diff --git a/include/picrin.h b/include/picrin.h
index 97b76051..65b12a56 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -84,6 +84,7 @@ struct pic_proc *pic_codegen(pic_state *, pic_value);
 void pic_abort(pic_state *, const char *);
 void pic_raise(pic_state *, pic_value);
 void pic_error(pic_state *, const char *);
+void pic_warn(pic_state *, const char *);
 
 void pic_debug(pic_state *, pic_value);
 
diff --git a/src/codegen.c b/src/codegen.c
index bb411cdf..7d2ed45a 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -139,6 +139,7 @@ scope_global_define(pic_state *pic, const char *name)
   struct xh_entry *e;
 
   if ((e = xh_get(pic->global_tbl, name))) {
+    pic_warn(pic, "redefining global");
     return e->val;
   }
   e = xh_put(pic->global_tbl, name, pic->glen++);
diff --git a/src/error.c b/src/error.c
index b963b3a9..fde4cf95 100644
--- a/src/error.c
+++ b/src/error.c
@@ -20,3 +20,9 @@ pic_abort(pic_state *pic, const char *msg)
   puts(msg);
   abort();
 }
+
+void
+pic_warn(pic_state *pic, const char *msg)
+{
+  fprintf(stderr, "warn: %s\n", msg);
+}