content: Markdown frontmatter -> metadata + 9 tests (469/469)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/content/md-import.sx

@@ -1,14 +1,15 @@
 ;; content-on-sx — Markdown import adapter (markdown text -> block document).
 ;;
 ;; A line-based parser, the inverse of markdown.sx's asMarkdown. Confined to the
-;; adapter boundary: the core knows nothing about Markdown. Handles ATX headings
-;; (#..######), fenced code (```lang), blockquotes (> ), unordered (- / * ) and
-;; ordered (1. ) lists, thematic breaks (--- / ***), pipe tables (header row +
-;; --- separator row + body rows), and paragraphs (consecutive plain lines joined
-;; with a space). Block ids are assigned sequentially b0,b1…
+;; adapter boundary: the core knows nothing about Markdown. Handles a leading
+;; --- frontmatter block (key: value -> doc metadata), ATX headings (#..######),
+;; fenced code (```lang), blockquotes (> ), unordered (- / * ) and ordered (1. )
+;; lists, thematic breaks (--- / ***), pipe tables (header + --- separator +
+;; body), and paragraphs (consecutive plain lines joined with a space). Block ids
+;; are assigned sequentially b0,b1…
 ;;
-;; Requires (loaded by harness): block.sx, doc.sx, table.sx (mk-table); and
-;; markdown.sx for the adapter's export side.
+;; Requires (loaded by harness): block.sx, doc.sx, table.sx (mk-table),
+;; meta.sx (doc-with-meta); markdown.sx for the adapter's export side.
 
 (define md/-id (fn (i) (str "b" i)))
 (define md/-blank? (fn (s) (= s "")))
@@ -35,6 +36,18 @@
   md/-drop
   (fn (s prefix) (substring s (string-length prefix) (string-length s))))
 
+(define
+  md/-drop-n
+  (fn
+    (xs n)
+    (if
+      (= n 0)
+      xs
+      (if
+        (= (len xs) 0)
+        xs
+        (md/-drop-n (rest xs) (- n 1))))))
+
 (define
   md/-join-with
   (fn
@@ -365,7 +378,72 @@
   md/parse
   (fn (text) (md/-walk (split text (str "\n")) 0 (list))))
 
+;; ── frontmatter (leading --- key: value --- block) ──
+(define
+  md/-frontmatter?
+  (fn (lines) (and (> (len lines) 0) (= (first lines) "---"))))
+(define
+  md/-fm-end
+  (fn
+    (lines i)
+    (cond
+      ((>= i (len lines)) -1)
+      ((= (nth lines i) "---") i)
+      (else (md/-fm-end lines (+ i 1))))))
+(define
+  md/-fm-add
+  (fn
+    (acc line)
+    (let
+      ((parts (split line ":")))
+      (if
+        (< (len parts) 2)
+        acc
+        (let
+          ((key (trim (first parts)))
+            (val (trim (md/-join-with ":" (rest parts)))))
+          (cond
+            ((= key "title") (assoc acc :title val))
+            ((= key "slug") (assoc acc :slug val))
+            ((= key "tags")
+              (assoc acc :tags (map (fn (t) (trim t)) (split val ","))))
+            (else acc)))))))
+(define
+  md/-fm-pairs
+  (fn
+    (lines start end acc)
+    (if
+      (>= start end)
+      acc
+      (md/-fm-pairs
+        lines
+        (+ start 1)
+        end
+        (md/-fm-add acc (nth lines start))))))
+
 ;; ── adapter ──
-(define md/import (fn (text doc-id) (doc-new doc-id (md/parse text))))
+(define
+  md/import
+  (fn
+    (text doc-id)
+    (let
+      ((lines (split text (str "\n"))))
+      (if
+        (md/-frontmatter? lines)
+        (let
+          ((end (md/-fm-end lines 1)))
+          (if
+            (= end -1)
+            (doc-new doc-id (md/-walk lines 0 (list)))
+            (doc-with-meta
+              (doc-new
+                doc-id
+                (md/-walk
+                  (md/-drop-n lines (+ end 1))
+                  0
+                  (list)))
+              (md/-fm-pairs lines 1 end {}))))
+        (doc-new doc-id (md/-walk lines 0 (list)))))))
+
 (define content/from-markdown md/import)
 (define markdown-adapter {:export (fn (doc) (asMarkdown doc)) :import md/import})