diff --git a/include/picrin.h b/include/picrin.h
index bfb72de7..fb5d964d 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -19,4 +19,6 @@ pic_value pic_cdr(pic_state *, pic_value);
 
 pic_value pic_intern_cstr(pic_state *, const char *);
 
+void pic_debug(pic_state *, pic_value);
+
 #endif
diff --git a/include/picrin/value.h b/include/picrin/value.h
index 86146585..85cb863a 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -14,6 +14,9 @@ typedef struct {
 } pic_value;
 
 enum pic_tt {
+  /* immediate */
+  PIC_TT_NIL,
+  /* heap */
   PIC_TT_PAIR,
   PIC_TT_SYMBOL
 };
@@ -36,6 +39,11 @@ struct pic_symbol {
   char *name;
 };
 
+#define pic_pair_ptr(o) ((struct pic_pair *)o->u->data)
+#define pic_symbol_ptr(o) ((struct pic_symbol *)o->u->data)
+
+enum pic_tt pic_type(pic_value v);
+
 pic_value pic_nil_value();
 pic_value pic_obj_value(struct pic_object *obj);
 
diff --git a/src/value.c b/src/value.c
index 1c60e3ce..b06007a1 100644
--- a/src/value.c
+++ b/src/value.c
@@ -1,5 +1,16 @@
 #include "picrin.h"
 
+enum pic_tt
+pic_type(pic_value v)
+{
+  switch (v->type) {
+  case PIC_VTYPE_NIL:
+    return PIC_TT_NIL;
+  case PIC_VTYPE_HEAP:
+    return v->u->data->tt;
+  }
+}
+
 pic_value
 pic_nil_value()
 {
diff --git a/src/write.c b/src/write.c
new file mode 100644
index 00000000..bc4483e3
--- /dev/null
+++ b/src/write.c
@@ -0,0 +1,26 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "picrin.h"
+
+void
+pic_debug(pic_state *pic, pic_value obj)
+{
+  switch (pic_type(obj)) {
+  case PIC_TT_NIL:
+    printf("()");
+    break;
+  case PIC_TT_PAIR:
+    printf("(");
+    pic_debug(pic, pic_car(pic, obj));
+    printf(" . ");
+    pic_debug(pic, pic_cdr(pic, obj));
+    printf(")");
+    break;
+  case PIC_TT_SYMBOL:
+    printf("%s", pic_symbol_ptr(obj)->name);
+    break;
+  default:
+    abort();
+  }
+}