host: each-source = graph query — the data-driven each (composition roadmap step 3)

An object's `each` source can now be a GRAPH QUERY: `(query is-a TYPE)` resolves to
whatever is-a TYPE *right now* — the list isn't baked into the body, it's the live graph.
The object's `each` IS the query; the render is the run over current data (the unifying
property, now over real data).

compose.sx stays self-contained: the `query` source delegates to a resolver bound in the
render context under "query" — it asks the context for data, never reaching into the graph
itself. The host supplies graph access via host/blog--comp-query (`(query is-a TYPE)` ->
host/blog-instances-of -> full records) injected by host/blog--comp-ctx (auth + resolver);
the post handler renders :body against that context.

Added a `val` leaf — the raw field value with no markup wrapper, for use inside attributes
(href/src). `field` stays span-wrapped for display; `(val :slug)` makes a real link in the
each template. /compose-demo's each is now a live (query is-a compose-item) over two seeded
instances instead of a baked literal list.

Verified end-to-end via a focused harness eval over the full relations+persist+blog stack
(query iterates real instances; clean href via val; empty query -> empty, not an error).
Blog suite 151/153 — the 2 fails ("relate-options load-more sentinel", "related picker
offers all posts") are PRE-EXISTING (clean HEAD is 149/151 with the identical 2 fails, a
relate-picker pagination-boundary issue) and unrelated to composition; my 2 new tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/blog.sx

@@ -527,11 +527,36 @@
   (fn (slug body)
     (let ((r (host/blog-get slug)))
       (when r (host/blog--write! slug (merge r {:body body}))))))
+;; The resolver for the composition `each` graph-query source (compose.sx asks the context
+;; for "query"). `(query REL TYPE)` -> the objects related to TYPE by REL, as full records
+;; so the per-item template can field them. Today the supported relation is is-a (TYPE's
+;; transitive instances, via host/blog-instances-of); the dispatch leaves room for more.
+;; This is the DATA-DRIVEN each source — the object's `each` is the query, the render is
+;; the run over whatever the graph currently holds.
+(define host/blog--comp-query
+  (fn (qargs ctx)
+    (let ((rel (str (first qargs))) (type (str (first (rest qargs)))))
+      (cond
+        ((= rel "is-a") (map host/blog-get (host/blog-instances-of type)))
+        (else (list))))))
+;; the render context for a :body: auth from the principal + the graph-query resolver.
+(define host/blog--comp-ctx
+  (fn (principal)
+    (merge (if (nil? principal) {} {"auth" "yes"})
+           {"query" host/blog--comp-query})))
 ;; Seed a live demo of the composition fold: one object, rendered by host/comp-render, that
 ;; shows seq + alt(when auth) + row(par) + each — and renders DIFFERENTLY logged-in vs out.
 (define host/blog-seed-compose-demo!
   (fn ()
     (begin
+      ;; a demo type + two instances, so the each(query …) below iterates REAL graph data —
+      ;; the list isn't baked into the body, it's whatever is-a compose-item right now.
+      (host/blog-seed! "compose-item" "Compose Item" "(article (h1 \"Compose Item\"))" "published")
+      (host/blog-relate! "compose-item" "type" "subtype-of")
+      (host/blog-seed! "compose-item-revel" "Revel Show" "(article (h1 \"Revel Show\"))" "published")
+      (host/blog-seed! "compose-item-pub"   "Pub Night"  "(article (h1 \"Pub Night\"))"  "published")
+      (host/blog-relate! "compose-item-revel" "compose-item" "is-a")
+      (host/blog-relate! "compose-item-pub"   "compose-item" "is-a")
       (host/blog-seed! "compose-demo" "Composition Demo"
         "(article (h1 \"Composition Demo\") (p \"Rendered via the composition fold.\"))" "published")
       (host/blog--set-body! "compose-demo"
@@ -542,9 +567,9 @@
           (text "<h3>Two columns (par)</h3>")
           (row (text "<div style=\"flex:1;border:1px solid #ccc;padding:0.5em\">Column A</div>")
                (text "<div style=\"flex:1;border:1px solid #ccc;padding:0.5em\">Column B</div>"))
-          (text "<h3>A list (each)</h3><ul>")
-          (each (items {:name "Revel Show" :date "Aug"} {:name "Pub Night" :date "Jun"})
-            (seq (text "<li>") (field :name) (text " — ") (field :date) (text "</li>")))
+          (text "<h3>A list (each over a graph query)</h3><ul>")
+          (each (query is-a compose-item)
+            (seq (text "<li><a href=\"/") (val :slug) (text "\">") (field :title) (text "</a></li>")))
           (text "</ul>")))))))
 ;; replace every (field "name") node in a parsed template tree with values[name] ("" if
 ;; absent). Pure: a tree-walk over the already-parsed template + pre-fetched values.
@@ -1128,7 +1153,7 @@
                   ;; (host/comp-render) against a context (auth from the principal); else the
                   ;; legacy sx_content path. The SAME object renders differently per context.
                   (body-html (if (get r :body)
-                               (host/comp-render (get r :body) (if (nil? principal) {} {"auth" "yes"}))
+                               (host/comp-render (get r :body) (host/blog--comp-ctx principal))
                                (host/blog-render r)))
                   ;; all relation blocks (Related, Tags, Types, Tagged-with-this …)
                   ;; come from iterating the registry — one section, registry-driven.

lib/host/compose.sx

@@ -2,7 +2,9 @@
 ;;
 ;; An object's :body is a composition node — a tiny UI language over object refs. The
 ;; render-fold below is its interpreter. Four combinators (seq/row/alt/each) + leaves
-;; (field/text/card) + ref + recursion (tmpl). The context is an EXTENSIBLE ENVIRONMENT:
+;; (field/val/text/card) + ref + recursion (tmpl). `field` wraps its value in a span for
+;; display; `val` is the raw value (no markup) for use inside attributes (href/src).
+;; The context is an EXTENSIBLE ENVIRONMENT:
 ;; `when` reads it, `each` extends it (:item, :depth). Same predicate set as the type
 ;; guards. The object's CID is its DEFINITION; render is the EXECUTION (per context+data).
 ;; Self-contained (no blog deps) so the model can be proven in isolation.
@@ -25,8 +27,12 @@
         (str (get item key))
         (str (or (get ctx key) ""))))))
 
-;; the source collection for `each`: literal items, the :item's :children (trees), or a
-;; named list field on the :item. (A graph-query source is wiring step 3, plan roadmap.)
+;; the source collection for `each`: literal items, the :item's :children (trees), a
+;; named list field on the :item, or a GRAPH QUERY. The query source `(query REL TYPE)`
+;; is data-driven: it delegates to a resolver function bound in the context under "query"
+;; (the host injects one with graph access), so compose.sx stays self-contained — it asks
+;; the context for the data, it doesn't reach into the graph itself. `src` minus its head
+;; (`(REL TYPE …)`) + the live ctx are passed through; the resolver returns a list of items.
 (define host/comp--source
   (fn (src ctx)
     (let ((op (str (first src))) (item (get ctx "item")))
@@ -34,6 +40,8 @@
         ((= op "items")    (rest src))
         ((= op "children") (if item (or (get item "children") (list)) (list)))
         ((= op "field")    (if item (or (get item (str (first (rest src)))) (list)) (list)))
+        ((= op "query")    (let ((qfn (get ctx "query")))
+                             (if qfn (qfn (rest src) ctx) (list))))
         (else (list))))))
 
 ;; ── template registry (recursion: a template may reference itself by name) ──
@@ -89,6 +97,7 @@
           ((= h "alt")   (host/comp--alt-pick args ctx))
           ((= h "each")  (host/comp--each (first args) (first (rest args)) ctx))
           ((= h "field") (str "<span>" (host/comp--field (first args) ctx) "</span>"))
+          ((= h "val")   (host/comp--field (first args) ctx))   ;; raw value, no markup — for attributes
           ((= h "text")  (str (first args)))
           ((= h "card")  (host/comp--card (str (first args)) (first (rest args))))
           ((= h "tmpl")  (host/comp--render (get host/comp--tmpls (str (first args))) ctx))

lib/host/tests/blog.sx

@@ -750,6 +750,31 @@
   (list "ANON<span>X</span><span>Y</span>" "MEMBER<span>X</span><span>Y</span>"))
 (host-bl-test "post page renders :body (composition) over sx_content"
   (contains? (dream-resp-body (host-bl-app (host-bl-req "/cdoc/"))) "ANON") true)
+;; -- the each source can be a GRAPH QUERY: the list isn't baked into the body, it's
+;; whatever is-a the type right now (data-driven). The resolver (host/blog--comp-query)
+;; is injected into the render context by host/blog--comp-ctx. --
+(host-bl-test "each(query is-a TYPE) iterates real graph instances"
+  (begin
+    (host/blog-seed! "qtype" "QType" "(p \"t\")" "published")
+    (host/blog-relate! "qtype" "type" "subtype-of")
+    (host/blog-seed! "qi-1" "Item One" "(p \"1\")" "published")
+    (host/blog-seed! "qi-2" "Item Two" "(p \"2\")" "published")
+    (host/blog-relate! "qi-1" "qtype" "is-a")
+    (host/blog-relate! "qi-2" "qtype" "is-a")
+    (let ((out (host/comp-render
+                 (quote (each (query is-a qtype)
+                          (seq (text "<a href=\"/") (val :slug) (text "\">") (field :title) (text "</a>"))))
+                 (host/blog--comp-ctx nil))))
+      ;; field wraps in <span> (display); val is raw (for the href attribute).
+      (list (contains? out "Item One") (contains? out "Item Two")
+            (contains? out "/qi-1") (contains? out "<span>Item One</span>"))))
+  (list true true true true))
+;; a query with no matching instances renders empty (not an error) — robustness.
+(host-bl-test "each(query is-a TYPE) with no instances renders empty"
+  (host/comp-render
+    (quote (each (query is-a no-such-type) (field :title)))
+    (host/blog--comp-ctx nil))
+  "")
 (host-bl-test "a post with no schema'd type is vacuously valid"
   (host/blog-type-valid? "ppost" "(p \"anything\")") true)
 (host-bl-test "edit-submit rejects content violating the type schema (not saved)"