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Guest Scheme call/cc is escape-only (re-entry hangs), so durable resume uses deterministic replay: suspend escapes to the driver; resume re-runs the flow and replays resolved suspends from a (tag value) log. No live continuation is ever serialized — persisted state is plain data, survives restart. Adds flow/start (now state-returning, backward compatible), flow/resume, flow/cancel, store.sx. Harness reuses one env with a per-test reset (full env rebuild 66x was too slow). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
52 lines
4.3 KiB
Plaintext
52 lines
4.3 KiB
Plaintext
;; lib/flow/spec.sx — flow combinators as a Scheme prelude.
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;;
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;; A flow is a Scheme procedure of one argument: the upstream value.
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;; node : input -> output
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;; A leaf node ignoring its argument is effectively a thunk. Combinators
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;; build composite nodes out of child nodes. The whole flow runs INSIDE the
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;; Scheme interpreter.
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;;
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;; Phase 1 combinators (flow-combinators-src):
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;; flow-node / flow-id / flow-const / sequence / parallel / defflow
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;;
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;; Phase 2 combinators (flow-control-src):
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;; branch / fail / failed? / fail-reason / try-catch / retry / timeout / tick
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;;
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;; Phase 3 suspend core (flow-suspend-src):
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;; The guest Scheme's call/cc is ESCAPE-ONLY (re-invoking a captured k after it
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;; returns hangs the runtime), so suspend/resume CANNOT re-enter a continuation.
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;; Instead, durability uses DETERMINISTIC REPLAY: a flow re-runs from the start
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;; on each resume; suspend points that have already been resolved replay their
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;; logged value, and the first unresolved suspend escapes back to the driver.
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;; The entire persisted state is the replay log (plain (tag value) data), which
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;; survives process restart — no live continuation is ever serialized.
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;;
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;; (suspend tag) — if tag is in the replay log, return its value; else escape
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;; to the driver as (flow-suspended tag). tags must be unique & deterministic
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;; across replays. ALL effects/non-determinism must go through suspend so their
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;; results are logged (otherwise they re-run on every replay).
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;; (flow-drive flow input log) — run flow with the given replay log; returns
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;; (flow-done result) or (flow-suspended tag).
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(define
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flow-combinators-src
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"(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) (define nm body))))")
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(define
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flow-control-src
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"(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)))))")
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(define
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flow-suspend-src
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"(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)))))")
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(define
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flow-load-combinators!
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(fn
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(env)
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(begin
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(scheme-eval-program (scheme-parse-all flow-combinators-src) env)
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(scheme-eval-program (scheme-parse-all flow-control-src) env)
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(scheme-eval-program (scheme-parse-all flow-suspend-src) env)
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env)))
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