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flow: railway attempt combinator — fail-value short-circuit + 10 tests (Phase 6)
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(attempt n1 n2 ...) threads like sequence but stops at the first node returning a
(fail ...) value, returning that failure. Makes the fail/recover error model
compose into validation/ETL pipelines (railway-oriented). 132/132 across 8 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
giles
2026-06-06 18:09:21 +00:00
parent 0a1b89c975
commit d9b9da3843
6 changed files with 95 additions and 7 deletions
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4
lib/flow/spec.sx
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@@ -19,6 +19,8 @@
;; branch / fail / failed? / fail-reason / try-catch / retry / timeout / tick
;; tap (Phase 5): side-effecting pass-through (returns input unchanged).
;; recover (Phase 5): the fail-VALUE counterpart of try-catch.
;; attempt (Phase 6): railway sequence — thread nodes left-to-right but stop at
;; the first node that returns a (fail ...) value, returning that failure.
;;
;; Phase 3 suspend core (flow-suspend-src):
;; The guest Scheme's call/cc is ESCAPE-ONLY (re-invoking a captured k after it
@@ -42,7 +44,7 @@
(define
flow-control-src
"(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)))\n (define (recover node handler)\n (lambda (input)\n (let ((r (node input)))\n (if (failed? r) (handler (fail-reason r)) r))))\n (define (tap effect)\n (lambda (input) (begin (effect input) input)))\n (define (try-catch node handler)\n (lambda (input) (guard (e (#t (handler e))) (node input))))\n (define (flow-retry-step n node input)\n (guard (e (#t (if (<= n 1) (raise e) (flow-retry-step (- n 1) node input))))\n (node input)))\n (define (retry n node) (lambda (input) (flow-retry-step n node input)))\n (define flow-timeout-budget -1)\n (define (tick)\n (if (< flow-timeout-budget 0)\n 0\n (begin\n (set! flow-timeout-budget (- flow-timeout-budget 1))\n (if (< flow-timeout-budget 0)\n (raise (quote flow-timeout))\n flow-timeout-budget))))\n (define (timeout budget node)\n (lambda (input)\n (let ((saved flow-timeout-budget))\n (set! flow-timeout-budget budget)\n (guard (e (#t (begin (set! flow-timeout-budget saved) (raise e))))\n (let ((result (node input)))\n (set! flow-timeout-budget saved)\n result)))))")
"(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)))\n (define (recover node handler)\n (lambda (input)\n (let ((r (node input)))\n (if (failed? r) (handler (fail-reason r)) r))))\n (define (tap effect)\n (lambda (input) (begin (effect input) input)))\n (define (flow-attempt-step ns v)\n (if (failed? v)\n v\n (if (null? ns) v (flow-attempt-step (cdr ns) ((car ns) v)))))\n (define attempt (lambda ns (lambda (input) (flow-attempt-step ns input))))\n (define (try-catch node handler)\n (lambda (input) (guard (e (#t (handler e))) (node input))))\n (define (flow-retry-step n node input)\n (guard (e (#t (if (<= n 1) (raise e) (flow-retry-step (- n 1) node input))))\n (node input)))\n (define (retry n node) (lambda (input) (flow-retry-step n node input)))\n (define flow-timeout-budget -1)\n (define (tick)\n (if (< flow-timeout-budget 0)\n 0\n (begin\n (set! flow-timeout-budget (- flow-timeout-budget 1))\n (if (< flow-timeout-budget 0)\n (raise (quote flow-timeout))\n flow-timeout-budget))))\n (define (timeout budget node)\n (lambda (input)\n (let ((saved flow-timeout-budget))\n (set! flow-timeout-budget budget)\n (guard (e (#t (begin (set! flow-timeout-budget saved) (raise e))))\n (let ((result (node input)))\n (set! flow-timeout-budget saved)\n result)))))")
(define
flow-suspend-src