1 files changed, 34 insertions, 0 deletions
diff --git a/lib/hook.ml b/lib/hook.ml
new file mode 100644
index 0000000000..1e2a2f279d
--- /dev/null
+++ b/lib/hook.ml
@@ -0,0 +1,34 @@
+(************************************************************************)
+(*         *   The Coq Proof Assistant / The Coq Development Team       *)
+(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
+(* <O___,, *       (see CREDITS file for the list of authors)           *)
+(*   \VV/  **************************************************************)
+(*    //   *    This file is distributed under the terms of the         *)
+(*         *     GNU Lesser General Public License Version 2.1          *)
+(*         *     (see LICENSE file for the text of the license)         *)
+(************************************************************************)
+
+type 'a content =
+| Unset
+| Default of 'a
+| Set of 'a
+
+type 'a t = 'a content ref
+
+type 'a value = 'a t
+
+let get (hook : 'a value) = match !hook with
+| Unset -> assert false
+| Default data | Set data -> data
+
+let set (hook : 'a t) data = match !hook with
+| Unset | Default _ -> hook := Set data
+| Set _ -> assert false
+
+let make ?default () =
+  let data = match default with
+  | None -> Unset
+  | Some data -> Default data
+  in
+  let ans = ref data in
+  (ans, ans)