From 7ef5669ce9536a36752d5ea28cf99ce5a8a8764a Mon Sep 17 00:00:00 2001
From: Vincent Balat <vincent.balat@vblt.org>
Date: Fri, 29 May 2026 15:24:45 +0200
Subject: [PATCH] demo_ref: pass a Deriving_Json codec to the persistent eref

Eliom 13 requires every Reference.eref ~persistent to carry a
Deriving_Json codec. Use [%json: Os.Date.local_calendar option],
which resolves to the codec exposed by Os.Date.
---
 demo_ref.eliom | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/demo_ref.eliom b/demo_ref.eliom
index c7eb5b1..78b559f 100644
--- a/demo_ref.eliom
+++ b/demo_ref.eliom
@@ -10,7 +10,8 @@ open%shared Eliom.Content.Html.F
    Ocsigen Start is creating a session group for each user.
 *)
 let%server last_visit =
-  Eliom.Reference.eref ~persistent:"demo_last_visit"
+  Eliom.Reference.eref
+    ~persistent:("demo_last_visit", [%json: Os.Date.local_calendar option])
     ~scope:Eliom.Common.default_group_scope None
 
 (* Read & reset last_visit *)