From a526d934cea17ff1b76ddad5c5f2934fb85c11cb Mon Sep 17 00:00:00 2001
From: Lukasz Kasprzak <lukas@labunix.xyz>
Date: Wed, 19 Aug 2026 08:09:12 +0200
Subject: fix(render): reject empty tag paths, sharpen the raw-form test

F1: test_no_raw_or_partial_form's first assertion only excluded one
literal shape (Ok [Var ["{name"]]), so it could not actually catch a
future raw/unescaped constructor under a different name. Replace it
with an assertion of the real parse result for {{{name}}}
(Ok [Var ["{name"]; Text "}"]), documented behaviour rather than a
guarantee this test cannot check -- the real guarantee is structural:
node has exactly four constructors and none of them is raw.

F2: {{.}}, {{#}}, {{^}} and {{/}} used to parse to a Var/Section/
Inverted with an empty path, reachable but never designed. This engine
has no "current context" for a bare dot to mean, so a bare-dot or
empty-sigil path is now a parse error at lex time, covering all four
sigil forms via one path helper. The existing "empty tag {{}}" branch
is unchanged and still reachable (a fully empty body is a distinct
case from a sigil with an empty path).
---
 lib/render/template.ml | 18 ++++++++++++++----
 test/test_template.ml  | 31 ++++++++++++++++++++++++++++---
 2 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/lib/render/template.ml b/lib/render/template.ml
index 8de91a6..3e35d53 100644
--- a/lib/render/template.ml
+++ b/lib/render/template.ml
@@ -38,13 +38,23 @@ let lex src =
             let b = trim body in
             if b = "" then Error "empty tag {{}}"
             else
+              (* Every sigil form (var, #, ^, /) resolves a dotted path; an
+                 empty path ("{{.}}", "{{#}}", "{{^}}", "{{/}}") names
+                 nothing -- there is no "current context" concept for a bare
+                 dot to mean, so it is a parse error rather than an invented
+                 behaviour Task 3 would have to define. *)
+              let with_path ctor rest =
+                match path rest with
+                | [] -> Error "empty tag path: {{.}} and {{#}} name nothing"
+                | p -> Ok (ctor p)
+              in
               match b.[0] with
-              | '#' -> Ok (TOpen (path (String.sub b 1 (String.length b - 1))))
-              | '^' -> Ok (TInv (path (String.sub b 1 (String.length b - 1))))
-              | '/' -> Ok (TClose (path (String.sub b 1 (String.length b - 1))))
+              | '#' -> with_path (fun p -> TOpen p) (String.sub b 1 (String.length b - 1))
+              | '^' -> with_path (fun p -> TInv p) (String.sub b 1 (String.length b - 1))
+              | '/' -> with_path (fun p -> TClose p) (String.sub b 1 (String.length b - 1))
               | '!' -> Ok TComment
               | '>' -> Error "partials are not supported: a template may not include another file"
-              | _ -> Ok (TVar (path b))
+              | _ -> with_path (fun p -> TVar p) b
           in
           (match tag with
            | Error e -> Error e
diff --git a/test/test_template.ml b/test/test_template.ml
index dee281b..cfb3b04 100644
--- a/test/test_template.ml
+++ b/test/test_template.ml
@@ -42,12 +42,36 @@ let test_unterminated_tag_is_an_error () =
 (* The safety promise (spec section 5): a template is data, never a program.
    There is no raw/triple-brace form to opt out of escaping, and no partial. *)
 let test_no_raw_or_partial_form () =
-  Alcotest.(check bool) "triple brace is not an unescaped var" true
-    (match T.parse "{{{name}}}" with Ok [ T.Var [ "{name" ] ] -> false | Ok _ -> true | Error _ -> true);
+  (* {{{name}}} is not a triple-brace "unescaped" form -- there is no such
+     form in this engine, so this assertion states what parsing it ACTUALLY
+     produces rather than gesturing at a guarantee it cannot check: the
+     lexer only recognises double braces, so it reads an ordinary var whose
+     path is the literal string "{name", then a leftover "}" as text. The
+     real guarantee against a raw form is structural, not this assertion:
+     [node] has exactly four constructors -- Text, Var, Section, Inverted --
+     and none of them is raw/unescaped. If [node] ever grows a fifth, raw
+     constructor, this test is not what will catch it. *)
+  Alcotest.(check bool) "triple brace parses as var + stray text" true
+    (parse_ok "{{{name}}}" = [ T.Var [ "{name" ]; T.Text "}" ]);
   match T.parse "{{>partial}}" with
   | Error _ -> ()
   | Ok _ -> Alcotest.fail "partials must not parse: a template may not pull in another file"
 
+(* No "current context" concept exists in this engine (sections push an
+   object; lookup falls back outward), so a bare-dot or empty-sigil path has
+   no meaning. Reject at parse time rather than inventing behaviour Task 3's
+   renderer would otherwise have to define for a case nobody designed. *)
+let test_empty_path_is_an_error () =
+  let expect_error label s =
+    match T.parse s with
+    | Error _ -> ()
+    | Ok _ -> Alcotest.failf "%s: an empty tag path must be a parse error" label
+  in
+  expect_error "{{.}}" "{{.}}";
+  expect_error "{{ . }}" "{{ . }}";
+  expect_error "{{#}}{{/}}" "{{#}}{{/}}";
+  expect_error "{{^}}{{/}}" "{{^}}{{/}}"
+
 let suite =
   ( "Template/parse",
     [ Alcotest.test_case "plain text" `Quick test_plain_text;
@@ -58,4 +82,5 @@ let suite =
       Alcotest.test_case "unclosed section errors" `Quick test_unclosed_section_is_an_error;
       Alcotest.test_case "mismatched close errors" `Quick test_mismatched_close_is_an_error;
       Alcotest.test_case "unterminated tag errors" `Quick test_unterminated_tag_is_an_error;
-      Alcotest.test_case "no raw or partial form" `Quick test_no_raw_or_partial_form ] )
+      Alcotest.test_case "no raw or partial form" `Quick test_no_raw_or_partial_form;
+      Alcotest.test_case "empty path errors" `Quick test_empty_path_is_an_error ] )
-- 
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