kernel(date): ISO-8601 rendering, validating parse, sexp converters

Adds sexplib and ppx_sexp_conv to the project and wires the ppx into the
kernel library. Date's sexp form is an ISO-8601 atom rather than the opaque
rata die, so data files stay human-editable and parsing revalidates the
1583..9999 domain.

7 files changed, 64 insertions, 6 deletions

diff --git a/colitur.opam b/colitur.opam
index d855e8e..3edcfa4 100644
--- a/colitur.opam
+++ b/colitur.opam
@@ -7,6 +7,8 @@ depends: [
   "alcotest"
   "qcheck"
   "qcheck-alcotest"
+  "sexplib"
+  "ppx_sexp_conv"
   "odoc" {with-doc}
 ]
 build: [
diff --git a/dune-project b/dune-project
index cd3af07..91824f4 100644
--- a/dune-project
+++ b/dune-project
@@ -6,4 +6,4 @@
 (package
  (name colitur)
  (synopsis "Deterministic liturgical calendar engine (computus liturgicus)")
- (depends ocaml dune alcotest qcheck qcheck-alcotest))
+ (depends ocaml dune alcotest qcheck qcheck-alcotest sexplib ppx_sexp_conv))
diff --git a/lib/kernel/date.ml b/lib/kernel/date.ml
index 525b35b..cf5aabd 100644
--- a/lib/kernel/date.ml
+++ b/lib/kernel/date.ml
@@ -1,4 +1,4 @@
-type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat
+type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat [@@deriving sexp]
 
 (* Internal representation: rata die = days since 1970-01-01 (proleptic
    Gregorian). Howard Hinnant's civil<->days algorithm; OCaml's `/` truncates
@@ -56,3 +56,26 @@ let weekday t =
   (* rata 0 = 1970-01-01 = Thursday (index 4 with Sun=0). *)
   let w = (((t + 4) mod 7) + 7) mod 7 in
   [| Sun; Mon; Tue; Wed; Thu; Fri; Sat |].(w)
+
+let weekday_to_string = function
+  | Sun -> "sunday" | Mon -> "monday" | Tue -> "tuesday" | Wed -> "wednesday"
+  | Thu -> "thursday" | Fri -> "friday" | Sat -> "saturday"
+
+let to_iso8601 t = Printf.sprintf "%04d-%02d-%02d" (year t) (month t) (day t)
+
+let of_iso8601 s =
+  match String.split_on_char '-' s with
+  | [ y; m; d ]
+    when String.length y = 4 && String.length m = 2 && String.length d = 2 -> (
+      match (int_of_string_opt y, int_of_string_opt m, int_of_string_opt d) with
+      | Some year, Some month, Some day -> make ~year ~month ~day
+      | _ -> Error (Printf.sprintf "date %S: not an ISO-8601 date" s))
+  | _ -> Error (Printf.sprintf "date %S: expected YYYY-MM-DD" s)
+
+let sexp_of_t t = Sexplib0.Sexp_conv.sexp_of_string (to_iso8601 t)
+
+let t_of_sexp sexp =
+  let s = Sexplib0.Sexp_conv.string_of_sexp sexp in
+  match of_iso8601 s with
+  | Ok t -> t
+  | Error msg -> Sexplib0.Sexp_conv.of_sexp_error msg sexp
diff --git a/lib/kernel/date.mli b/lib/kernel/date.mli
index b684d61..2f632c8 100644
--- a/lib/kernel/date.mli
+++ b/lib/kernel/date.mli
@@ -3,7 +3,7 @@
     arithmetic and comparison are total and cheap. *)
 
 type t
-type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat
+type weekday = Sun | Mon | Tue | Wed | Thu | Fri | Sat [@@deriving sexp]
 
 (** [make ~year ~month ~day] validates the date; rejects a month/day out of
     range and any year outside 1583..9999. *)
@@ -21,3 +21,16 @@ val to_rata : t -> int
 val of_rata : int -> t
 val add_days : t -> int -> t
 val compare : t -> t -> int
+
+(** ISO-8601 rendering and validating parsing. [of_iso8601] enforces the same
+    domain as {!make}. *)
+val to_iso8601 : t -> string
+val of_iso8601 : string -> (t, string) result
+
+val weekday_to_string : weekday -> string
+
+(** Sexp form is an ISO-8601 atom, e.g. [2026-04-05]. [t_of_sexp] validates and
+    raises [Of_sexp_error] on a malformed or out-of-domain date; loaders convert
+    that to a [result]. *)
+val t_of_sexp : Sexplib0.Sexp.t -> t
+val sexp_of_t : t -> Sexplib0.Sexp.t
diff --git a/lib/kernel/dune b/lib/kernel/dune
index b63c476..b909ac5 100644
--- a/lib/kernel/dune
+++ b/lib/kernel/dune
@@ -1,2 +1,5 @@
 (library
- (name colitur_kernel))
+ (name colitur_kernel)
+ (libraries sexplib)
+ (preprocess
+  (pps ppx_sexp_conv)))
diff --git a/test/dune b/test/dune
index 8ee8b87..3d8fa59 100644
--- a/test/dune
+++ b/test/dune
@@ -1,6 +1,6 @@
 (test
  (name test_colitur)
- (libraries colitur_kernel alcotest qcheck qcheck-alcotest))
+ (libraries colitur_kernel alcotest qcheck qcheck-alcotest sexplib))
 
 (cram
  (deps %{bin:colitur}))
diff --git a/test/test_date.ml b/test/test_date.ml
index 2458659..e3c2dfb 100644
--- a/test/test_date.ml
+++ b/test/test_date.ml
@@ -52,11 +52,28 @@ let prop_weekday_cycle =
   QCheck.Test.make ~name:"weekday(add_days d 7) = weekday d" arb_date
     (fun d -> D.weekday (D.add_days d 7) = D.weekday d)
 
+let test_iso8601 () =
+  Alcotest.(check string) "to_iso8601" "2026-04-05" (D.to_iso8601 (mk 2026 4 5));
+  (match D.of_iso8601 "2026-04-05" with
+   | Ok d -> Alcotest.(check int) "of_iso8601 roundtrip" 0 (D.compare d (mk 2026 4 5))
+   | Error e -> Alcotest.failf "of_iso8601: %s" e);
+  Alcotest.(check bool) "reject garbage" true (Result.is_error (D.of_iso8601 "nope"));
+  Alcotest.(check bool) "reject 2026-02-30" true (Result.is_error (D.of_iso8601 "2026-02-30"))
+
+let test_date_sexp () =
+  let d = mk 2026 4 5 in
+  Alcotest.(check string) "sexp_of_t" "2026-04-05" (Sexplib.Sexp.to_string (D.sexp_of_t d));
+  Alcotest.(check int) "t_of_sexp roundtrip" 0
+    (D.compare (D.t_of_sexp (D.sexp_of_t d)) d);
+  Alcotest.(check string) "weekday_to_string" "sunday" (D.weekday_to_string (D.weekday d))
+
 let suite =
   ( "Date",
     [ Alcotest.test_case "weekday" `Quick test_weekday;
       Alcotest.test_case "roundtrip" `Quick test_roundtrip;
       Alcotest.test_case "add_days boundaries" `Quick test_add_days;
-      Alcotest.test_case "make rejects invalid" `Quick test_make_reject ]
+      Alcotest.test_case "make rejects invalid" `Quick test_make_reject;
+      Alcotest.test_case "iso8601" `Quick test_iso8601;
+      Alcotest.test_case "sexp" `Quick test_date_sexp ]
     @ List.map QCheck_alcotest.to_alcotest
         [ prop_roundtrip; prop_add_inverse; prop_weekday_cycle ] )