_PUSHUNDEF");
+    break;
   case OP_PUSHNIL:
     puts("OP_PUSHNIL");
     break;
diff --git a/extlib/benz/include/picrin/value.h b/extlib/benz/include/picrin/value.h
index 9790926b..c69e09b0 100644
--- a/extlib/benz/include/picrin/value.h
+++ b/extlib/benz/include/picrin/value.h
@@ -242,7 +242,6 @@ PIC_INLINE pic_value pic_float_value(double);
 PIC_INLINE pic_value pic_int_value(int);
 PIC_INLINE pic_value pic_size_value(size_t);
 PIC_INLINE pic_value pic_char_value(char c);
-PIC_INLINE pic_value pic_none_value();
 
 PIC_INLINE bool pic_eq_p(pic_value, pic_value);
 PIC_INLINE bool pic_eqv_p(pic_value, pic_value);
@@ -517,16 +516,6 @@ pic_invalid_value()
   return v;
 }
 
-PIC_INLINE pic_value
-pic_none_value()
-{
-#if PIC_NONE_IS_FALSE
-  return pic_false_value();
-#else
-# error enable PIC_NONE_IS_FALSE
-#endif
-}
-
 #if PIC_NAN_BOXING || PIC_WORD_BOXING
 
 PIC_INLINE bool
diff --git a/extlib/benz/lib.c b/extlib/benz/lib.c
index 8eff0bb7..6ece6a1f 100644
--- a/extlib/benz/lib.c
+++ b/extlib/benz/lib.c
@@ -246,7 +246,7 @@ pic_lib_condexpand(pic_state *pic)
     }
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -261,7 +261,7 @@ pic_lib_import(pic_state *pic)
     import(pic, argv[i]);
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -276,7 +276,7 @@ pic_lib_export(pic_state *pic)
     export(pic, argv[i]);
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -306,7 +306,7 @@ pic_lib_define_library(pic_state *pic)
     pic_raise(pic, pic->err);
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -318,7 +318,7 @@ pic_lib_in_library(pic_state *pic)
 
   pic_in_library(pic, spec);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/extlib/benz/macro.c b/extlib/benz/macro.c
index ed7625fd..fbda5faf 100644
--- a/extlib/benz/macro.c
+++ b/extlib/benz/macro.c
@@ -105,7 +105,7 @@ macroexpand_list(pic_state *pic, pic_value obj, struct pic_env *env)
 static pic_value
 macroexpand_defer(pic_state *pic, pic_value expr, struct pic_env *env)
 {
-  pic_value skel = pic_list1(pic, pic_none_value()); /* (#<none>) */
+  pic_value skel = pic_list1(pic, pic_invalid_value()); /* (#<invalid>) */
 
   pic_push(pic, pic_cons(pic, expr, skel), env->defer);
 
@@ -238,7 +238,7 @@ macroexpand_defsyntax(pic_state *pic, pic_value expr, struct pic_env *env)
 
   define_macro(pic, rename, pic_proc_ptr(val));
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/pair.c b/extlib/benz/pair.c
index c0b031af..b3da3b6d 100644
--- a/extlib/benz/pair.c
+++ b/extlib/benz/pair.c
@@ -492,7 +492,7 @@ pic_pair_set_car(pic_state *pic)
 
   pic_set_car(pic, v, w);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -504,7 +504,7 @@ pic_pair_set_cdr(pic_state *pic)
 
   pic_set_cdr(pic, v, w);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -531,7 +531,7 @@ static pic_value
 pic_pair_make_list(pic_state *pic)
 {
   size_t i;
-  pic_value fill = pic_none_value();
+  pic_value fill = pic_undef_value();
 
   pic_get_args(pic, "k|o", &i, &fill);
 
@@ -621,7 +621,7 @@ pic_pair_list_set(pic_state *pic)
 
   pic_list_set(pic, list, i, obj);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -692,7 +692,7 @@ pic_pair_for_each(pic_state *pic)
     pic_apply(pic, proc, pic_reverse(pic, arg));
   } while (1);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/port.c b/extlib/benz/port.c
index ab5c39d8..5b04f89b 100644
--- a/extlib/benz/port.c
+++ b/extlib/benz/port.c
@@ -327,7 +327,7 @@ pic_port_close_port(pic_state *pic)
 
   pic_close_port(pic, port);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 #define assert_port_profile(port, flgs, stat, caller) do {              \
@@ -675,7 +675,7 @@ pic_port_newline(pic_state *pic)
   assert_port_profile(port, PIC_PORT_OUT | PIC_PORT_TEXT, PIC_PORT_OPEN, "newline");
 
   xfputs("\n", port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -689,7 +689,7 @@ pic_port_write_char(pic_state *pic)
   assert_port_profile(port, PIC_PORT_OUT | PIC_PORT_TEXT, PIC_PORT_OPEN, "write-char");
 
   xfputc(c, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -714,7 +714,7 @@ pic_port_write_string(pic_state *pic)
   for (i = start; i < end && str[i] != '\0'; ++i) {
     xfputc(str[i], port->file);
   }
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -728,7 +728,7 @@ pic_port_write_byte(pic_state *pic)
   assert_port_profile(port, PIC_PORT_OUT | PIC_PORT_BINARY, PIC_PORT_OPEN, "write-u8");
 
   xfputc(i, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -754,7 +754,7 @@ pic_port_write_blob(pic_state *pic)
   for (i = start; i < end; ++i) {
     xfputc(blob->data[i], port->file);
   }
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -767,7 +767,7 @@ pic_port_flush(pic_state *pic)
   assert_port_profile(port, PIC_PORT_OUT, PIC_PORT_OPEN, "flush-output-port");
 
   xfflush(port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/extlib/benz/read.c b/extlib/benz/read.c
index a43b28de..8320af38 100644
--- a/extlib/benz/read.c
+++ b/extlib/benz/read.c
@@ -633,7 +633,7 @@ read_label_set(pic_state *pic, struct pic_port *port, int i)
     {
       pic_value tmp;
 
-      val = pic_cons(pic, pic_none_value(), pic_none_value());
+      val = pic_cons(pic, pic_undef_value(), pic_undef_value());
 
       xh_put_int(&pic->reader->labels, i, &val);
 
diff --git a/extlib/benz/record.c b/extlib/benz/record.c
index dc35e223..55c98f14 100644
--- a/extlib/benz/record.c
+++ b/extlib/benz/record.c
@@ -96,7 +96,7 @@ pic_record_record_set(pic_state *pic)
 
   pic_record_set(pic, rec, slot, val);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void
diff --git a/extlib/benz/string.c b/extlib/benz/string.c
index ee1f3527..1e1e083c 100644
--- a/extlib/benz/string.c
+++ b/extlib/benz/string.c
@@ -646,7 +646,7 @@ pic_str_string_for_each(pic_state *pic)
     pic_apply(pic, proc, vals);
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/var.c b/extlib/benz/var.c
index 08e1fb89..95da6b16 100644
--- a/extlib/benz/var.c
+++ b/extlib/benz/var.c
@@ -43,7 +43,7 @@ var_set(pic_state *pic, struct pic_proc *var, pic_value val)
 
   pic_dict_set(pic, dict, id, val);
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/vector.c b/extlib/benz/vector.c
index 08d2f72a..c3b914c1 100644
--- a/extlib/benz/vector.c
+++ b/extlib/benz/vector.c
@@ -14,7 +14,7 @@ pic_make_vec(pic_state *pic, size_t len)
   vec->len = len;
   vec->data = (pic_value *)pic_malloc(pic, sizeof(pic_value) * len);
   for (i = 0; i < len; ++i) {
-    vec->data[i] = pic_none_value();
+    vec->data[i] = pic_undef_value();
   }
   return vec;
 }
@@ -119,7 +119,7 @@ pic_vec_vector_set(pic_state *pic)
     pic_errorf(pic, "vector-set!: index out of range");
   }
   v->data[k] = o;
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -144,14 +144,14 @@ pic_vec_vector_copy_i(pic_state *pic)
     while (start < end) {
       to->data[--at] = from->data[--end];
     }
-    return pic_none_value();
+    return pic_undef_value();
   }
 
   while (start < end) {
     to->data[at++] = from->data[start++];
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -231,7 +231,7 @@ pic_vec_vector_fill_i(pic_state *pic)
     vec->data[start++] = obj;
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -292,7 +292,7 @@ pic_vec_vector_for_each(pic_state *pic)
     pic_apply(pic, proc, vals);
   }
 
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
diff --git a/extlib/benz/vm.c b/extlib/benz/vm.c
index 17bf655f..825b63fd 100644
--- a/extlib/benz/vm.c
+++ b/extlib/benz/vm.c
@@ -682,8 +682,8 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value args)
 
 #if PIC_DIRECT_THREADED_VM
   static void *oplabels[] = {
-    &&L_OP_NOP, &&L_OP_POP, &&L_OP_PUSHNIL, &&L_OP_PUSHTRUE, &&L_OP_PUSHFALSE,
-    &&L_OP_PUSHINT, &&L_OP_PUSHCHAR, &&L_OP_PUSHCONST,
+    &&L_OP_NOP, &&L_OP_POP, &&L_OP_PUSHUNDEF, &&L_OP_PUSHNIL, &&L_OP_PUSHTRUE,
+    &&L_OP_PUSHFALSE, &&L_OP_PUSHINT, &&L_OP_PUSHCHAR, &&L_OP_PUSHCONST,
     &&L_OP_GREF, &&L_OP_GSET, &&L_OP_LREF, &&L_OP_LSET, &&L_OP_CREF, &&L_OP_CSET,
     &&L_OP_JMP, &&L_OP_JMPIF, &&L_OP_NOT, &&L_OP_CALL, &&L_OP_TAILCALL, &&L_OP_RET,
     &&L_OP_LAMBDA, &&L_OP_CONS, &&L_OP_CAR, &&L_OP_CDR, &&L_OP_NILP,
@@ -729,6 +729,10 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value args)
       (void)(POP());
       NEXT;
     }
+    CASE(OP_PUSHUNDEF) {
+      PUSH(pic_undef_value());
+      NEXT;
+    }
     CASE(OP_PUSHNIL) {
       PUSH(pic_nil_value());
       NEXT;
@@ -1197,7 +1201,7 @@ pic_apply_trampoline(pic_state *pic, struct pic_proc *proc, pic_value args)
   ci->retc = (int)pic_length(pic, args);
 
   if (ci->retc == 0) {
-    return pic_none_value();
+    return pic_undef_value();
   } else {
     return pic_car(pic, args);
   }
diff --git a/extlib/benz/write.c b/extlib/benz/write.c
index 32aa3e8c..73ee11f5 100644
--- a/extlib/benz/write.c
+++ b/extlib/benz/write.c
@@ -449,7 +449,7 @@ pic_write_write(pic_state *pic)
 
   pic_get_args(pic, "o|p", &v, &port);
   write(pic, v, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -460,7 +460,7 @@ pic_write_write_simple(pic_state *pic)
 
   pic_get_args(pic, "o|p", &v, &port);
   write_simple(pic, v, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -471,7 +471,7 @@ pic_write_write_shared(pic_state *pic)
 
   pic_get_args(pic, "o|p", &v, &port);
   write_shared(pic, v, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 static pic_value
@@ -482,7 +482,7 @@ pic_write_display(pic_state *pic)
 
   pic_get_args(pic, "o|p", &v, &port);
   display(pic, v, port->file);
-  return pic_none_value();
+  return pic_undef_value();
 }
 
 void