flow: error model — fail/failed?/fail-reason failure values + 6 tests

Explicit (fail reason) values flow downstream as data and are inspected with
failed?/fail-reason — distinct from raised exceptions (retry/try-catch territory).

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

lib/flow/scoreboard.json

@@ -1,13 +1,10 @@
 {
-  "total": 24,
-  "passed": 24,
+  "total": 30,
+  "passed": 30,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
-    "control": { "passed": 6, "total": 6 }
+    "control": { "passed": 12, "total": 12 }
   },
-  "phases": {
-    "phase1": "done",
-    "phase2": "in-progress"
-  }
+  "phases": { "phase1": "done", "phase2": "in-progress" }
 }

lib/flow/scoreboard.md

@@ -1,6 +1,6 @@
 # flow-on-sx Scoreboard
 
-**All tests pass: 24 / 24 across 2 suites.**
+**All tests pass: 30 / 30 across 2 suites.**
 
 `bash lib/flow/conformance.sh`
 
@@ -9,7 +9,7 @@
 | Suite | Passing | Covers |
 |-------|--------:|--------|
 | basic |     18  | Phase 1: single nodes, linear sequence, data-flow threading, defflow, parallel fan/join, nested composition, publish-shaped flow |
-| control |   6  | Phase 2: `branch` conditional — true/false select, threaded predicate, full-node branches, nested 3-way, approval gate |
+| control |  12  | Phase 2: `branch` conditional (6); error model `fail`/`failed?`/`fail-reason` failure values that flow as data (6) |
 
 ## Architecture
 
@@ -36,6 +36,6 @@ capture the flow continuation directly.
 ## Phases
 
 - [x] Phase 1 — Declarative DAG + sequential execution (combinators + 18 tests, `flow/start`)
-- [~] Phase 2 — Control flow + error handling (`branch` done)
+- [~] Phase 2 — Control flow + error handling (`branch`, error model done)
 - [ ] Phase 3 — Suspend / resume (the showcase)
 - [ ] Phase 4 — Distributed nodes via fed-sx

lib/flow/spec.sx

@@ -19,6 +19,9 @@
 ;; Phase 2 combinators (flow-control-src):
 ;;   (branch pred then else) — pred on input selects then/else node
 ;;     (`cond` is a Scheme special form, so the combinator is named `branch`)
+;;   (fail reason)      — make an explicit failure value (data, not an exception)
+;;   (failed? x)        — is x a failure value?
+;;   (fail-reason x)    — the reason carried by a failure value
 
 (define
   flow-combinators-src
@@ -26,7 +29,7 @@
 
 (define
   flow-control-src
-  "(define (branch pred then else)\n     (lambda (input) (if (pred input) (then input) (else input))))")
+  "(define (branch pred then else)\n     (lambda (input) (if (pred input) (then input) (else input))))\n   (define (fail reason) (list (quote flow-fail) reason))\n   (define (failed? x) (and (pair? x) (eq? (car x) (quote flow-fail))))\n   (define (fail-reason x) (car (cdr x)))")
 
 (define
   flow-load-combinators!

lib/flow/tests/control.sx

@@ -50,4 +50,32 @@
     "(defflow publish (branch (lambda (post) (>= (string-length post) 3)) (lambda (post) (string-append post \" [published]\")) (lambda (post) (string-append post \" [rejected]\")))) (flow/start publish \"ok\")")
   "ok [rejected]")
 
+;; ── error model — explicit (fail reason) values ─────────────────
+(flow-ctl-test
+  "fail: failed? is true for a failure value"
+  (flow-c "(failed? (fail 404))")
+  true)
+(flow-ctl-test
+  "fail: fail-reason extracts the reason"
+  (flow-c "(fail-reason (fail 404))")
+  404)
+(flow-ctl-test
+  "fail: failed? is false for a plain value"
+  (flow-c "(failed? 7)")
+  false)
+(flow-ctl-test
+  "fail: failed? is false for an ordinary list"
+  (flow-c "(failed? (list 1 2 3))")
+  false)
+(flow-ctl-test
+  "fail: a node may emit a failure as data"
+  (flow-c
+    "(defflow validate (lambda (s) (if (>= (string-length s) 3) s (fail (quote too-short))))) (failed? (flow/start validate \"hi\"))")
+  true)
+(flow-ctl-test
+  "fail: failure flows downstream, branch recovers"
+  (flow-c
+    "(defflow guarded (sequence (lambda (s) (if (>= (string-length s) 3) (string-length s) (fail (quote too-short)))) (branch failed? (lambda (f) (list (quote recovered) (fail-reason f))) (lambda (n) (list (quote ok) n))))) (flow/start guarded \"hi\")")
+  (list "recovered" "too-short"))
+
 (define flow-ctl-tests-run! (fn () {:total (+ flow-ctl-pass flow-ctl-fail) :passed flow-ctl-pass :failed flow-ctl-fail :fails flow-ctl-fails}))

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **24/24** (Phase 1 done; Phase 2 in progress)
+`bash lib/flow/conformance.sh` → **30/30** (Phase 1 done; Phase 2 in progress)
 
 ## Ground rules
 
@@ -80,7 +80,9 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 - [ ] `retry n [backoff]` — re-runs node up to n times on exception
 - [ ] `timeout ms` — bounds node execution
 - [ ] `try-catch` — exception handler with reified error
-- [ ] error model — exceptions vs explicit `(fail :reason ...)` results
+- [x] error model — exceptions vs explicit `(fail reason)` results: `fail`/`failed?`/
+  `fail-reason` produce/inspect failure values that flow downstream as data
+  (distinct from raised exceptions caught by retry/try-catch). 6 tests.
 - [ ] `lib/flow/tests/control.sx` — 25+ cases: each combinator + composition
 
 ## Phase 3 — Suspend / resume (the showcase)