diff --git a/mrbgems/mruby-binding/src/binding.c b/mrbgems/mruby-binding/src/binding.c
index 311765171..7850afaca 100644
--- a/mrbgems/mruby-binding/src/binding.c
+++ b/mrbgems/mruby-binding/src/binding.c
@@ -8,8 +8,10 @@
 #include <mruby/internal.h>
 #include <mruby/presym.h>
 
+/* provided by mruby-binding-core */
 const struct RProc *mrb_binding_extract_proc(mrb_state *mrb, mrb_value binding);
 struct REnv *mrb_binding_extract_env(mrb_state *mrb, mrb_value binding);
+/* provided by mruby-compiler vid mruby-eval */
 typedef mrb_bool mrb_parser_foreach_top_variable_func(mrb_state *mrb, mrb_sym sym, void *user);
 void mrb_parser_foreach_top_variable(mrb_state *mrb, struct mrb_parser_state *p, mrb_parser_foreach_top_variable_func *func, void *user);