;; lib/flow/spec.sx — flow combinators as a Scheme prelude.
;;
;; A flow is a Scheme procedure of one argument: the upstream value.
;;   node : input -> output
;; A leaf node ignoring its argument is effectively a thunk. Combinators
;; build composite nodes out of child nodes. The whole flow runs INSIDE the
;; Scheme interpreter.
;;
;; Phase 1 combinators (flow-combinators-src):
;;   flow-node / flow-id / flow-const / sequence / parallel / defflow
;;   defflow both binds the flow and registers it by name (flow-register!, in
;;   store.sx) so it can be re-resolved after a process restart.
;;
;; Phase 2 combinators (flow-control-src):
;;   branch / fail / failed? / fail-reason / try-catch / retry / timeout / tick
;;
;; Phase 3 suspend core (flow-suspend-src):
;;   The guest Scheme's call/cc is ESCAPE-ONLY (re-invoking a captured k after it
;;   returns hangs the runtime), so suspend/resume CANNOT re-enter a continuation.
;;   Instead, durability uses DETERMINISTIC REPLAY: a flow re-runs from the start
;;   on each resume; suspend points that have already been resolved replay their
;;   logged value, and the first unresolved suspend escapes back to the driver.
;;   The entire persisted state is the replay log (plain (tag value) data), which
;;   survives process restart — no live continuation is ever serialized.
;;
;;   (suspend tag)  — if tag is in the replay log, return its value; else escape
;;     to the driver as (flow-suspended tag). tags must be unique & deterministic
;;     across replays. ALL effects/non-determinism must go through suspend so their
;;     results are logged (otherwise they re-run on every replay).
;;   (flow-drive flow input log) — run flow with the given replay log; returns
;;     (flow-done result) or (flow-suspended tag).

(define
  flow-combinators-src
  "(define (flow-node f) f)\n   (define (flow-id input) input)\n   (define (flow-const v) (lambda (input) v))\n   (define (flow-seq-step ns v)\n     (if (null? ns) v (flow-seq-step (cdr ns) ((car ns) v))))\n   (define sequence (lambda ns (lambda (input) (flow-seq-step ns input))))\n   (define parallel (lambda ns (lambda (input) (map (lambda (n) (n input)) ns))))\n   (define-syntax defflow\n     (syntax-rules ()\n       ((defflow nm body)\n        (begin (define nm body) (flow-register! (quote nm) nm)))))")

(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 (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
  "(define flow-replay-log (list))\n   (define flow-suspend-k #f)\n   (define (flow-log-lookup tag log)\n     (if (null? log)\n         (list #f #f)\n         (if (eq? (car (car log)) tag)\n             (list #t (car (cdr (car log))))\n             (flow-log-lookup tag (cdr log)))))\n   (define (suspend tag)\n     (let ((hit (flow-log-lookup tag flow-replay-log)))\n       (if (car hit)\n           (car (cdr hit))\n           (flow-suspend-k (list (quote flow-suspended) tag)))))\n   (define (flow-drive flow input log)\n     (set! flow-replay-log log)\n     (call/cc\n       (lambda (k)\n         (set! flow-suspend-k k)\n         (list (quote flow-done) (flow input)))))")

(define
  flow-load-combinators!
  (fn
    (env)
    (begin
      (scheme-eval-program (scheme-parse-all flow-combinators-src) env)
      (scheme-eval-program (scheme-parse-all flow-control-src) env)
      (scheme-eval-program (scheme-parse-all flow-suspend-src) env)
      env)))