;; lib/events/notify.sx — durable notification delivery flows over an injected
;; transport (lib/flow).
;;
;; Reminders and digests are durable `flow`s: a flow `request`s delivery (a
;; suspend point), the HOST performs the actual send via an injected `dispatch`
;; (the transport — email/push/etc.), and resumes the flow with the outcome.
;; Because flow uses deterministic replay, a completed delivery is never re-run
;; on recovery; the host owns IO and persistence.
;;
;; Delivery is AT-LEAST-ONCE with idempotency. Each message carries an id (the
;; idempotency key). Two protections stop double-delivery:
;;   1. The transport dedups by id — a re-send of a delivered id is a no-op
;;      that still reports ok, so a retry never produces two pings.
;;   2. flow's replay log records each resolved request, so recovery replays the
;;      logged outcome instead of re-issuing the send.
;;
;; Retry/backoff rides flow suspend/resume: each attempt issues a request with a
;; DISTINCT tag `(deliver <id> <n>)` — distinct tags keep deterministic replay
;; correct across retries. The dispatch returns (ok info) to finish or
;; (retry reason) to try again, bounded by `maxn` (then (failed id reason)).
;;
;; A message is a 3-element list (id recipient body). The transport is generic
;; and injected — when feed/notify lands, both consumers share one transport,
;; so this delivery core is a candidate for extraction to `delivery-on-sx`.
;;
;; The Scheme flow source below loads into a flow env (see lib/flow/api.sx).
;; `ev/notify-run` prepends it to a caller program and evaluates in the shared
;; flow env.

(define
  ev-notify-flows-src
  "(define (ev-msg-id m) (car m))\n   (define (ev-msg-recipient m) (car (cdr m)))\n   (define (ev-msg-body m) (car (cdr (cdr m))))\n   (define (ev-mem x xs)\n     (if (null? xs) #f (if (equal? x (car xs)) #t (ev-mem x (cdr xs)))))\n   (define (ev-notify-attempt m n maxn)\n     (let ((r (request (list (quote deliver) (ev-msg-id m) n) m)))\n       (if (eq? (car r) (quote ok))\n           (list (quote delivered) (ev-msg-id m) n)\n           (if (>= n maxn)\n               (list (quote failed) (ev-msg-id m) (car (cdr r)))\n               (ev-notify-attempt m (+ n 1) maxn)))))\n   (define (ev-deliver-reminder maxn)\n     (flow-node (lambda (m) (ev-notify-attempt m 1 maxn))))\n   (define (ev-digest-step ms maxn)\n     (if (null? ms)\n         (list)\n         (cons (ev-notify-attempt (car ms) 1 maxn)\n               (ev-digest-step (cdr ms) maxn))))\n   (define (ev-deliver-digest maxn)\n     (flow-node (lambda (ms) (ev-digest-step ms maxn))))")

;; Run a Scheme flow program with the notify flows preloaded, in the shared
;; flow env. Returns the program's value (SX-native).
(define
  ev/notify-run
  (fn (prog) (flow-run (str ev-notify-flows-src "\n" prog))))