host P0.2: publish-DAG + execute-fold runner + capability check (hypothesis confirmed)

The hypothesis test. FINDING: a synchronous business flow expresses NATURALLY as an EXECUTE-FOLD
composition (host/execute.sx: seq/effect/alt — the category branch IS 'alt'), NOT an artdag
DATAFLOW DAG (which has no control flow). So 'business logic = art-dag' holds at the ABSTRACTION
(both content-addressed op-DAGs) and is REFINED at the vocabulary: the synchronous control-flow
runner is the execute-fold (caps {effect,branch,each}); artdag is the dataflow sibling. Two
instances of one thing, run very differently — exactly the framing.

lib/host/flows.sx: capability typing (host/flow--node-cap/required-caps derive a DAG's capability
set from its node vocabulary; effect→effect, alt→branch, each→each, wait→suspend), the execute-fold
seam runner (advertises {effect,branch,each}), and host/flow--bind (required ⊆ advertised → derive
the runner, else fail-fast). host/blog--publish-dag (the publish workflow) + publish-ctx.

Verified: publish-DAG required-caps = {effect,branch} → binds to the sync runner; runs →
newsletter→[validate,digest] / urgent→[validate,notify] / other→[validate,skip]; a  node →
{suspend} → binds FAIL-FAST against the exec-runner (would need the Erlang runner, RA). Runner is
DERIVED, not chosen. flows 7/7, blog 203/203, full host conformance 591/591.

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

lib/host/blog.sx

@@ -128,6 +128,22 @@
          :id (host/blog-cid slug)                     ;; the object's content CID
          :object {:type "article" :slug slug
                   :category (host/blog--post-category slug)}}))))
+;; P0.2: the publish WORKFLOW as an EXECUTE-FOLD composition (host/execute.sx) — the SYNCHRONOUS
+;; business flow. Validate, then BRANCH on category (newsletter → build a digest, urgent → notify
+;; now, else skip). Content flow (effect/alt), NOT dataflow — so it's the execute-fold, not artdag.
+;; Its required capabilities are {effect, branch} (host/flow--required-caps) → binds to the sync
+;; execute-fold runner (which advertises {effect, branch, each}). A `wait` node would add {suspend}
+;; and fail-fast against that runner (requiring the Erlang runner, RA). Runs against a ctx built
+;; from the activity's object.
+(define host/blog--publish-dag
+  (quote (seq
+    (effect validate (field "slug"))
+    (alt (when (eq "category" "newsletter") (effect digest (field "slug")))
+         (when (eq "category" "urgent")     (effect notify (field "slug")))
+         (else (effect skip))))))
+;; the ctx a publish activity presents to the publish-DAG (string keys — preds read ctx by key).
+(define host/blog--publish-ctx
+  (fn (activity) (let ((o (get activity :object))) {"category" (get o :category) "slug" (get o :slug)})))
 
 ;; ── render ──────────────────────────────────────────────────────────
 ;; A post's sx_content is SX element markup -> HTML via render-page (which supplies

lib/host/conformance.sh

@@ -95,6 +95,7 @@ MODULES=(
   "lib/host/compose.sx"
   "lib/host/execute.sx"
   "lib/host/behavior.sx"
+  "lib/host/flows.sx"
   "lib/host/htmlsx.sx"
   "lib/host/blog.sx"
   "lib/host/page.sx"
@@ -114,6 +115,7 @@ SUITES=(
   "blog        host-bl-tests-run!  lib/host/tests/blog.sx"
   "htmlsx      host-ht-tests-run!  lib/host/tests/htmlsx.sx"
   "behavior    host-be-tests-run!  lib/host/tests/behavior.sx"
+  "flows       host-fl-tests-run!  lib/host/tests/flows.sx"
   "compose     host-cp-tests-run!  lib/host/tests/compose.sx"
   "execute     host-ex-tests-run!  lib/host/tests/execute.sx"
   "session     host-se-tests-run!  lib/host/tests/session.sx"

lib/host/flows.sx

@@ -0,0 +1,44 @@
+;; lib/host/flows.sx — behavior DAGs + CAPABILITY-typed nodes / capability-advertising runners
+;; (plans/business-logic-fed-flows.md). P0.2 finding: a SYNCHRONOUS business flow is an EXECUTE-FOLD
+;; composition (host/execute.sx: effect/alt/each — content-addressed control flow), NOT an artdag
+;; DATAFLOW DAG (which has no branch). Both are "content-addressed op-DAGs" — two instances of one
+;; abstraction, run very differently: the execute-fold runner (control flow, synchronous) vs the
+;; artdag runner (dataflow, memoized/parallel). The DIFFERENCE is which capabilities their nodes
+;; need. A node declares its capability; a runner ADVERTISES what it supports; the binder checks
+;; required ⊆ advertised (fail fast); so the sync/durable/distributed choice is DERIVED from the DAG.
+
+;; ── capability typing: a node kind → the capability it needs ──────────
+(define host/flow--node-cap
+  (fn (h)
+    (cond ((= h "effect") "effect")
+          ((= h "alt") "branch")
+          ((= h "each") "each")
+          ((= h "wait") "suspend")      ;; a timer/suspend node — the execute-fold canNOT run it
+          (else nil))))
+(define host/flow--uniq-concat
+  (fn (a b) (reduce (fn (acc x) (if (contains? acc x) acc (concat acc (list x)))) a b)))
+;; the capability SET a composition requires — the union of its nodes' caps (walked recursively).
+(define host/flow--required-caps
+  (fn (node)
+    (if (not (= (type-of node) "list")) (list)
+      (let ((self (host/flow--node-cap (str (first node))))
+            (kids (reduce (fn (acc c) (host/flow--uniq-concat acc (host/flow--required-caps c)))
+                          (list) (rest node))))
+        (if (nil? self) kids (host/flow--uniq-concat (list self) kids))))))
+(define host/flow--subset? (fn (a b) (reduce (fn (ok x) (and ok (contains? b x))) true a)))
+
+;; ── the SYNCHRONOUS op-table runner = the execute-fold ────────────────
+;; a seam runner {:capabilities :run}. It ADVERTISES {effect, branch, each} — the execute-fold
+;; vocabulary. run: fold the composition (dag) against the env's :ctx → the effect log (as data).
+(define host/flow--exec-runner
+  {:capabilities (list "effect" "branch" "each")
+   :run (fn (dag env) {:status "done" :effects (host/exec-run dag (or (get env :ctx) {}))})})
+
+;; DERIVE the runner: bind a DAG to a runner iff its required capabilities ⊆ the runner's advertised.
+;; Fails fast (a {:bind-error …}) rather than mysteriously at run time. This is where "simple in SX
+;; / durable in Erlang / distributed in celery-sx" becomes a checkable property of the DAG.
+(define host/flow--bind
+  (fn (runner dag)
+    (let ((need (host/flow--required-caps dag)) (have (get runner :capabilities)))
+      (if (host/flow--subset? need have) {:ok true :runner runner}
+        {:ok false :bind-error {:needs need :has have}}))))

lib/host/serve.sh

@@ -110,6 +110,7 @@ MODULES=(
   "lib/host/compose.sx"
   "lib/host/execute.sx"
   "lib/host/behavior.sx"
+  "lib/host/flows.sx"
   "lib/host/htmlsx.sx"
   "lib/host/blog.sx"
   "lib/host/server.sx"

lib/host/tests/blog.sx

@@ -1182,6 +1182,24 @@
   (list "urgent" "urgent"))
 (host-bl-test "publish-activity of a missing post is nil"
   (host/blog--publish-activity "nope-nope-nope") nil)
+;; P0.2: the publish WORKFLOW as an execute-fold DAG — branches on category, needs {effect,branch},
+;; binds to the synchronous execute-fold runner (derived, not chosen).
+(host-bl-test "publish-DAG: category branch (newsletter→digest) via the execute-fold"
+  (begin
+    (host/blog-put! "pdag1" "P" "(article (h1 \"x\"))" "published")
+    (host/blog--set-field-values! "pdag1" {"category" "newsletter"})
+    (let ((act (host/blog--publish-activity "pdag1")))
+      (map (fn (e) (get e :verb))
+        (get ((get host/flow--exec-runner :run) host/blog--publish-dag {:ctx (host/blog--publish-ctx act)}) :effects))))
+  (list "validate" "digest"))
+(host-bl-test "publish-DAG: urgent→notify now, other→skip"
+  (list (map (fn (e) (get e :verb)) (get ((get host/flow--exec-runner :run) host/blog--publish-dag {:ctx {"category" "urgent" "slug" "s"}}) :effects))
+        (map (fn (e) (get e :verb)) (get ((get host/flow--exec-runner :run) host/blog--publish-dag {:ctx {"category" "draft" "slug" "s"}}) :effects)))
+  (list (list "validate" "notify") (list "validate" "skip")))
+(host-bl-test "publish-DAG requires {effect,branch} and binds to the sync runner (derived)"
+  (list (host/flow--required-caps host/blog--publish-dag)
+        (get (host/flow--bind host/flow--exec-runner host/blog--publish-dag) :ok))
+  (list (list "effect" "branch") true))
 
 (define
   host-bl-tests-run!

lib/host/tests/flows.sx

@@ -0,0 +1,50 @@
+;; lib/host/tests/flows.sx — P0.2: capability-typed nodes + the execute-fold (synchronous) runner.
+;; A composition's required capabilities are DERIVED from its node vocabulary; a runner advertises
+;; what it supports; the binder checks required ⊆ advertised (fail fast) — so the runner is derived.
+
+(define host-fl-pass 0)
+(define host-fl-fail 0)
+(define host-fl-fails (list))
+(define host-fl-test
+  (fn (name actual expected)
+    (if (= actual expected)
+      (set! host-fl-pass (+ host-fl-pass 1))
+      (begin (set! host-fl-fail (+ host-fl-fail 1))
+        (append! host-fl-fails {:name name :actual actual :expected expected})))))
+
+;; ── required-caps: the node vocabulary → the capability set ──
+(host-fl-test "required-caps: effect + alt → {effect, branch}"
+  (host/flow--required-caps (quote (seq (effect a) (alt (when (eq "k" "v") (effect b)) (else (effect c))))))
+  (list "effect" "branch"))
+(host-fl-test "required-caps: each adds :each; wait adds :suspend"
+  (list (host/flow--required-caps (quote (each (query is-a t) (effect x))))
+        (host/flow--required-caps (quote (seq (effect a) (wait morning)))))
+  (list (list "each" "effect") (list "effect" "suspend")))
+(host-fl-test "required-caps: a plain effect-only DAG needs only {effect} (zero ceremony)"
+  (host/flow--required-caps (quote (effect notify (field "to"))))
+  (list "effect"))
+
+;; ── the binder DERIVES the runner: required ⊆ advertised, or fail fast ──
+(host-fl-test "subset?: required ⊆ advertised"
+  (list (host/flow--subset? (list "effect" "branch") (list "effect" "branch" "each"))
+        (host/flow--subset? (list "suspend") (list "effect" "branch" "each")))
+  (list true false))
+(host-fl-test "bind: an {effect,branch} DAG binds to the exec runner; a {suspend} DAG fails FAST"
+  (let ((ok  (host/flow--bind host/flow--exec-runner (quote (alt (when (eq "k" "v") (effect a)) (else (effect b))))))
+        (bad (host/flow--bind host/flow--exec-runner (quote (seq (effect a) (wait m))))))
+    (list (get ok :ok) (get bad :ok) (get (get bad :bind-error) :needs)))
+  (list true false (list "effect" "suspend")))
+
+;; ── the execute-fold runner folds a composition against the env :ctx → effect-as-data ──
+(host-fl-test "exec runner: run a composition → {:status done :effects […]}"
+  (let ((r ((get host/flow--exec-runner :run) (quote (effect notify (field "to"))) {:ctx {"to" "alice"}})))
+    (list (get r :status) (map (fn (e) (get e :verb)) (get r :effects)) (get (first (get r :effects)) :args)))
+  (list "done" (list "notify") (list "alice")))
+(host-fl-test "exec runner advertises {effect, branch, each}"
+  (get host/flow--exec-runner :capabilities)
+  (list "effect" "branch" "each"))
+
+(define host-fl-tests-run!
+  (fn ()
+    {:total (+ host-fl-pass host-fl-fail)
+     :passed host-fl-pass :failed host-fl-fail :fails host-fl-fails}))