diff --git a/include/picrin/var.h b/include/picrin/var.h
index 69eac2ca..d3bbaf4e 100644
--- a/include/picrin/var.h
+++ b/include/picrin/var.h
@@ -12,12 +12,13 @@ extern "C" {
 struct pic_var {
   PIC_OBJECT_HEADER
   pic_value stack;
+  struct pic_proc *conv;
 };
 
 #define pic_var_p(o) (pic_type(o) == PIC_TT_VAR)
 #define pic_var_ptr(o) ((struct pic_var *)pic_ptr(o))
 
-struct pic_var *pic_var_new(pic_state *, pic_value);
+struct pic_var *pic_var_new(pic_state *, pic_value, struct pic_proc * /* = NULL */);
 
 pic_value pic_var_ref(pic_state *, struct pic_var *);
 void pic_var_set(pic_state *, struct pic_var *, pic_value);
diff --git a/src/gc.c b/src/gc.c
index e673f045..3d28aa96 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -469,6 +469,9 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
   case PIC_TT_VAR: {
     struct pic_var *var = (struct pic_var *)obj;
     gc_mark(pic, var->stack);
+    if (var->conv) {
+      gc_mark_object(pic, (struct pic_object *)var->conv);
+    }
     break;
   }
   case PIC_TT_IREP: {
diff --git a/src/var.c b/src/var.c
index 5bf215b6..a5df8bb0 100644
--- a/src/var.c
+++ b/src/var.c
@@ -7,12 +7,13 @@
 #include "picrin/pair.h"
 
 struct pic_var *
-pic_var_new(pic_state *pic, pic_value init)
+pic_var_new(pic_state *pic, pic_value init, struct pic_proc *conv)
 {
   struct pic_var *var;
 
   var = (struct pic_var *)pic_obj_alloc(pic, sizeof(struct pic_var), PIC_TT_VAR);
   var->stack = pic_nil_value();
+  var->conv = conv;
 
   pic_var_push(pic, var, init);
 
@@ -28,12 +29,18 @@ pic_var_ref(pic_state *pic, struct pic_var *var)
 void
 pic_var_set(pic_state *pic, struct pic_var *var, pic_value value)
 {
+  if (var->conv != NULL) {
+    value = pic_apply1(pic, var->conv, value);
+  }
   pic_set_car(pic, var->stack, value);
 }
 
 void
 pic_var_push(pic_state *pic, struct pic_var *var, pic_value value)
 {
+  if (var->conv != NULL) {
+    value = pic_apply1(pic, var->conv, value);
+  }
   var->stack = pic_cons(pic, value, var->stack);
 }
 
@@ -46,11 +53,12 @@ pic_var_pop(pic_state *pic, struct pic_var *var)
 static pic_value
 pic_var_make_var(pic_state *pic)
 {
+  struct pic_proc *conv = NULL;
   pic_value init;
 
-  pic_get_args(pic, "o", &init);
+  pic_get_args(pic, "o|l", &init, &conv);
 
-  return pic_obj_value(pic_var_new(pic, init));
+  return pic_obj_value(pic_var_new(pic, init, conv));
 }
 
 static pic_value