From da9cf402ceed8102aec1a9d008f8e918a23d39f3 Mon Sep 17 00:00:00 2001
From: Lukasz Kasprzak <lukas@labunix.xyz>
Date: Wed, 19 Aug 2026 08:17:30 +0200
Subject: feat(render): template renderer with mandatory escaping

Every interpolated value is escaped for the template's flavour; the
template's own literal text never is, because that is the author's
markup. There is no raw form, so a template cannot opt out.

Scope is a stack with outward fallback, so a grid template can reach the
year number from inside a week without the view duplicating it into
every cell.

A missing key renders empty -- the one deliberate silence, so a template
survives a rite that does not set every optional field.

Mutation-tested: dropping the Escape.apply call reddens the
data-cannot-escape-flavour case.
---
 lib/render/template.ml  | 55 +++++++++++++++++++++++++++++++++++++++++++++++++
 lib/render/template.mli |  9 ++++++++
 2 files changed, 64 insertions(+)

(limited to 'lib')

diff --git a/lib/render/template.ml b/lib/render/template.ml
index 3e35d53..7efa92b 100644
--- a/lib/render/template.ml
+++ b/lib/render/template.ml
@@ -94,3 +94,58 @@ let build items =
   go [] [] items
 
 let parse src = match lex src with Error e -> Error e | Ok items -> build items
+
+(* Scope is a STACK, innermost first: a section pushes its own object, and a
+   lookup falls back outward. Without the fallback a grid template could not
+   reach the year number from inside a week. *)
+let rec lookup stack path =
+  match stack with
+  | [] -> None
+  | top :: rest -> (
+      match descend top path with Some v -> Some v | None -> lookup rest path)
+
+and descend v path =
+  match (v, path) with
+  | _, [] -> Some v
+  | Obj kvs, k :: tl -> (
+      match List.assoc_opt k kvs with Some v' -> descend v' tl | None -> None)
+  | _ -> None
+
+let truthy = function
+  | Bool b -> b
+  | Str "" -> false
+  | Str _ -> true
+  | List [] -> false
+  | List _ -> true
+  | Obj _ -> true
+
+let render ~flavour nodes value =
+  let b = Buffer.create 4096 in
+  let rec go stack nodes =
+    List.iter
+      (fun node ->
+        match node with
+        | Text t -> Buffer.add_string b t
+        | Var p -> (
+            match lookup stack p with
+            | Some (Str s) -> Buffer.add_string b (Escape.apply flavour s)
+            | Some (Bool true) -> Buffer.add_string b "true"
+            | Some (Bool false) -> ()
+            | Some (List _) | Some (Obj _) | None -> ())
+        | Section (p, body) -> (
+            match lookup stack p with
+            | None -> ()
+            | Some (List items) -> List.iter (fun it -> go (it :: stack) body) items
+            | Some v when truthy v -> go (v :: stack) body
+            | Some _ -> ())
+        | Inverted (p, body) -> (
+            match lookup stack p with
+            | None -> go stack body
+            | Some v -> if not (truthy v) then go stack body))
+      nodes
+  in
+  go [ value ] nodes;
+  Buffer.contents b
+
+let render_string ~flavour src value =
+  match parse src with Error e -> Error e | Ok nodes -> Ok (render ~flavour nodes value)
diff --git a/lib/render/template.mli b/lib/render/template.mli
index 3f9ef97..4118496 100644
--- a/lib/render/template.mli
+++ b/lib/render/template.mli
@@ -26,3 +26,12 @@ type node =
 (** Never raises; a malformed template is an [Error] with a human-readable
     reason, because the template is user input. *)
 val parse : string -> (node list, string) result
+
+(** Render against a value. Escaping is applied to every interpolated value and
+    NEVER to the template's own literal text, which is the author's markup.
+    A missing key renders as the empty string -- the one deliberate silence, so
+    that a template survives a rite that does not set every optional field. *)
+val render : flavour:Escape.flavour -> node list -> value -> string
+
+(** [parse] then [render]. *)
+val render_string : flavour:Escape.flavour -> string -> value -> (string, string) result
-- 
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