1	# Durable job runner: a journaled scheduler queue, bounded by a resources pool, with retry and a breaker per target. Source: https://github.com/paymog/bend-kit/tree/main/runner
2	import Base
3	import bend-kit-bytes@0.3.2.0/bytes.bend as Bytes
4	import bend-kit-journal@0.2.0.0/journal.bend as J
5	import bend-kit-scheduler@0.3.0.0/queue.bend as Q
6	import bend-kit-resources@0.1.0.0/resources.bend as Res
7	import bend-kit-retry@0.2.0.0/retry.bend as Retry
8	import bend-kit-time@0.2.0.0/time.bend as Time
9	import bend-kit-collections@0.1.3.0/heap.bend as H
10	import bend-kit-collections@0.1.3.0/omap.bend as O
11	import bend-kit-files@0.2.0.0/files.bend as Files
12	
13	# Why retry gave up on a ticket: its budget was spent, its deadline passed, its failure must not repeat,
14	# or another try would repeat its failure.
15	type Why is Data:
16	  WhySpent{}
17	  WhyLate{}
18	  WhyUnsafe{}
19	  WhyFailed{}
20	
21	# A ticket that retry gave up on, with its payload. It stays until Revive or Cancel takes it.
22	type Dead<-A: Data> is Data:
23	  Dead{ticket: Q.Ticket<A>, why: Why}
24	
25	# One durable transition. Each one is one journal record, appended before the state changes.
26	# Select: the queue's select at now. Replay runs it at the same now, so it takes the same ticket.
27	# Requeue: the attempt failed; it counts one failure. Defer: the breaker held the ticket back; no attempt ran.
28	# Abandon: retry gave up; the running ticket becomes dead. Revive: a dead ticket is pending again at deadline, with a new budget.
29	# Break: the breaker of target changed to state.
30	type Op<-A: Data> is Data:
31	  Enqueue{deadline: Nat, payload: A}
32	  Select{now: Nat}
33	  Complete{id: Nat}
34	  Cancel{id: Nat}
35	  Requeue{id: Nat, deadline: Nat}
36	  Defer{id: Nat, deadline: Nat}
37	  Abandon{id: Nat, why: Why}
38	  Revive{id: Nat, deadline: Nat}
39	  Break{target: String, state: Retry.State}
40	
41	# The failed attempts of a ticket, and its first deadline, from which the policy deadline counts.
42	type Try is Data:
43	  Try{fails: Nat, start: Nat}
44	
45	# A Try for each ticket that is not yet complete, cancelled, or dead, by ticket id.
46	def Tries() -> Data:
47	  O.OMap<Nat, Try>
48	
49	# The breaker state of each target that has one, by target. An absent target is closed with no failures.
50	def Breakers() -> Data:
51	  Map<&2, Retry.State>
52	
53	# The durable state: the queue, the tries, the dead tickets, newest first, and the breakers.
54	type State<-A: Data> is Data:
55	  State{queue: Q.State<A>, tries: Tries(), dead: List<&2, Dead<A>>, breakers: Breakers()}
56	
57	def new(-A: Data) -> State<A>:
58	  State{Q.new(A), O.OMap.new(Nat, Try), Nil{}, Map.new(&2, Retry.State)}
59	
60	def queue(-A: Data, s: State<A>) -> Q.State<A>:
61	  State{q, ts, ds, bs} = s
62	  q
63	
64	def tries(-A: Data, s: State<A>) -> Tries():
65	  State{q, ts, ds, bs} = s
66	  ts
67	
68	# The dead tickets, newest first. A Revive of one makes it pending again; a Cancel drops it.
69	def dead(-A: Data, s: State<A>) -> List<&2, Dead<A>>:
70	  State{q, ts, ds, bs} = s
71	  ds
72	
73	def breakers(-A: Data, s: State<A>) -> Breakers():
74	  State{q, ts, ds, bs} = s
75	  bs
76	
77	# ---- dead tickets
78	
79	def dead.id(-A: Data, d: Dead<A>) -> Nat:
80	  match d:
81	    case Dead{t, w}:
82	      Q.id(A, t)
83	
84	def dead.remove.keep(-A: Data, b: Bool, d: Dead<A>, rest: List<&2, Dead<A>>) -> List<&2, Dead<A>>:
85	  match b:
86	    case True{}:
87	      rest
88	    case False{}:
89	      Con{d, rest}
90	
91	# The dead tickets without ticket i.
92	def dead.remove(-A: Data, ds: List<&2, Dead<A>>, +i: Nat) -> List<&2, Dead<A>>:
93	  match ds:
94	    case Nil{}:
95	      Nil{}
96	    case Con{+d, rest}:
97	      dead.remove.keep(A, Nat.is_eq(dead.id(A, d), i), d, dead.remove(A, rest, i))
98	
99	def dead.find.choose(-A: Data, b: Bool, d: Dead<A>, rest: Maybe<&2, Q.Ticket<A>>) -> Maybe<&2, Q.Ticket<A>>:
100	  match b:
101	    case True{}:
102	      match d:
103	        case Dead{t, w}:
104	          Some{t}
105	    case False{}:
106	      rest
107	
108	# The ticket of dead ticket i.
109	def dead.find(-A: Data, ds: List<&2, Dead<A>>, +i: Nat) -> Maybe<&2, Q.Ticket<A>>:
110	  match ds:
111	    case Nil{}:
112	      None{}
113	    case Con{+d, rest}:
114	      dead.find.choose(A, Nat.is_eq(dead.id(A, d), i), d, dead.find(A, rest, i))
115	
116	def dead.bury(-A: Data, m: Maybe<&2, Q.Ticket<A>>, ds: List<&2, Dead<A>>, w: Why) -> List<&2, Dead<A>>:
117	  match m:
118	    case None{}:
119	      ds
120	    case Some{t}:
121	      Con{Dead{t, w}, ds}
122	
123	# ---- tries
124	
125	def tries.drop(ts: Tries(), +i: Nat) -> Tries():
126	  O.OMap.del(~Nat, ~Try, ~Nat.cmp, ts, i)
127	
128	def tries.get(ts: Tries(), +i: Nat) -> Maybe<&2, Try>:
129	  O.OMap.get(~Nat, ~Try, ~Nat.cmp, ts, i)
130	
131	def tries.bump.got(m: Maybe<&2, Try>, ts: Tries(), +i: Nat) -> Tries():
132	  match m:
133	    case None{}:
134	      ts
135	    case Some{Try{f, st}}:
136	      O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{1n+f, st})
137	
138	def tries.bump(+ts: Tries(), +i: Nat) -> Tries():
139	  tries.bump.got(tries.get(ts, i), ts, i)
140	
141	# ---- breakers
142	
143	def brk.got(r: Breakers() & Retry.State) -> Retry.State:
144	  (m, b) = r
145	  b
146	
147	# The breaker state of target key.
148	def brk.get(-A: Data, s: State<A>, key: String) -> Retry.State:
149	  brk.got(Map.get(Retry.State, Retry.CircuitClosed{Nil{}}, breakers(A, s), key))
150	
151	def brk.state(br: Retry.Breaker, old: Retry.State) -> Retry.State:
152	  match br:
153	    case Retry.BreakerOff{}:
154	      old
155	    case Retry.BreakerOn{c, st}:
156	      st
157	
158	def brk.nats(a: List<&2, Nat>, b: List<&2, Nat>) -> Bool:
159	  match a b:
160	    case Nil{} Nil{}:
161	      True{}
162	    case Con{+x, xs} Con{+y, ys}:
163	      Bool.and(Nat.is_eq(x, y), brk.nats(xs, ys))
164	    case p q:
165	      False{}
166	
167	def brk.same(a: Retry.State, b: Retry.State) -> Bool:
168	  match a b:
169	    case Retry.CircuitClosed{xs} Retry.CircuitClosed{ys}:
170	      brk.nats(xs, ys)
171	    case Retry.CircuitOpen{+x} Retry.CircuitOpen{+y}:
172	      Nat.is_eq(x, y)
173	    case Retry.CircuitHalf{+x} Retry.CircuitHalf{+y}:
174	      Nat.is_eq(x, y)
175	    case p q:
176	      False{}
177	
178	def calm.state(b: Retry.State) -> Retry.State:
179	  match b:
180	    case Retry.CircuitClosed{fs}:
181	      Retry.CircuitClosed{fs}
182	    case Retry.CircuitOpen{u}:
183	      Retry.CircuitOpen{u}
184	    case Retry.CircuitHalf{k}:
185	      Retry.CircuitHalf{0n}
186	
187	def calm.map(m: Breakers()) -> Breakers():
188	  match m:
189	    case MTip{}:
190	      MTip{}
191	    case MLeaf{k, v}:
192	      MLeaf{k, calm.state(v)}
193	    case MNode{p, lo, hi}:
194	      MNode{p, calm.map(lo), calm.map(hi)}
195	
196	# The probes of a half-open breaker died with the process that ran them, so open counts none in flight.
197	def calm(-A: Data, s: State<A>) -> State<A>:
198	  State{q, ts, ds, bs} = s
199	  State{q, ts, ds, calm.map(bs)}
200	
201	# ---- the transition
202	
203	def step.enqueued(-A: Data, r: Q.State<A> & Nat, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers(), d: Nat) -> State<A>:
204	  (q, i) = r
205	  State{q, O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{0n, d}), ds, bs}
206	
207	def step.selected(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers()) -> State<A>:
208	  (q, t) = r
209	  State{q, ts, ds, bs}
210	
211	def step.requeued(-A: Data, r: Q.State<A> & Bool, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers()) -> State<A>:
212	  (q, b) = r
213	  State{q, ts, ds, bs}
214	
215	# Complete takes the ticket from the running list only: O(running), not O(pending).
216	def step.completed(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers()) -> State<A>:
217	  (q, t) = r
218	  State{q, ts, ds, bs}
219	
220	# Abandon takes the ticket from the running list, as Complete does, and keeps it with its payload.
221	def step.abandoned(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers(), w: Why) -> State<A>:
222	  (q, t) = r
223	  State{q, ts, dead.bury(A, t, ds, w), bs}
224	
225	def step.revived(~A: Data, m: Maybe<&2, Q.Ticket<A>>, n: Nat, h: H.Heap<Q.Ticket<A>>, r: List<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, bs: Breakers(), +d: Nat) -> State<A>:
226	  match m:
227	    case None{}:
228	      State{Q.State{n, h, r}, ts, ds, bs}
229	    case Some{Q.Ticket{+i, old, x}}:
230	      State{Q.State{n, H.Heap.push(~Q.Ticket<A>, ~(a => b => Q.le(A, a, b)), Q.Ticket{i, d, x}, h), r}, O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{0n, d}), ds, bs}
231	
232	# The state after op. Live and replay both run it, so they agree.
233	def step(~A: Data, s: State<A>, op: Op<A>) -> State<A>:
234	  match s op:
235	    case State{q, ts, ds, bs} Enqueue{+d, x}:
236	      step.enqueued(A, Q.enqueue(~A, d, x, q), ts, ds, bs, d)
237	    case State{q, ts, ds, bs} Select{now}:
238	      step.selected(A, Q.select(~A, now, q), ts, ds, bs)
239	    case State{q, ts, ds, bs} Complete{+i}:
240	      step.completed(A, Q.complete(A, i, q), tries.drop(ts, i), ds, bs)
241	    case State{q, ts, ds, bs} Cancel{+i}:
242	      State{Q.cancel(~A, i, q), tries.drop(ts, i), dead.remove(A, ds, i), bs}
243	    case State{q, ts, ds, bs} Requeue{+i, d}:
244	      step.requeued(A, Q.requeue(~A, i, d, q), tries.bump(ts, i), ds, bs)
245	    case State{q, ts, ds, bs} Defer{i, d}:
246	      step.requeued(A, Q.requeue(~A, i, d, q), ts, ds, bs)
247	    case State{q, ts, ds, bs} Abandon{+i, w}:
248	      step.abandoned(A, Q.complete(A, i, q), tries.drop(ts, i), ds, bs, w)
249	    case State{Q.State{n, h, r}, ts, +ds, bs} Revive{+i, d}:
250	      step.revived(~A, dead.find(A, ds, i), n, h, r, ts, dead.remove(A, ds, i), bs, d)
251	    case State{q, ts, ds, bs} Break{k, b}:
252	      State{q, ts, ds, Map.set(&2, Retry.State, bs, k, b)}
253	
254	def apply.op(~A: Data, m: Maybe<&2, Op<A>>, s: State<A>) -> Maybe<&2, State<A>>:
255	  match m:
256	    case None{}:
257	      None{}
258	    case Some{op}:
259	      Some{step(~A, s, op)}
260	
261	# Folds one record into the replayed state. A record that does not decode ends the replay with None.
262	def apply(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Op<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
263	  match acc:
264	    case None{}:
265	      None{}
266	    case Some{s}:
267	      apply.op(~A, dec(r), s)
268	
269	# A journal record. Compaction writes one Snap of the whole state; every other record is one op.
270	type Rec<-A: Data> is Data:
271	  Snap{state: State<A>}
272	  Do{op: Op<A>}
273	
274	def load.rec(~A: Data, m: Maybe<&2, Rec<A>>, s: State<A>) -> Maybe<&2, State<A>>:
275	  match m:
276	    case None{}:
277	      None{}
278	    case Some{Snap{t}}:
279	      Some{t}
280	    case Some{Do{op}}:
281	      Some{step(~A, s, op)}
282	
283	# Folds one record into the replayed state: a Snap replaces it, and an op steps it, as apply does.
284	# A record that does not decode ends the replay with None.
285	def load(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Rec<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
286	  match acc:
287	    case None{}:
288	      None{}
289	    case Some{s}:
290	      load.rec(~A, dec(r), s)
291	
292	# ---- pool
293	
294	# Picked: a due ticket and one unit of the pool for it. Idle: no due ticket, or no free unit; the pool is unchanged.
295	type Pick<-A: Data> is Type:
296	  Picked{pool: Res.Pool, ticket: Q.Ticket<A>, grant: Res.Grant}
297	  Idle{pool: Res.Pool}
298	
299	def pick.took(-A: Data, r: Res.Reserve, t: Q.Ticket<A>) -> Pick<A>:
300	  match r:
301	    case Res.Took{p, g}:
302	      Picked{p, t, g}
303	    case Res.Refused{p}:
304	      Idle{p}
305	
306	def pick.due(-A: Data, m: Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
307	  match m:
308	    case None{}:
309	      Idle{p}
310	    case Some{t}:
311	      pick.took(A, Res.reserve(p, 1n), t)
312	
313	def pick.sel(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
314	  (q, m) = r
315	  pick.due(A, m, p)
316	
317	# The ticket that Select{now} takes, with one unit of p, when there is a ticket due and a unit free.
318	def pick(~A: Data, p: Res.Pool, s: State<A>, now: Nat) -> Pick<A>:
319	  State{q, ts, ds, bs} = s
320	  pick.sel(A, Q.select(~A, now, q), p)
321	
322	# ---- retry
323	
324	# The budget left for ticket i: the policy's attempts less its failures, until its start plus the policy deadline.
325	def budget.of(+attempts: Nat, t: Maybe<&2, Try>, d: Maybe<&2, Nat>) -> Retry.Budget:
326	  match t d:
327	    case None{} m:
328	      Retry.Budget{attempts, None{}}
329	    case Some{Try{f, st}} None{}:
330	      Retry.Budget{Nat.sub(attempts, f), None{}}
331	    case Some{Try{f, +st}} Some{ms}:
332	      Retry.Budget{Nat.sub(attempts, f), Some{Nat.add(st, ms)}}
333	
334	def budget(p: Retry.Policy, t: Maybe<&2, Try>) -> Retry.Budget:
335	  Retry.Policy{a, base, cap, d} = p
336	  budget.of(a, t, d)
337	
338	# The failures so far: the index of the next retry.
339	def fails(t: Maybe<&2, Try>) -> Nat:
340	  match t:
341	    case None{}:
342	      0n
343	    case Some{Try{f, st}}:
344	      f
345	
346	# Why retry stopped a transient failure: no attempt is left, or the next wait would reach the deadline.
347	def gave(b: Retry.Budget) -> Why:
348	  Retry.Budget{left, d} = b
349	  match left:
350	    case 0n:
351	      WhySpent{}
352	    case 1n+k:
353	      WhyLate{}
354	
355	# A settled attempt completes its ticket; retry gave up on any other.
356	def stopped(-A: Data, o: Retry.Outcome, b: Retry.Budget, +id: Nat) -> Op<A>:
357	  match o:
358	    case Retry.Settled{}:
359	      Complete{id}
360	    case Retry.Failed{}:
361	      Abandon{id, WhyFailed{}}
362	    case Retry.Unsafe{}:
363	      Abandon{id, WhyUnsafe{}}
364	    case Retry.Transient{w}:
365	      Abandon{id, gave(b)}
366	
367	def after.plan(-A: Data, pl: Retry.Plan, o: Retry.Outcome, b: Retry.Budget, +id: Nat, +now: Nat) -> Op<A>:
368	  match pl:
369	    case Retry.PlanStop{}:
370	      stopped(A, o, b, id)
371	    case Retry.PlanWait{w}:
372	      Requeue{id, Nat.add(now, w)}
373	
374	# The record after an attempt of ticket id ends at now with o: requeue at now plus retry's wait,
375	# complete when o is settled, or abandon when retry gave up.
376	def after(-A: Data, +o: Retry.Outcome, p: Retry.Policy, +b: Retry.Budget, +id: Nat, +n: Nat, +now: Nat, +r: Nat) -> Op<A>:
377	  after.plan(A, Retry.plan(o, p, b, n, now, r), o, b, id, now)
378	
379	# When an open breaker lets attempts start again.
380	def until(br: Retry.Breaker, +now: Nat) -> Nat:
381	  match br:
382	    case Retry.BreakerOn{c, Retry.CircuitOpen{u}}:
383	      u
384	    case Retry.BreakerOn{c, Retry.CircuitClosed{fs}}:
385	      Nat.add(now, Retry.Circuit.cooldown(c))
386	    case Retry.BreakerOn{c, Retry.CircuitHalf{k}}:
387	      Nat.add(now, Retry.Circuit.cooldown(c))
388	    case Retry.BreakerOff{}:
389	      now
390	
391	# The record for a ticket that admit refused: Defer to when the breaker may admit, or Abandon when its budget is spent or late.
392	def held(-A: Data, why: Retry.Deny, br: Retry.Breaker, +id: Nat, +now: Nat) -> Op<A>:
393	  match why:
394	    case Retry.DenyOpen{}:
395	      Defer{id, until(br, now)}
396	    case Retry.DenyLate{}:
397	      Abandon{id, WhyLate{}}
398	    case Retry.DenySpent{}:
399	      Abandon{id, WhySpent{}}
400	
401	# ---- the record codec: a u32le tag, then u64le fields; an Enqueue then holds the payload. An Abandon's second field is why.code.
402	# A Break is tag 10, then its target and its breaker state.
403	
404	def why.code(w: Why) -> Nat:
405	  match w:
406	    case WhySpent{}:
407	      0n
408	    case WhyLate{}:
409	      1n
410	    case WhyUnsafe{}:
411	      2n
412	    case WhyFailed{}:
413	      3n
414	
415	def W() -> Nat:
416	  Nat.mul(65536n, 65536n)
417	
418	# Nat fields hold values under 2^64.
419	def put.u64(b: Bytes.Bytes, +at: U32, +n: Nat) -> Bytes.Bytes:
420	  Bytes.set.u32le(Bytes.set.u32le(b, at, U32.from_nat(Nat.mod(n, W()))), (at + 4 : U32), U32.from_nat(Nat.div(n, W())))
421	
422	def head(+tag: U32, +a: Nat, +c: Nat) -> Bytes.Bytes:
423	  put.u64(put.u64(Bytes.set.u32le(Bytes.new(20), 0, tag), 4, a), 12, c)
424	
425	def snap.u32(out: Bytes.Bytes, +v: U32) -> Bytes.Bytes:
426	  Bytes.append(out, Bytes.set.u32le(Bytes.new(4), 0, v))
427	
428	def snap.u64(out: Bytes.Bytes, +n: Nat) -> Bytes.Bytes:
429	  Bytes.append(out, put.u64(Bytes.new(8), 0, n))
430	
431	def snap.chars(s: String, out: Bytes.Bytes) -> Bytes.Bytes:
432	  match s:
433	    case SNil{}:
434	      out
435	    case SCon{Chr{c}, t}:
436	      snap.chars(t, snap.u32(out, c))
437	
438	# A target is a u32 count of chars, then each char as a u32.
439	def snap.key(out: Bytes.Bytes, +k: String) -> Bytes.Bytes:
440	  snap.chars(k, snap.u32(out, Bytes.count(k, 0)))
441	
442	def snap.nats(xs: List<&2, Nat>, out: Bytes.Bytes) -> Bytes.Bytes:
443	  match xs:
444	    case Nil{}:
445	      out
446	    case Con{x, rest}:
447	      snap.nats(rest, snap.u64(out, x))
448	
449	# A breaker state is a u32 kind. 0 is closed: a u32 count, then each failure as a u64. 1 is open: until. 2 is half-open: busy.
450	def snap.brk(out: Bytes.Bytes, b: Retry.State) -> Bytes.Bytes:
451	  match b:
452	    case Retry.CircuitClosed{+fs}:
453	      snap.nats(fs, snap.u32(snap.u32(out, 0), U32.from_nat(List.length(&2, Nat, fs))))
454	    case Retry.CircuitOpen{u}:
455	      snap.u64(snap.u32(out, 1), u)
456	    case Retry.CircuitHalf{k}:
457	      snap.u64(snap.u32(out, 2), k)
458	
459	def encode(~A: Data, ~penc: A -> Bytes.Bytes, op: Op<A>) -> Bytes.Bytes:
460	  match op:
461	    case Enqueue{+d, x}:
462	      Bytes.append(put.u64(Bytes.set.u32le(Bytes.new(12), 0, 1), 4, d), penc(x))
463	    case Select{+now}:
464	      head(2, now, 0n)
465	    case Complete{+i}:
466	      head(3, i, 0n)
467	    case Cancel{+i}:
468	      head(4, i, 0n)
469	    case Requeue{+i, +d}:
470	      head(5, i, d)
471	    case Defer{+i, +d}:
472	      head(6, i, d)
473	    case Abandon{+i, w}:
474	      head(8, i, why.code(w))
475	    case Revive{+i, +d}:
476	      head(9, i, d)
477	    case Break{k, b}:
478	      snap.brk(snap.key(snap.u32(Bytes.new(0), 10), k), b)
479	
480	def get.field(m: Maybe<&2, U32>) -> U32:
481	  match m:
482	    case None{}:
483	      0
484	    case Some{x}:
485	      x
486	
487	def get.u32(r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & U32:
488	  (b, m) = r
489	  (b, get.field(m))
490	
491	def get.hi(+lo: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
492	  (b, m) = r
493	  (b, Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(get.field(m)), W())))
494	
495	def get.lo(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
496	  (b, m) = r
497	  +lo = get.field(m)
498	  get.hi(lo, Bytes.get.u32le(b, (at + 4 : U32)))
499	
500	def get.u64(b: Bytes.Bytes, +at: U32) -> Bytes.Bytes & Nat:
501	  get.lo(at, Bytes.get.u32le(b, at))
502	
503	# ---- the snapshot codec: tag 7 and the next id, then the heap, the running list, the tries, the dead tickets, and the breakers.
504	# A u32 flag starts each tree node and list cell (1) and marks each end (0), so decode rebuilds the same shape.
505	# A ticket is its u64 id and deadline, then a u32 length and the payload. A dead ticket is its ticket, then a u32 why.code.
506	# A breaker is its target, then its state, as in a Break record.
507	
508	def snap.payload(out: Bytes.Bytes, p: Bytes.Bytes) -> Bytes.Bytes:
509	  Bytes.Bytes{+len, buf} = p
510	  Bytes.append(snap.u32(out, len), Bytes.Bytes{len, buf})
511	
512	def snap.ticket(~A: Data, ~penc: A -> Bytes.Bytes, out: Bytes.Bytes, t: Q.Ticket<A>) -> Bytes.Bytes:
513	  Q.Ticket{+i, +d, x} = t
514	  snap.payload(snap.u64(snap.u64(out, i), d), penc(x))
515	
516	def snap.heap(~A: Data, ~penc: A -> Bytes.Bytes, h: H.Heap<Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
517	  match h:
518	    case H.HLeaf{}:
519	      snap.u32(out, 0)
520	    case H.HNode{+n, t, l, r}:
521	      snap.heap(~A, ~penc, r, snap.heap(~A, ~penc, l, snap.ticket(~A, ~penc, snap.u64(snap.u32(out, 1), n), t)))
522	
523	def snap.run(~A: Data, ~penc: A -> Bytes.Bytes, xs: List<&2, Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
524	  match xs:
525	    case Nil{}:
526	      snap.u32(out, 0)
527	    case Con{t, rest}:
528	      snap.run(~A, ~penc, rest, snap.ticket(~A, ~penc, snap.u32(out, 1), t))
529	
530	def snap.tries(ts: Tries(), out: Bytes.Bytes) -> Bytes.Bytes:
531	  match ts:
532	    case O.OTip{}:
533	      snap.u32(out, 0)
534	    case O.OBin{+n, +k, Try{+f, +st}, lo, hi}:
535	      snap.tries(hi, snap.tries(lo, snap.u64(snap.u64(snap.u64(snap.u32(snap.u32(out, 1), n), k), f), st)))
536	
537	def snap.dead(~A: Data, ~penc: A -> Bytes.Bytes, ds: List<&2, Dead<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
538	  match ds:
539	    case Nil{}:
540	      snap.u32(out, 0)
541	    case Con{Dead{t, w}, rest}:
542	      snap.dead(~A, ~penc, rest, snap.u32(snap.ticket(~A, ~penc, snap.u32(out, 1), t), U32.from_nat(why.code(w))))
543	
544	def snap.brks(xs: List<&2, Sigma<&2, &2, String, _ => Retry.State>>, out: Bytes.Bytes) -> Bytes.Bytes:
545	  match xs:
546	    case Nil{}:
547	      snap.u32(out, 0)
548	    case Con{(k, b), rest}:
549	      snap.brks(rest, snap.brk(snap.key(snap.u32(out, 1), k), b))
550	
551	# The Snap record of s.
552	def encode.snap(~A: Data, ~penc: A -> Bytes.Bytes, s: State<A>) -> Bytes.Bytes:
553	  State{Q.State{+n, h, run}, ts, ds, bs} = s
554	  snap.brks(Map.to_list(&2, Retry.State, bs),
555	    snap.dead(~A, ~penc, ds, snap.tries(ts, snap.run(~A, ~penc, run, snap.heap(~A, ~penc, h, snap.u64(snap.u32(Bytes.new(0), 7), n))))))
556	
557	def encode.rec(~A: Data, ~penc: A -> Bytes.Bytes, r: Rec<A>) -> Bytes.Bytes:
558	  match r:
559	    case Snap{s}:
560	      encode.snap(~A, ~penc, s)
561	    case Do{op}:
562	      encode(~A, ~penc, op)
563	
564	# A snapshot reader: the record, and the offset of the next field.
565	type Rd is Type:
566	  Rd{b: Bytes.Bytes, at: U32}
567	
568	# One step of the snapshot decoder. None: a field runs past the record or does not decode.
569	def Dec(-T: Type) -> Type:
570	  Rd -> Maybe<&1, Rd & T>
571	
572	def Dec.next(-A: Type, -B: Type, m: Maybe<&1, Rd & A>, f: A -> Dec(B)) -> Maybe<&1, Rd & B>:
573	  match m:
574	    case None{}:
575	      None{}
576	    case Some{(r, x)}:
577	      f(x)(r)
578	
579	def Dec.bind(-A: Type, -B: Type, m: Dec(A), f: A -> Dec(B)) -> Dec(B):
580	  r => Dec.next(A, B, m(r), f)
581	
582	def Dec.pure(-A: Type, x: A) -> Dec(A):
583	  r => Some{(r, x)}
584	
585	def Dec.fail(-A: Type) -> Dec(A):
586	  r => None{}
587	
588	def rd.u32.of(m: Maybe<&2, U32>, b: Bytes.Bytes, +at: U32) -> Maybe<&1, Rd & U32>:
589	  match m:
590	    case None{}:
591	      None{}
592	    case Some{x}:
593	      Some{(Rd{b, (at + 4 : U32)}, x)}
594	
595	def rd.u32.got(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Maybe<&1, Rd & U32>:
596	  (b, m) = r
597	  rd.u32.of(m, b, at)
598	
599	def rd.u32.at(r: Rd) -> Maybe<&1, Rd & U32>:
600	  Rd{b, +at} = r
601	  rd.u32.got(at, Bytes.get.u32le(b, at))
602	
603	def rd.u32() -> Dec(U32):
604	  r => rd.u32.at(r)
605	
606	def rd.u64.of(+lo: U32, +hi: U32) -> Nat:
607	  Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(hi), W()))
608	
609	def rd.u64() -> Dec(Nat):
610	  do Dec<Nat>:
611	    lo : U32 <- rd.u32()
612	    hi : U32 <- rd.u32()
613	    return rd.u64.of(lo, hi)
614	
615	def rd.cut.got(+at: U32, +n: U32, r: Bytes.Bytes & Bytes.Bytes) -> Maybe<&1, Rd & Bytes.Bytes>:
616	  (b, p) = r
617	  Some{(Rd{b, (at + n : U32)}, p)}
618	
619	def rd.cut.if(ok: Bool, b: Bytes.Bytes, +at: U32, +n: U32) -> Maybe<&1, Rd & Bytes.Bytes>:
620	  match ok:
621	    case False{}:
622	      None{}
623	    case True{}:
624	      rd.cut.got(at, n, Bytes.slice(b, at, n))
625	
626	def rd.cut.at(+n: U32, r: Rd) -> Maybe<&1, Rd & Bytes.Bytes>:
627	  Rd{Bytes.Bytes{+len, buf}, +at} = r
628	  rd.cut.if(Bytes.fits(len, at, n), Bytes.Bytes{len, buf}, at, n)
629	
630	# The next n octets.
631	def rd.cut(+n: U32) -> Dec(Bytes.Bytes):
632	  r => rd.cut.at(n, r)
633	
634	def rd.flag.of(+m: U32) -> Dec(Bool):
635	  Bool.pick(Dec(Bool), U32.is_eq(m, 0), Dec.pure(Bool, False{}), Bool.pick(Dec(Bool), U32.is_eq(m, 1), Dec.pure(Bool, True{}), Dec.fail(Bool)))
636	
637	# A u32 flag: 1 is True, 0 is False, and any other value fails.
638	def rd.flag() -> Dec(Bool):
639	  Dec.bind(U32, Bool, rd.u32(), m => rd.flag.of(m))
640	
641	def rd.payload(-A: Data, +i: Nat, +d: Nat, m: Maybe<&2, A>) -> Dec(Q.Ticket<A>):
642	  match m:
643	    case None{}:
644	      Dec.fail(Q.Ticket<A>)
645	    case Some{x}:
646	      Dec.pure(Q.Ticket<A>, Q.Ticket{i, d, x})
647	
648	def rd.ticket(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>) -> Dec(Q.Ticket<A>):
649	  do Dec<Q.Ticket<A>>:
650	    i : Nat <- rd.u64()
651	    d : Nat <- rd.u64()
652	    n : U32 <- rd.u32()
653	    p : Bytes.Bytes <- rd.cut(n)
654	    rd.payload(A, i, d, pdec(p))
655	
656	# Each rd.* tree reader takes the flag already read. fuel bounds the depth: every node holds at least one 4-octet flag.
657	def rd.heap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, node: Bool) -> Dec(H.Heap<Q.Ticket<A>>):
658	  match fuel node:
659	    case 0n b:
660	      Dec.fail(H.Heap<Q.Ticket<A>>)
661	    case 1n+f False{}:
662	      Dec.pure(H.Heap<Q.Ticket<A>>, H.HLeaf{})
663	    case 1n+f True{}:
664	      do Dec<H.Heap<Q.Ticket<A>>>:
665	        n : Nat <- rd.u64()
666	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
667	        lf : Bool <- rd.flag()
668	        l : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, lf)
669	        rf : Bool <- rd.flag()
670	        r : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, rf)
671	        return H.HNode{n, t, l, r}
672	
673	def rd.run(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, Q.Ticket<A>>):
674	  match fuel more:
675	    case 0n b:
676	      Dec.fail(List<&2, Q.Ticket<A>>)
677	    case 1n+f False{}:
678	      Dec.pure(List<&2, Q.Ticket<A>>, Nil{})
679	    case 1n+f True{}:
680	      do Dec<List<&2, Q.Ticket<A>>>:
681	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
682	        m : Bool <- rd.flag()
683	        rest : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, f, m)
684	        return Con{t, rest}
685	
686	def rd.tries(+fuel: Nat, node: Bool) -> Dec(O.OMap<Nat, Try>):
687	  match fuel node:
688	    case 0n b:
689	      Dec.fail(O.OMap<Nat, Try>)
690	    case 1n+f False{}:
691	      Dec.pure(O.OMap<Nat, Try>, O.OTip{})
692	    case 1n+f True{}:
693	      do Dec<O.OMap<Nat, Try>>:
694	        n : U32 <- rd.u32()
695	        k : Nat <- rd.u64()
696	        fails : Nat <- rd.u64()
697	        st : Nat <- rd.u64()
698	        lf : Bool <- rd.flag()
699	        lo : O.OMap<Nat, Try> <- rd.tries(f, lf)
700	        hf : Bool <- rd.flag()
701	        hi : O.OMap<Nat, Try> <- rd.tries(f, hf)
702	        return O.OBin{n, k, Try{fails, st}, lo, hi}
703	
704	def rd.why(+c: U32) -> Dec(Why):
705	  Bool.pick(Dec(Why), U32.is_eq(c, 0), Dec.pure(Why, WhySpent{}),
706	  Bool.pick(Dec(Why), U32.is_eq(c, 1), Dec.pure(Why, WhyLate{}),
707	  Bool.pick(Dec(Why), U32.is_eq(c, 2), Dec.pure(Why, WhyUnsafe{}),
708	  Bool.pick(Dec(Why), U32.is_eq(c, 3), Dec.pure(Why, WhyFailed{}), Dec.fail(Why)))))
709	
710	def rd.dead(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, Dead<A>>):
711	  match fuel more:
712	    case 0n b:
713	      Dec.fail(List<&2, Dead<A>>)
714	    case 1n+f False{}:
715	      Dec.pure(List<&2, Dead<A>>, Nil{})
716	    case 1n+f True{}:
717	      do Dec<List<&2, Dead<A>>>:
718	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
719	        c : U32 <- rd.u32()
720	        w : Why <- rd.why(c)
721	        m : Bool <- rd.flag()
722	        rest : List<&2, Dead<A>> <- rd.dead(~A, ~pdec, f, m)
723	        return Con{Dead{t, w}, rest}
724	
725	# Each rd.* sequence reader takes the count or flag already read. fuel bounds its length: every item holds at least 4 octets.
726	def rd.chars(+fuel: Nat, end: Bool, +n: U32) -> Dec(String):
727	  match fuel end:
728	    case 0n b:
729	      Dec.fail(String)
730	    case 1n+f True{}:
731	      Dec.pure(String, SNil{})
732	    case 1n+f False{}:
733	      +k = (n - 1 : U32)
734	      do Dec<String>:
735	        c : U32 <- rd.u32()
736	        rest : String <- rd.chars(f, U32.is_eq(k, 0), k)
737	        return SCon{Chr{c}, rest}
738	
739	def rd.key(+fuel: Nat) -> Dec(String):
740	  do Dec<String>:
741	    +n : U32 <- rd.u32()
742	    rd.chars(fuel, U32.is_eq(n, 0), n)
743	
744	def rd.nats(+fuel: Nat, end: Bool, +n: U32) -> Dec(List<&2, Nat>):
745	  match fuel end:
746	    case 0n b:
747	      Dec.fail(List<&2, Nat>)
748	    case 1n+f True{}:
749	      Dec.pure(List<&2, Nat>, Nil{})
750	    case 1n+f False{}:
751	      +k = (n - 1 : U32)
752	      do Dec<List<&2, Nat>>:
753	        x : Nat <- rd.u64()
754	        rest : List<&2, Nat> <- rd.nats(f, U32.is_eq(k, 0), k)
755	        return Con{x, rest}
756	
757	def rd.closed(+fuel: Nat) -> Dec(Retry.State):
758	  do Dec<Retry.State>:
759	    +n : U32 <- rd.u32()
760	    fs : List<&2, Nat> <- rd.nats(fuel, U32.is_eq(n, 0), n)
761	    return Retry.CircuitClosed{fs}
762	
763	def rd.brk.of(+fuel: Nat, +kind: U32) -> Dec(Retry.State):
764	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 0), rd.closed(fuel),
765	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 1), Dec.bind(Nat, Retry.State, rd.u64(), u => Dec.pure(Retry.State, Retry.CircuitOpen{u})),
766	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 2), Dec.bind(Nat, Retry.State, rd.u64(), k => Dec.pure(Retry.State, Retry.CircuitHalf{k})), Dec.fail(Retry.State))))
767	
768	def rd.brk(+fuel: Nat) -> Dec(Retry.State):
769	  Dec.bind(U32, Retry.State, rd.u32(), k => rd.brk.of(fuel, k))
770	
771	def rd.break(-A: Data, +fuel: Nat) -> Dec(Op<A>):
772	  do Dec<Op<A>>:
773	    k : String <- rd.key(fuel)
774	    b : Retry.State <- rd.brk(fuel)
775	    return Break{k, b}
776	
777	def rd.brks(+fuel: Nat, more: Bool, acc: Map<&2, Retry.State>) -> Dec(Map<&2, Retry.State>):
778	  match fuel more:
779	    case 0n b:
780	      Dec.fail(Map<&2, Retry.State>)
781	    case 1n+f False{}:
782	      Dec.pure(Map<&2, Retry.State>, acc)
783	    case 1n+f True{}:
784	      do Dec<Map<&2, Retry.State>>:
785	        k : String <- rd.key(f)
786	        b : Retry.State <- rd.brk(f)
787	        m : Bool <- rd.flag()
788	        rd.brks(f, m, Map.set(&2, Retry.State, acc, k, b))
789	
790	def rd.state(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat) -> Dec(State<A>):
791	  do Dec<State<A>>:
792	    n : Nat <- rd.u64()
793	    hf : Bool <- rd.flag()
794	    h : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, fuel, hf)
795	    rf : Bool <- rd.flag()
796	    run : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, fuel, rf)
797	    tf : Bool <- rd.flag()
798	    ts : O.OMap<Nat, Try> <- rd.tries(fuel, tf)
799	    df : Bool <- rd.flag()
800	    ds : List<&2, Dead<A>> <- rd.dead(~A, ~pdec, fuel, df)
801	    bf : Bool <- rd.flag()
802	    bs : Map<&2, Retry.State> <- rd.brks(fuel, bf, Map.new(&2, Retry.State))
803	    return State{Q.State{n, h, run}, ts, ds, bs}
804	
805	# The value read, when the reader ended at len, the record's end.
806	def rd.done(-T: Data, +len: U32, m: Maybe<&1, Rd & T>) -> Maybe<&2, T>:
807	  match m:
808	    case None{}:
809	      None{}
810	    case Some{(Rd{b, +at}, +x)}:
811	      Bool.pick(Maybe<&2, T>, U32.is_eq(at, len), Some{x}, None{})
812	
813	# The Abandon of ticket a whose why.code is c, or None for any other code.
814	def decode.abandon(-A: Data, +a: Nat, +c: Nat) -> Maybe<&2, Op<A>>:
815	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 0n), Some{Abandon{a, WhySpent{}}},
816	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 1n), Some{Abandon{a, WhyLate{}}},
817	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 2n), Some{Abandon{a, WhyUnsafe{}}},
818	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 3n), Some{Abandon{a, WhyFailed{}}}, None{}))))
819	
820	def decode.small(-A: Data, ok: Bool, +tag: U32, +a: Nat, +c: Nat) -> Maybe<&2, Op<A>>:
821	  match ok:
822	    case False{}:
823	      None{}
824	    case True{}:
825	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 2), Some{Select{a}},
826	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 3), Some{Complete{a}},
827	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 4), Some{Cancel{a}},
828	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 5), Some{Requeue{a, c}},
829	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 6), Some{Defer{a, c}},
830	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 8), decode.abandon(A, a, c),
831	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 9), Some{Revive{a, c}}, None{})))))))
832	
833	def decode.payload(-A: Data, m: Maybe<&2, A>, +d: Nat) -> Maybe<&2, Op<A>>:
834	  match m:
835	    case None{}:
836	      None{}
837	    case Some{x}:
838	      Some{Enqueue{d, x}}
839	
840	def decode.c(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +tag: U32, +len: U32, +a: Nat, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
841	  (b, +c) = r
842	  decode.small(A, U32.is_eq(len, 20), tag, a, c)
843	
844	def decode.kind(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, enq: Bool, +tag: U32, +len: U32, +a: Nat, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
845	  match enq:
846	    case True{}:
847	      decode.payload(A, pdec(Bytes.snd(Bytes.slice(b, 12, (len - 12 : U32)))), a)
848	    case False{}:
849	      decode.c(~A, ~pdec, tag, len, a, get.u64(b, 12))
850	
851	def decode.a(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +tag: U32, +len: U32, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
852	  (b, +a) = r
853	  decode.kind(~A, ~pdec, U32.is_eq(tag, 1), tag, len, a, b)
854	
855	def decode.tag.if(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, brk: Bool, +tag: U32, +len: U32, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
856	  match brk:
857	    case True{}:
858	      rd.done(Op<A>, len, rd.break(A, U32.to_nat((len / 4 + 1 : U32)))(Rd{b, 4}))
859	    case False{}:
860	      decode.a(~A, ~pdec, tag, len, get.u64(b, 4))
861	
862	def decode.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +len: U32, r: Bytes.Bytes & U32) -> Maybe<&2, Op<A>>:
863	  (b, +tag) = r
864	  decode.tag.if(~A, ~pdec, U32.is_eq(tag, 10), tag, len, b)
865	
866	def decode.len(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, ok: Bool, +len: U32, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
867	  match ok:
868	    case False{}:
869	      None{}
870	    case True{}:
871	      decode.tag(~A, ~pdec, len, get.u32(Bytes.get.u32le(b, 0)))
872	
873	# The op of a record, or None when it is not one that encode writes.
874	def decode(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
875	  Bytes.Bytes{+len, buf} = b
876	  decode.len(~A, ~pdec, U32.is_ge(len, 12), len, Bytes.Bytes{len, buf})
877	
878	# The state of a Snap record whose tag is already checked. The decoder must end at the record's end.
879	def decode.snap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
880	  Bytes.Bytes{+len, buf} = b
881	  rd.done(State<A>, len, rd.state(~A, ~pdec, U32.to_nat((len / 4 + 1 : U32)))(Rd{Bytes.Bytes{len, buf}, 4}))
882	
883	def decode.snapped(-A: Data, m: Maybe<&2, State<A>>) -> Maybe<&2, Rec<A>>:
884	  match m:
885	    case None{}:
886	      None{}
887	    case Some{s}:
888	      Some{Snap{s}}
889	
890	def decode.did(-A: Data, m: Maybe<&2, Op<A>>) -> Maybe<&2, Rec<A>>:
891	  match m:
892	    case None{}:
893	      None{}
894	    case Some{op}:
895	      Some{Do{op}}
896	
897	def decode.rec.if(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, snap: Bool, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
898	  match snap:
899	    case True{}:
900	      decode.snapped(A, decode.snap(~A, ~pdec, b))
901	    case False{}:
902	      decode.did(A, decode(~A, ~pdec, b))
903	
904	def decode.rec.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, r: Bytes.Bytes & U32) -> Maybe<&2, Rec<A>>:
905	  (b, +tag) = r
906	  decode.rec.if(~A, ~pdec, U32.is_eq(tag, 7), b)
907	
908	# The Rec of a journal record, or None when it is neither a Snap nor an op that encode.rec writes.
909	def decode.rec(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
910	  decode.rec.tag(~A, ~pdec, get.u32(Bytes.get.u32le(b, 0)))
911	
912	# ---- the runner
913	
914	# policy: retry's policy for each ticket. circuit: the breaker of each target. idle: ms that drain waits when no attempt started.
915	# compact: tick compacts the journal once records past this many octets were appended since open or the last compaction; 0 never.
916	type Config is Data:
917	  Config{policy: Retry.Policy, circuit: Retry.Circuit, idle: U32, compact: U32}
918	
919	# The journal, its path, its file cap, and its length at open or at the last compaction.
920	type Log is Type:
921	  Log{journal: J.Journal, path: String, limit: U32, base: U32}
922	
923	# An attempt that runs: its unit of the pool, the budget that admit left, and its target.
924	type Flight<-A: Data> is Type:
925	  Flight{ticket: Q.Ticket<A>, grant: Res.Grant, budget: Retry.Budget, target: String}
926	
927	def Out(-X: Type) -> Type:
928	  Result<&1, &1, U32 & String, X>
929	
930	# A message to the fiber that owns the runner. Ended: an attempt finished. Add and Drop: another fiber asks for an enqueue or a cancel.
931	type Msg<-A: Data> is Type:
932	  Ended{id: Nat, outcome: Retry.Outcome}
933	  Add{deadline: Nat, payload: A, reply: Chan(Out(Nat))}
934	  Drop{id: Nat, reply: Chan(Out(Unit))}
935	
936	# An open runner. Replaying its journal gives its state; its pool bounds the attempts that run at once.
937	# Each attempt sends its outcome, and each other fiber its ask, on inbox. flights are the attempts that run.
938	type Runner<-A: Data> is Type:
939	  Runner{log: Log, state: State<A>, pool: Res.Pool, config: Config, inbox: Chan(Msg<A>), flights: List<&1, Flight<A>>}
940	
941	# The runner, the claim fuel left, and the first error.
942	def Claimed(-A: Data) -> Type:
943	  Runner<A> & Nat & Out(Unit)
944	
945	def Ran(-A: Data) -> Type:
946	  Runner<A> & Out(Unit)
947	
948	# Wall-clock ms since the Unix epoch. Deadlines must keep their meaning across a restart.
949	def clock() -> IO(Nat):
950	  do IO<Nat>:
951	    t : Time.Instant <- Time.now()
952	    return Time.Duration.to_ms_nat(Time.Instant.since_epoch(t))
953	
954	def first(a: Out(Unit), b: Out(Unit)) -> Out(Unit):
955	  match a:
956	    case Fail{e}:
957	      Fail{e}
958	    case Done{u}:
959	      b
960	
961	def log.appended(+path: String, +limit: U32, +base: U32, x: J.Journal & Out(Unit)) -> Log & Out(Unit):
962	  (j, o) = x
963	  (Log{j, path, limit, base}, o)
964	
965	def log.append(l: Log, p: Bytes.Bytes) -> IO(Log & Out(Unit)):
966	  Log{j, +path, +limit, +base} = l
967	  do IO<Log & Out(Unit)>:
968	    x : J.Journal & Out(Unit) <- J.append(j, p)
969	    return log.appended(path, limit, base, x)
970	
971	def log.close(l: Log) -> IO(Unit):
972	  Log{j, path, limit, base} = l
973	  J.close(j)
974	
975	# The journal reads with limit as its record cap, so that a snapshot fits; appends keep max.
976	def log.of(j: J.Journal, +max: U32, path: String, +limit: U32) -> Log:
977	  J.Journal{file, cap, +end, ok} = j
978	  Log{J.Journal{file, max, end, ok}, path, limit, end}
979	
980	def commit.done(~A: Data, x: Log & Out(Unit), s: State<A>, +op: Op<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Ran(A):
981	  match x:
982	    case (j, Done{u}):
983	      (Runner{j, step(~A, s, op), p, c, ib, fs}, Done{u})
984	    case (j, Fail{e}):
985	      (Runner{j, s, p, c, ib, fs}, Fail{e})
986	
987	# Appends op, and applies it once the append is durable. After a failed append the state is as it was,
988	# and the journal refuses every later append until the runner is reopened.
989	def commit(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +op: Op<A>) -> IO(Ran(A)):
990	  Runner{j, s, p, c, ib, fs} = r
991	  do IO<Ran(A)>:
992	    x : Log & Out(Unit) <- log.append(j, encode(~A, ~penc, op))
993	    return commit.done(~A, x, s, op, p, c, ib, fs)
994	
995	def enqueue.done(-A: Data, +id: Nat, y: Ran(A)) -> Runner<A> & Out(Nat):
996	  match y:
997	    case (r, Done{u}):
998	      (r, Done{id})
999	    case (r, Fail{e}):
1000	      (r, Fail{e})
1001	
1002	# Adds a job that is due at deadline (wall-clock ms) and answers its ticket id once the record is durable.
1003	def enqueue(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +deadline: Nat, x: A) -> IO(Runner<A> & Out(Nat)):
1004	  Runner{j, +s, p, c, ib, fs} = r
1005	  do IO<Runner<A> & Out(Nat)>:
1006	    y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, c, ib, fs}, Enqueue{deadline, x})
1007	    return enqueue.done(A, Q.next_id(A, queue(A, s)), y)
1008	
1009	# Cancels ticket id. A running attempt finishes, but its ticket is never selected again. A dead ticket is dropped.
1010	def cancel(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat) -> IO(Ran(A)):
1011	  commit(~A, ~penc, r, Cancel{id})
1012	
1013	# Makes dead ticket id pending again at deadline (wall-clock ms), with its payload and a new retry budget.
1014	# A ticket that is not dead does not change.
1015	def revive(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat, +deadline: Nat) -> IO(Ran(A)):
1016	  commit(~A, ~penc, r, Revive{id, deadline})
1017	
1018	# ---- asks from other fibers
1019	
1020	# The runner back, and its inbox. Another fiber passes the inbox to ask.enqueue and ask.cancel.
1021	def inbox(-A: Data, r: Runner<A>) -> Runner<A> & Chan(Msg<A>):
1022	  Runner{j, s, p, c, +ib, fs} = r
1023	  (Runner{j, s, p, c, ib, fs}, ib)
1024	
1025	def ask.answer(-X: Type, m: Maybe<&1, Out(X)>) -> Out(X):
1026	  match m:
1027	    case None{}:
1028	      Fail{(32, "runner: the runner closed before it answered")}
1029	    case Some{o}:
1030	      o
1031	
1032	def ask.sent(-A: Data, -X: Type, x: Result<&1, &1, Msg<A>, Unit>, +rep: Chan(Out(X))) -> IO(Out(X)):
1033	  match x:
1034	    case Fail{m}:
1035	      IO.pure(Out(X), Fail{(32, "runner: the inbox is closed")})
1036	    case Done{u}:
1037	      do IO<Out(X)>:
1038	        a : Maybe<&1, Out(X)> <- Chan.recv(Out(X), rep)
1039	        return ask.answer(X, a)
1040	
1041	# Sends m, built with a new reply channel, on the inbox and waits for the reply.
1042	def ask(-A: Data, -X: Type, +ib: Chan(Msg<A>), m: Chan(Out(X)) -> Msg<A>) -> IO(Out(X)):
1043	  do IO<Out(X)>:
1044	    +rep : Chan(Out(X)) <- Chan.new(Out(X), 1)
1045	    x : Result<&1, &1, Msg<A>, Unit> <- Chan.send(Msg<A>, ib, m(rep))
1046	    ask.sent(A, X, x, rep)
1047	
1048	# From another fiber: adds a job due at deadline, and answers its ticket id once the record is durable.
1049	# The answer comes when the fiber that owns the runner reads the inbox, in tick or drain.
1050	def ask.enqueue(-A: Data, +ib: Chan(Msg<A>), +deadline: Nat, x: A) -> IO(Out(Nat)):
1051	  ask(A, Nat, ib, rep => Add{deadline, x, rep})
1052	
1053	# From another fiber: cancels ticket id, as cancel does. The answer comes when the owner reads the inbox.
1054	def ask.cancel(-A: Data, +ib: Chan(Msg<A>), +id: Nat) -> IO(Out(Unit)):
1055	  ask(A, Unit, ib, rep => Drop{id, rep})
1056	
1057	# ---- one round
1058	
1059	# A grant of this runner's pool always fits it, so Kept does not happen.
1060	def release.got(-A: Data, x: Res.Release, j: Log, s: State<A>, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Runner<A>:
1061	  match x:
1062	    case Res.Released{p}:
1063	      Runner{j, s, p, c, ib, fs}
1064	    case Res.Kept{p, g}:
1065	      Runner{j, s, p, c, ib, fs}
1066	
1067	def release(-A: Data, r: Runner<A>, g: Res.Grant) -> Runner<A>:
1068	  Runner{j, s, p, c, ib, fs} = r
1069	  release.got(A, Res.release(p, g), j, s, c, ib, fs)
1070	
1071	def claim.held(-A: Data, y: Ran(A), g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1072	  match y:
1073	    case (r, Fail{e}):
1074	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1075	    case (r, Done{u}):
1076	      next(release(A, r, g))
1077	
1078	def claim.tag(-A: Data, +id: Nat, o: Retry.Outcome) -> IO(Msg<A>):
1079	  IO.pure(Msg<A>, Ended{id, o})
1080	
1081	def trip.if(~A: Data, ~penc: A -> Bytes.Bytes, same: Bool, r: Runner<A>, +key: String, st: Retry.State) -> IO(Ran(A)):
1082	  match same:
1083	    case True{}:
1084	      IO.pure(Ran(A), (r, Done{Unit{}}))
1085	    case False{}:
1086	      commit(~A, ~penc, r, Break{key, st})
1087	
1088	# Appends a Break when the target's breaker left old, so that a reopen restores it.
1089	def trip(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +key: String, +old: Retry.State, br: Retry.Breaker) -> IO(Ran(A)):
1090	  +st = brk.state(br, old)
1091	  trip.if(~A, ~penc, brk.same(old, st), r, key, st)
1092	
1093	def claim.go(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, gate: Retry.Gate, +br: Retry.Breaker, b: Retry.Budget, +key: String,
1094	    r: Runner<A>, +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1095	  match gate:
1096	    case Retry.GateGo{}:
1097	      Runner{j, s, p, c, +ib, fs} = r
1098	      do IO<Claimed(A)>:
1099	        u : Chan(Msg<A>) <- IO.fork.go(Msg<A>, IO.bind(Retry.Outcome, Msg<A>, work(x), o => claim.tag(A, id, o)), ib)
1100	        next(Runner{j, s, p, c, ib, Con{Flight{Q.Ticket{id, d, x}, g, b, key}, fs}})
1101	    case Retry.GateNo{why}:
1102	      do IO<Claimed(A)>:
1103	        y : Ran(A) <- commit(~A, ~penc, r, held(A, why, br, id, now))
1104	        claim.held(A, y, g, next)
1105	
1106	def claim.tripped(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, y: Ran(A), gate: Retry.Gate, +br: Retry.Breaker, b: Retry.Budget, +key: String,
1107	    +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1108	  match y:
1109	    case (r, Fail{e}):
1110	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1111	    case (r, Done{u}):
1112	      claim.go(~A, ~penc, ~work, now, gate, br, b, key, r, id, d, x, g, next)
1113	
1114	# A change to the breaker is durable before the attempt starts.
1115	def claim.gate(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, a: Retry.Breaker & Retry.Budget & Retry.Gate, +key: String, +old: Retry.State,
1116	    r: Runner<A>, +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1117	  (+br, b, gate) = a
1118	  do IO<Claimed(A)>:
1119	    y : Ran(A) <- trip(~A, ~penc, r, key, old, br)
1120	    claim.tripped(~A, ~penc, ~work, now, y, gate, br, b, key, id, d, x, g, next)
1121	
1122	# The selected ticket asks its target's breaker and its own budget for an attempt.
1123	def claim.admit(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, r: Runner<A>, t: Q.Ticket<A>, g: Res.Grant,
1124	    next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1125	  Runner{j, +s, p, +c, ib, fs} = r
1126	  Config{pol, circ, idle, every} = c
1127	  Q.Ticket{+id, d, +x} = t
1128	  +key = target(x)
1129	  +old = brk.get(A, s, key)
1130	  claim.gate(~A, ~penc, ~work, now, Retry.admit(Retry.BreakerOn{circ, old}, budget(pol, tries.get(tries(A, s), id)), now), key, old,
1131	    Runner{j, s, p, c, ib, fs}, id, d, x, g, next)
1132	
1133	def claim.selected(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, y: Ran(A),
1134	    t: Q.Ticket<A>, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1135	  match y:
1136	    case (r, Fail{e}):
1137	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1138	    case (r, Done{u}):
1139	      claim.admit(~A, ~penc, ~target, ~work, now, r, t, g, next)
1140	
1141	def claim.pick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +left: Nat, +now: Nat, k: Pick<A>, j: Log, s: State<A>,
1142	    c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1143	  match k:
1144	    case Idle{p}:
1145	      IO.pure(Claimed(A), (Runner{j, s, p, c, ib, fs}, Nat.add(1n, left), Done{Unit{}}))
1146	    case Picked{p, t, g}:
1147	      do IO<Claimed(A)>:
1148	        y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, c, ib, fs}, Select{now})
1149	        claim.selected(~A, ~penc, ~target, ~work, now, y, t, g, next)
1150	
1151	# Starts an attempt for each due ticket while the pool has a free unit; each attempt sends its outcome on the inbox.
1152	# fuel bounds the tickets taken, and the fuel left comes back.
1153	def claim(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +now: Nat, r: Runner<A>) -> IO(Claimed(A)):
1154	  match fuel:
1155	    case 0n:
1156	      IO.pure(Claimed(A), (r, 0n, Done{Unit{}}))
1157	    case 1n+n:
1158	      Runner{j, +s, p, c, ib, fs} = r
1159	      +m = n
1160	      claim.pick(~A, ~penc, ~target, ~work, m, now, pick(~A, p, s, now), j, s, c, ib, fs, (r2 => claim(~A, ~penc, ~target, ~work, m, now, r2)))
1161	
1162	def settle.done(-A: Data, y: Ran(A), g: Res.Grant, err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1163	  (r, res) = y
1164	  next(release(A, r, g), first(err, res))
1165	
1166	def settle.br(~A: Data, ~penc: A -> Bytes.Bytes, +op: Op<A>, y: Ran(A), g: Res.Grant, err: Out(Unit),
1167	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1168	  match y:
1169	    case (r, Fail{e}):
1170	      next(release(A, r, g), first(err, Fail{e}))
1171	    case (r, Done{u}):
1172	      do IO<Runner<A> & Out(Bool)>:
1173	        z : Ran(A) <- commit(~A, ~penc, r, op)
1174	        settle.done(A, z, g, err, next)
1175	
1176	# Tells the target's breaker, appends the outcome's op, and returns the attempt's unit to the pool.
1177	# The op of a cancelled ticket changes no state: the queue no longer holds it.
1178	def settle.op(~A: Data, ~penc: A -> Bytes.Bytes, +o: Retry.Outcome, b: Retry.Budget, +id: Nat, +key: String, +now: Nat, +rnd: Nat, r: Runner<A>, g: Res.Grant,
1179	    err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1180	  Runner{j, +s, p, +c, ib, fs} = r
1181	  Config{pol, circ, idle, every} = c
1182	  +old = brk.get(A, s, key)
1183	  do IO<Runner<A> & Out(Bool)>:
1184	    y : Ran(A) <- trip(~A, ~penc, Runner{j, s, p, c, ib, fs}, key, old, Retry.Breaker.done(Retry.BreakerOn{circ, old}, now, Retry.failed(o)))
1185	    settle.br(~A, ~penc, after(A, o, pol, b, id, fails(tries.get(tries(A, s), id)), now, rnd), y, g, err, next)
1186	
1187	def random(r: Out(U32)) -> Nat:
1188	  match r:
1189	    case Done{x}:
1190	      U32.to_nat(x)
1191	    case Fail{e}:
1192	      0n
1193	
1194	def take.keep(-A: Data, m: Maybe<&1, Flight<A>>, xs: List<&1, Flight<A>>) -> List<&1, Flight<A>>:
1195	  match m:
1196	    case None{}:
1197	      xs
1198	    case Some{f}:
1199	      Con{f, xs}
1200	
1201	def take.at(-A: Data, eq: Bool, f: Flight<A>, r: Maybe<&1, Flight<A>> & List<&1, Flight<A>>) -> Maybe<&1, Flight<A>> & List<&1, Flight<A>>:
1202	  match eq:
1203	    case True{}:
1204	      (m, xs) = r
1205	      (Some{f}, take.keep(A, m, xs))
1206	    case False{}:
1207	      (m, xs) = r
1208	      (m, Con{f, xs})
1209	
1210	# The flight of ticket id, and the other flights.
1211	def take(-A: Data, fs: List<&1, Flight<A>>, +id: Nat) -> Maybe<&1, Flight<A>> & List<&1, Flight<A>>:
1212	  match fs:
1213	    case Nil{}:
1214	      (None{}, Nil{})
1215	    case Con{Flight{Q.Ticket{+i, d, x}, g, b, k}, rest}:
1216	      take.at(A, Nat.is_eq(i, id), Flight{Q.Ticket{i, d, x}, g, b, k}, take(A, rest, id))
1217	
1218	def settle.flight(~A: Data, ~penc: A -> Bytes.Bytes, mf: Maybe<&1, Flight<A>>, +o: Retry.Outcome, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1219	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1220	  match mf:
1221	    case None{}:
1222	      next(r, err)
1223	    case Some{Flight{Q.Ticket{id, d, x}, g, b, key}}:
1224	      settle.op(~A, ~penc, o, b, id, key, now, rnd, r, g, err, next)
1225	
1226	def settle.take(~A: Data, ~penc: A -> Bytes.Bytes, t: Maybe<&1, Flight<A>> & List<&1, Flight<A>>, +o: Retry.Outcome, +now: Nat, +rnd: Nat,
1227	    j: Log, s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), err: Out(Unit),
1228	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1229	  (mf, fs) = t
1230	  settle.flight(~A, ~penc, mf, o, now, rnd, Runner{j, s, p, c, ib, fs}, err, next)
1231	
1232	# Settles the flight of ticket id with outcome o.
1233	def settle(~A: Data, ~penc: A -> Bytes.Bytes, +id: Nat, +o: Retry.Outcome, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1234	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1235	  Runner{j, s, p, c, ib, fs} = r
1236	  settle.take(~A, ~penc, take(A, fs, id), o, now, rnd, j, s, p, c, ib, err, next)
1237	
1238	# Sends the failure to the asker, and keeps it as the round's error.
1239	def asked.failed(-A: Data, -X: Type, +rep: Chan(Out(X)), r: Runner<A>, +c: U32, +m: String, err: Out(Unit),
1240	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1241	  do IO<Runner<A> & Out(Bool)>:
1242	    x : Result<&1, &1, Out(X), Unit> <- Chan.send(Out(X), rep, Fail{(c, m)})
1243	    next(r, 0n, first(err, Fail{(c, m)}))
1244	
1245	def asked.fail(-A: Data, -X: Type, +rep: Chan(Out(X)), r: Runner<A>, e: U32 & String, err: Out(Unit),
1246	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1247	  (c, m) = e
1248	  asked.failed(A, X, rep, r, c, m, err, next)
1249	
1250	def added(-A: Data, +rep: Chan(Out(Nat)), y: Runner<A> & Out(Nat), err: Out(Unit),
1251	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1252	  match y:
1253	    case (r, Done{+i}):
1254	      do IO<Runner<A> & Out(Bool)>:
1255	        x : Result<&1, &1, Out(Nat), Unit> <- Chan.send(Out(Nat), rep, Done{i})
1256	        next(r, 1n, err)
1257	    case (r, Fail{e}):
1258	      asked.fail(A, Nat, rep, r, e, err, next)
1259	
1260	def dropped(-A: Data, +rep: Chan(Out(Unit)), y: Ran(A), err: Out(Unit),
1261	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1262	  match y:
1263	    case (r, Done{u}):
1264	      do IO<Runner<A> & Out(Bool)>:
1265	        x : Result<&1, &1, Out(Unit), Unit> <- Chan.send(Out(Unit), rep, Done{Unit{}})
1266	        next(r, 0n, err)
1267	    case (r, Fail{e}):
1268	      asked.fail(A, Unit, rep, r, e, err, next)
1269	
1270	# Settles an ended attempt, or appends an ask's op and answers it. next gets the claim fuel the message adds: 1 for an enqueue.
1271	def handle(~A: Data, ~penc: A -> Bytes.Bytes, m: Msg<A>, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1272	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1273	  match m:
1274	    case Ended{+id, +o}:
1275	      settle(~A, ~penc, id, o, now, rnd, r, err, (r2 => e2 => next(r2, 0n, e2)))
1276	    case Add{+d, x, +rep}:
1277	      IO.bind(Runner<A> & Out(Nat), Runner<A> & Out(Bool), enqueue(~A, ~penc, r, d, x), y => added(A, rep, y, err, next))
1278	    case Drop{+i, +rep}:
1279	      IO.bind(Ran(A), Runner<A> & Out(Bool), cancel(~A, ~penc, r, i), y => dropped(A, rep, y, err, next))
1280	
1281	def merged(-A: Data, e: Out(Unit), k: Claimed(A)) -> Claimed(A):
1282	  (r, n, err) = k
1283	  (r, n, first(e, err))
1284	
1285	def empty.heap(-A: Data, h: H.Heap<Q.Ticket<A>>) -> Bool:
1286	  match h:
1287	    case H.HLeaf{}:
1288	      True{}
1289	    case H.HNode{n, t, l, r}:
1290	      False{}
1291	
1292	def empty.list(-A: Data, xs: List<&2, Q.Ticket<A>>) -> Bool:
1293	  match xs:
1294	    case Nil{}:
1295	      True{}
1296	    case Con{t, rest}:
1297	      False{}
1298	
1299	# No ticket is pending or running. Dead tickets wait for a Revive, so they do not count.
1300	def idle(-A: Data, s: State<A>) -> Bool:
1301	  State{Q.State{n, h, run}, ts, ds, bs} = s
1302	  Bool.and(empty.heap(A, h), empty.list(A, run))
1303	
1304	def pending(-A: Data, s: State<A>) -> Nat:
1305	  State{Q.State{n, h, run}, ts, ds, bs} = s
1306	  H.Heap.size(Q.Ticket<A>, h)
1307	
1308	# The flights back, and whether none runs.
1309	def flights.none(-A: Data, fs: List<&1, Flight<A>>) -> List<&1, Flight<A>> & Bool:
1310	  match fs:
1311	    case Nil{}:
1312	      (Nil{}, True{})
1313	    case Con{f, rest}:
1314	      (Con{f, rest}, False{})
1315	
1316	# ---- compaction
1317	
1318	# Kept: compaction failed before the rename, and the old journal stands.
1319	# Lost: the rename happened, but the new journal did not open. Moved: the journal at path is the snapshot.
1320	type Swap is Type:
1321	  Kept{err: U32 & String}
1322	  Lost{err: U32 & String}
1323	  Moved{journal: J.Journal}
1324	
1325	def swap.reopened(x: Out(J.Journal & Unit)) -> Swap:
1326	  match x:
1327	    case Fail{e}:
1328	      Lost{e}
1329	    case Done{(j, u)}:
1330	      Moved{j}
1331	
1332	# The reopen fsyncs the directory, so the rename is durable before the next append.
1333	def swap.renamed(+path: String, +limit: U32, x: Out(Unit)) -> IO(Swap):
1334	  match x:
1335	    case Fail{e}:
1336	      IO.pure(Swap, Kept{e})
1337	    case Done{u}:
1338	      do IO<Swap>:
1339	        y : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), path, limit, limit, Unit{})
1340	        return swap.reopened(y)
1341	
1342	def swap.written(+path: String, +tmp: String, +limit: U32, x: J.Journal & Out(Unit)) -> IO(Swap):
1343	  match x:
1344	    case (t, Fail{e}):
1345	      do IO<Swap>:
1346	        J.close(t)
1347	        return Kept{e}
1348	    case (t, Done{u}):
1349	      do IO<Swap>:
1350	        J.close(t)
1351	        y : Out(Unit) <- Files.rename(tmp, path)
1352	        swap.renamed(path, limit, y)
1353	
1354	def swap.opened(+path: String, +tmp: String, +limit: U32, x: Out(J.Journal & Unit), snap: Bytes.Bytes) -> IO(Swap):
1355	  match x:
1356	    case Fail{e}:
1357	      IO.pure(Swap, Kept{e})
1358	    case Done{(t, u)}:
1359	      do IO<Swap>:
1360	        y : J.Journal & Out(Unit) <- J.append(t, snap)
1361	        swap.written(path, tmp, limit, y)
1362	
1363	# Writes snap as the only record of path.compact, fsyncs it, and renames it over path.
1364	def swap(+path: String, +limit: U32, snap: Bytes.Bytes) -> IO(Swap):
1365	  +tmp = path ++ ".compact"
1366	  do IO<Swap>:
1367	    gone : Out(Unit) <- Files.remove(tmp)
1368	    x : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), tmp, limit, limit, Unit{})
1369	    swap.opened(path, tmp, limit, x, snap)
1370	
1371	def log.stop(j: J.Journal) -> J.Journal:
1372	  J.Journal{file, max, end, ok} = j
1373	  J.Journal{file, max, end, False{}}
1374	
1375	def compact.moved(x: Swap, j: J.Journal, +max: U32, path: String, +limit: U32, +base: U32) -> IO(Log & Out(Unit)):
1376	  match x:
1377	    case Kept{e}:
1378	      IO.pure(Log & Out(Unit), (Log{j, path, limit, base}, Fail{e}))
1379	    case Lost{e}:
1380	      IO.pure(Log & Out(Unit), (Log{log.stop(j), path, limit, base}, Fail{e}))
1381	    case Moved{k}:
1382	      do IO<Log & Out(Unit)>:
1383	        J.close(j)
1384	        return (log.of(k, max, path, limit), Done{Unit{}})
1385	
1386	def compact.if(~A: Data, ~penc: A -> Bytes.Bytes, ok: Bool, j: J.Journal, +max: U32, +path: String, +limit: U32, +base: U32, s: State<A>) -> IO(Log & Out(Unit)):
1387	  match ok:
1388	    case False{}:
1389	      IO.pure(Log & Out(Unit), (Log{j, path, limit, base}, Fail{(5, "runner: an append failed; reopen the runner")}))
1390	    case True{}:
1391	      do IO<Log & Out(Unit)>:
1392	        x : Swap <- swap(path, limit, encode.snap(~A, ~penc, s))
1393	        compact.moved(x, j, max, path, limit, base)
1394	
1395	def compact.done(-A: Data, x: Log & Out(Unit), s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Ran(A):
1396	  (l, o) = x
1397	  (Runner{l, s, p, c, ib, fs}, o)
1398	
1399	# Writes the state as one Snap record into a new journal, fsyncs it, and renames it over the old one.
1400	# A crash leaves one of the two journals, and both replay to this state. After a failure the old
1401	# journal stands; when the failure came after the rename, appends stop until the runner is reopened.
1402	def compact(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>) -> IO(Ran(A)):
1403	  Runner{Log{j, path, limit, base}, +s, p, c, ib, fs} = r
1404	  J.Journal{file, +max, end, +ok} = j
1405	  do IO<Ran(A)>:
1406	    x : Log & Out(Unit) <- compact.if(~A, ~penc, ok, J.Journal{file, max, end, ok}, max, path, limit, base, s)
1407	    return compact.done(A, x, s, p, c, ib, fs)
1408	
1409	def tick.ran(e: Out(Unit), b: Bool) -> Out(Bool):
1410	  match e:
1411	    case Fail{x}:
1412	      Fail{x}
1413	    case Done{u}:
1414	      Done{b}
1415	
1416	def tick.compacted(-A: Data, +b: Bool, y: Ran(A)) -> Runner<A> & Out(Bool):
1417	  (r, e) = y
1418	  (r, tick.ran(e, b))
1419	
1420	def tick.due(~A: Data, ~penc: A -> Bytes.Bytes, due: Bool, +b: Bool, r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1421	  match due:
1422	    case False{}:
1423	      IO.pure(Runner<A> & Out(Bool), (r, Done{b}))
1424	    case True{}:
1425	      do IO<Runner<A> & Out(Bool)>:
1426	        y : Ran(A) <- compact(~A, ~penc, r)
1427	        return tick.compacted(A, b, y)
1428	
1429	def tick.size(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +b: Bool) -> IO(Runner<A> & Out(Bool)):
1430	  Runner{Log{J.Journal{file, max, +end, ok}, path, limit, +base}, s, p, +c, ib, fs} = r
1431	  Config{pol, circ, idle, +every} = c
1432	  tick.due(~A, ~penc, Bool.and(Bool.not(U32.is_eq(every, 0)), U32.is_gt((end - base : U32), every)), b,
1433	    Runner{Log{J.Journal{file, max, end, ok}, path, limit, base}, s, p, c, ib, fs})
1434	
1435	# Compacts once the journal grew past Config.compact octets since open or the last compaction.
1436	def tick.grown(~A: Data, ~penc: A -> Bytes.Bytes, t: Runner<A> & Out(Bool)) -> IO(Runner<A> & Out(Bool)):
1437	  match t:
1438	    case (r, Fail{e}):
1439	      IO.pure(Runner<A> & Out(Bool), (r, Fail{e}))
1440	    case (r, Done{b}):
1441	      tick.size(~A, ~penc, r, b)
1442	
1443	def tick.polled(-A: Data, x: Poll<&1, &1, Maybe<&1, Msg<A>>, Unit>) -> Maybe<&1, Msg<A>>:
1444	  match x:
1445	    case Ready{m}:
1446	      m
1447	    case Wait{u}:
1448	      None{}
1449	
1450	def tick.wait(-A: Data, t: List<&1, Flight<A>> & Bool, j: Log, s: State<A>, p: Res.Pool, c: Config, +ib: Chan(Msg<A>)) ->
1451	    IO(Runner<A> & Maybe<&1, Msg<A>>):
1452	  (fs, none) = t
1453	  match none:
1454	    case False{}:
1455	      do IO<Runner<A> & Maybe<&1, Msg<A>>>:
1456	        m : Maybe<&1, Msg<A>> <- Chan.recv(Msg<A>, ib)
1457	        return (Runner{j, s, p, c, ib, fs}, m)
1458	    case True{}:
1459	      do IO<Runner<A> & Maybe<&1, Msg<A>>>:
1460	        x : Poll<&1, &1, Maybe<&1, Msg<A>>, Unit> <- Chan.try_recv(Msg<A>, ib, 0)
1461	        return (Runner{j, s, p, c, ib, fs}, tick.polled(A, x))
1462	
1463	# The next message. While an attempt runs, it waits for one; when none runs, it takes one only when the inbox holds one.
1464	def tick.next(-A: Data, r: Runner<A>) -> IO(Runner<A> & Maybe<&1, Msg<A>>):
1465	  Runner{j, s, p, c, ib, fs} = r
1466	  tick.wait(A, flights.none(A, fs), j, s, p, c, ib)
1467	
1468	# Handles the message, then claims due tickets into free units with the claim fuel left. None ends the round.
1469	def tick.got(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), x: Runner<A> & Maybe<&1, Msg<A>>, +ran: Bool, +left: Nat,
1470	    err: Out(Unit), next: Claimed(A) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1471	  (r, m) = x
1472	  match m:
1473	    case None{}:
1474	      IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1475	    case Some{msg}:
1476	      do IO<Runner<A> & Out(Bool)>:
1477	        +now : Nat <- clock()
1478	        rnd : Out(U32) <- IO.random_u32()
1479	        handle(~A, ~penc, msg, now, random(rnd), r, err, (r2 => more => e2 =>
1480	          IO.bind(Claimed(A), Runner<A> & Out(Bool), claim(~A, ~penc, ~target, ~work, Nat.add(left, more), now, r2), k2 => next(merged(A, e2, k2)))))
1481	
1482	# Handles messages until no attempt runs and the inbox is empty. fuel bounds the messages; attempts that still run
1483	# when it ends stay in the runner, and the next tick settles them.
1484	def tick.run(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +ran: Bool, k: Claimed(A)) ->
1485	    IO(Runner<A> & Out(Bool)):
1486	  match fuel:
1487	    case 0n:
1488	      (r, left, err) = k
1489	      IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1490	    case 1n+n:
1491	      (r, +left, err) = k
1492	      do IO<Runner<A> & Out(Bool)>:
1493	        x : Runner<A> & Maybe<&1, Msg<A>> <- tick.next(A, r)
1494	        tick.got(~A, ~penc, ~target, ~work, x, ran, left, err, (k2 => tick.run(~A, ~penc, ~target, ~work, n, True{}, k2)))
1495	
1496	# Fuel for each pending ticket, each attempt that may already run, and the last look at the inbox.
1497	def tick.at(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1498	  Runner{j, +s, +p, c, ib, fs} = r
1499	  +fuel = Nat.add(2n, Nat.add(pending(A, s), Res.capacity(p)))
1500	  do IO<Runner<A> & Out(Bool)>:
1501	    k : Claimed(A) <- claim(~A, ~penc, ~target, ~work, fuel, now, Runner{j, s, p, c, ib, fs})
1502	    tick.run(~A, ~penc, ~target, ~work, fuel, False{}, k)
1503	
1504	# One round: start an attempt for each due ticket the pool admits. Then read the inbox: as each attempt finishes, record its outcome,
1505	# return its unit, and start the next due ticket in that unit; append and answer each ask from another fiber.
1506	# The round ends when no attempt runs and the inbox is empty. Then compact the journal when it grew past Config.compact.
1507	# Done{True} when an attempt ran or an ask was answered. work runs on its own; target names the breaker that guards it.
1508	def tick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1509	  do IO<Runner<A> & Out(Bool)>:
1510	    now : Nat <- clock()
1511	    t : Runner<A> & Out(Bool) <- tick.at(~A, ~penc, ~target, ~work, now, r)
1512	    tick.grown(~A, ~penc, t)
1513	
1514	def finished.of(-A: Data, +quiet: Bool, t: List<&1, Flight<A>> & Bool, j: Log, s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>)) ->
1515	    Runner<A> & Bool:
1516	  (fs, none) = t
1517	  (Runner{j, s, p, c, ib, fs}, Bool.and(quiet, none))
1518	
1519	# The runner back, and whether no ticket is pending or running and no attempt runs.
1520	def finished(-A: Data, r: Runner<A>) -> Runner<A> & Bool:
1521	  Runner{j, +s, p, c, ib, fs} = r
1522	  finished.of(A, idle(A, s), flights.none(A, fs), j, s, p, c, ib)
1523	
1524	def drain.ticked(-A: Data, +idle: U32, t: Runner<A> & Out(Bool), next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1525	  match t:
1526	    case (r, Fail{e}):
1527	      IO.pure(Ran(A), (r, Fail{e}))
1528	    case (r, Done{True{}}):
1529	      next(r)
1530	    case (r, Done{False{}}):
1531	      do IO<Ran(A)>:
1532	        IO.sleep(idle)
1533	        next(r)
1534	
1535	def drain.if(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), done: Bool, r: Runner<A>, next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1536	  match done:
1537	    case True{}:
1538	      IO.pure(Ran(A), (r, Done{Unit{}}))
1539	    case False{}:
1540	      Runner{j, s, p, +c, ib, fs} = r
1541	      Config{pol, circ, +idle, every} = c
1542	      do IO<Ran(A)>:
1543	        t : Runner<A> & Out(Bool) <- tick(~A, ~penc, ~target, ~work, Runner{j, s, p, c, ib, fs})
1544	        drain.ticked(A, idle, t, next)
1545	
1546	def drain.at(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), x: Runner<A> & Bool, next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1547	  (r, done) = x
1548	  drain.if(~A, ~penc, ~target, ~work, done, r, next)
1549	
1550	def drain.end(-A: Data, x: Runner<A> & Bool) -> Ran(A):
1551	  (r, done) = x
1552	  (r, Bool.pick(Out(Unit), done, Done{Unit{}}, Fail{(62, "runner: drain ran out of rounds")}))
1553	
1554	# Ticks until no ticket is pending or running and no attempt runs. It waits idle ms after a round that did nothing.
1555	# Fails when fuel rounds leave work.
1556	def drain(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, r: Runner<A>) -> IO(Ran(A)):
1557	  match fuel:
1558	    case 0n:
1559	      IO.pure(Ran(A), drain.end(A, finished(A, r)))
1560	    case 1n+n:
1561	      drain.at(~A, ~penc, ~target, ~work, finished(A, r), (r2 => drain(~A, ~penc, ~target, ~work, n, r2)))
1562	
1563	def orphans.next(-A: Data, y: Ran(A), err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Ran(A))) -> IO(Ran(A)):
1564	  (r, e) = y
1565	  next(r, first(err, e))
1566	
1567	# A running ticket at open lost its attempt in a crash: it is requeued at now, as a failed attempt.
1568	def orphans(~A: Data, ~penc: A -> Bytes.Bytes, ts: List<&2, Q.Ticket<A>>, +now: Nat, r: Runner<A>, err: Out(Unit)) -> IO(Ran(A)):
1569	  match ts:
1570	    case Nil{}:
1571	      IO.pure(Ran(A), (r, err))
1572	    case Con{Q.Ticket{id, d, x}, rest}:
1573	      do IO<Ran(A)>:
1574	        y : Ran(A) <- commit(~A, ~penc, r, Requeue{id, now})
1575	        orphans.next(A, y, err, (r2 => e2 => orphans(~A, ~penc, rest, now, r2, e2)))
1576	
1577	def close.answer(-A: Data, m: Msg<A>) -> IO(Unit):
1578	  match m:
1579	    case Ended{id, o}:
1580	      IO.pure(Unit, Unit{})
1581	    case Add{d, x, rep}:
1582	      do IO<Unit>:
1583	        y : Result<&1, &1, Out(Nat), Unit> <- Chan.send(Out(Nat), rep, Fail{(32, "runner: the runner closed")})
1584	        return Unit{}
1585	    case Drop{i, rep}:
1586	      do IO<Unit>:
1587	        y : Result<&1, &1, Out(Unit), Unit> <- Chan.send(Out(Unit), rep, Fail{(32, "runner: the runner closed")})
1588	        return Unit{}
1589	
1590	def close.polled(-A: Data, x: Poll<&1, &1, Maybe<&1, Msg<A>>, Unit>, next: Unit -> IO(Unit)) -> IO(Unit):
1591	  match x:
1592	    case Ready{Some{m}}:
1593	      do IO<Unit>:
1594	        close.answer(A, m)
1595	        next(Unit{})
1596	    case Ready{None{}}:
1597	      IO.pure(Unit, Unit{})
1598	    case Wait{u}:
1599	      IO.pure(Unit, Unit{})
1600	
1601	# Answers each ask left in the closed inbox with a failure. fuel bounds the messages it reads.
1602	def close.drain(-A: Data, fuel: Nat, +ib: Chan(Msg<A>)) -> IO(Unit):
1603	  match fuel:
1604	    case 0n:
1605	      IO.pure(Unit, Unit{})
1606	    case 1n+n:
1607	      do IO<Unit>:
1608	        x : Poll<&1, &1, Maybe<&1, Msg<A>>, Unit> <- Chan.try_recv(Msg<A>, ib, 0)
1609	        close.polled(A, x, (u => close.drain(A, n, ib)))
1610	
1611	# Closes the inbox and the journal. Each ask still in the inbox gets a failure, and no op is appended for it.
1612	def close(-A: Data, r: Runner<A>) -> IO(Unit):
1613	  Runner{j, s, +p, c, +ib, fs} = r
1614	  do IO<Unit>:
1615	    Chan.close(Msg<A>, ib)
1616	    close.drain(A, Nat.add(2n, Res.capacity(p)), ib)
1617	    log.close(j)
1618	
1619	def open.done(-A: Data, y: Ran(A)) -> IO(Out(Runner<A>)):
1620	  match y:
1621	    case (r, Fail{e}):
1622	      do IO<Out(Runner<A>)>:
1623	        close(A, r)
1624	        return Fail{e}
1625	    case (r, Done{u}):
1626	      IO.pure(Out(Runner<A>), Done{r})
1627	
1628	def open.got(~A: Data, ~penc: A -> Bytes.Bytes, x: Out(J.Journal & Maybe<&2, State<A>>), +max: U32, path: String, +limit: U32, c: Config, +capacity: Nat) ->
1629	    IO(Out(Runner<A>)):
1630	  match x:
1631	    case Fail{e}:
1632	      IO.pure(Out(Runner<A>), Fail{e})
1633	    case Done{(j, None{})}:
1634	      do IO<Out(Runner<A>)>:
1635	        J.close(j)
1636	        return Fail{(22, "runner: a journal record is not a runner record")}
1637	    case Done{(j, Some{+s})}:
1638	      do IO<Out(Runner<A>)>:
1639	        now : Nat <- clock()
1640	        ib : Chan(Msg<A>) <- Chan.new(Msg<A>, (U32.from_nat(capacity) + 1 : U32))
1641	        y : Ran(A) <- orphans(~A, ~penc, Q.running(A, queue(A, s)), now, Runner{log.of(j, max, path, limit), calm(A, s), Res.new(0n, capacity), c, ib, Nil{}}, Done{Unit{}})
1642	        open.done(A, y)
1643	
1644	def replay.step(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, acc: Maybe<&2, State<A>>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
1645	  load(~A, ~Bytes.Bytes, ~(y => decode.rec(~A, ~pdec, y)), acc, b)
1646	
1647	# Opens the runner at path: replays its journal into the state, then requeues each ticket that was running.
1648	# capacity bounds the attempts that run at once. max caps an op record's octets; limit caps the journal file and a snapshot.
1649	# Fails when the journal fails to open, or a committed record is not a snapshot or an op.
1650	def open(~A: Data, ~penc: A -> Bytes.Bytes, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +path: String, +max: U32, +limit: U32, c: Config, +capacity: Nat) -> IO(Out(Runner<A>)):
1651	  do IO<Out(Runner<A>)>:
1652	    x : Out(J.Journal & Maybe<&2, State<A>>) <- J.open(~Maybe<&2, State<A>>, ~(acc => b => replay.step(~A, ~pdec, acc, b)), path, limit, limit, Some{new(A)})
1653	    open.got(~A, ~penc, x, max, path, limit, c, capacity)
1654	
1655	# The runner back, and a copy of its state.
1656	def state(-A: Data, r: Runner<A>) -> Runner<A> & State<A>:
1657	  Runner{j, +s, p, c, ib, fs} = r
1658	  (Runner{j, s, p, c, ib, fs}, s)