1	# Hairpin: an HTTP client with a base URL, default headers, a pool, cookies, mTLS, redirects, and proven retries. Source: https://github.com/paymog/bend-kit/tree/main/hairpin
2	import Base
3	import bend-kit-http@0.38.1.1/http.bend as Http
4	import bend-kit-time@0.3.0.0/time.bend as Time
5	import bend-kit-url@0.4.1.0/url.bend as Url
6	import bend-kit-json@0.7.0.0/json.bend as Json
7	import bend-kit-bytes@0.3.3.0/bytes.bend as Bytes
8	import bend-kit-retry@0.2.0.0/retry.bend as Retry
9	
10	# base: "" for none. headers: lowercase names. ms: the step timeout. policy: attempts, backoff, and deadline of each request.
11	# ca: a PEM CA file to trust; "" trusts the default store.
12	type Opts is Data:
13	  Opts{base: String, headers: Map<&2, List<&2, String>>, ms: U32, mode: Http.Mode, policy: Retry.Policy, cert: Maybe<&2, Http.Cert>, ca: String}
14	
15	# breaker: the circuit breaker of every request on this client.
16	type Client is Type:
17	  Client{opts: Opts, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker}
18	
19	# ErrHttp: the last attempt failed. ErrDenied: no attempt started, for why.
20	type Err is Data:
21	  ErrHttp{error: Http.Err}
22	  ErrDenied{why: Retry.Deny}
23	
24	# One attempt, the first included; backoff from 500 ms, doubled to at most 30 s; no deadline.
25	def policy() -> Retry.Policy:
26	  Retry.Policy{1n, 500n, 30000, None{}}
27	
28	# A pool of up to cap idle sockets per origin, no base, no default headers, 30 s steps, follow redirects,
29	# no retries, and no circuit breaker.
30	def new.with(+cap: U32) -> Client:
31	  Client{Opts{"", Http.empty(), 30000, Http.ModeFollow{}, policy(), None{}, ""}, Http.pool.new.with(cap), None{}, Retry.BreakerOff{}}
32	
33	def new() -> Client:
34	  new.with(8)
35	
36	def base(c: Client, url: String) -> Client:
37	  Client{Opts{b, h, ms, mode, p, cert, ca}, pool, j, br} = c
38	  Client{Opts{url, h, ms, mode, p, cert, ca}, pool, j, br}
39	
40	# A default header. It replaces an earlier default of the same name; a request header of that name replaces it.
41	def header(c: Client, k: String, v: String) -> Client:
42	  Client{Opts{b, h, ms, mode, p, cert, ca}, pool, j, br} = c
43	  Client{Opts{b, Http.set(h, String.to_lower(k), v), ms, mode, p, cert, ca}, pool, j, br}
44	
45	def timeout(c: Client, ms: U32) -> Client:
46	  Client{Opts{b, h, old, mode, p, cert, ca}, pool, j, br} = c
47	  Client{Opts{b, h, ms, mode, p, cert, ca}, pool, j, br}
48	
49	def redirect(c: Client, mode: Http.Mode) -> Client:
50	  Client{Opts{b, h, ms, old, p, cert, ca}, pool, j, br} = c
51	  Client{Opts{b, h, ms, mode, p, cert, ca}, pool, j, br}
52	
53	# Up to n retries after the first attempt: n + 1 attempts in all.
54	def retry(c: Client, n: U32) -> Client:
55	  Client{Opts{b, h, ms, mode, Retry.Policy{old, base, cap, d}, cert, ca}, pool, j, br} = c
56	  Client{Opts{b, h, ms, mode, Retry.Policy{1n+U32.to_nat(n), base, cap, d}, cert, ca}, pool, j, br}
57	
58	# No attempt of a request starts ms or more after the request starts.
59	def deadline(c: Client, ms: U32) -> Client:
60	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, old}, cert, ca}, pool, j, br} = c
61	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, Some{U32.to_nat(ms)}}, cert, ca}, pool, j, br}
62	
63	# A circuit breaker in front of every request, starting closed.
64	def circuit(c: Client, circ: Retry.Circuit) -> Client:
65	  Client{o, pool, j, old} = c
66	  Client{o, pool, j, Retry.Breaker.new(circ)}
67	
68	# The breaker, for inspection.
69	def breaker.of(c: Client) -> Client & Retry.Breaker:
70	  Client{o, pool, j, +br} = c
71	  (Client{o, pool, j, br}, br)
72	
73	# Paths to a PEM client chain and its private key, presented to every HTTPS origin.
74	def cert(c: Client, chain: String, key: String) -> Client:
75	  Client{Opts{b, h, ms, mode, p, old, ca}, pool, j, br} = c
76	  Client{Opts{b, h, ms, mode, p, Some{Http.Cert{chain, key}}, ca}, pool, j, br}
77	
78	# Path to a PEM CA file, trusted in place of the default store. Like the certificate, it follows same-origin redirects only.
79	def ca(c: Client, path: String) -> Client:
80	  Client{Opts{b, h, ms, mode, p, cert, old}, pool, j, br} = c
81	  Client{Opts{b, h, ms, mode, p, cert, path}, pool, j, br}
82	
83	# Every request stores and sends cookies through jar. Its clock moves to Time.now() before each try.
84	def jar(c: Client, jar: Http.Jar) -> Client:
85	  Client{o, pool, old, br} = c
86	  Client{o, pool, Some{jar}, br}
87	
88	# The jar, if the client has one.
89	def jar.of(c: Client) -> Client & Maybe<&2, Http.Jar>:
90	  Client{o, pool, +j, br} = c
91	  (Client{o, pool, j, br}, j)
92	
93	def close(c: Client) -> IO(Unit):
94	  Client{o, pool, j, br} = c
95	  Http.pool.close(pool)
96	
97	# Close a stream from open.as. A body read to its end gives its connection to the client's pool.
98	def stream.close(c: Client, st: Http.Stream) -> IO(Client):
99	  Client{o, pool, j, br} = c
100	  do IO<Client>:
101	    p : Http.Pool <- Http.pool.stream.close(pool, st)
102	    return Client{o, p, j, br}
103	
104	def target.pick(none: Bool, b: String, ref: String) -> Maybe<&2, String>:
105	  match none:
106	    case True{}:
107	      Some{ref}
108	    case False{}:
109	      Http.resolve(b, ref)
110	
111	# ref against the base (RFC 3986 §5.2); with no base, ref as given. None when the base is not absolute.
112	def target(+b: String, ref: String) -> Maybe<&2, String>:
113	  target.pick(String.is_empty(b), b, ref)
114	
115	def target.abs(m: Maybe<&2, String>) -> Maybe<&2, Url.Abs>:
116	  match m:
117	    case None{}:
118	      None{}
119	    case Some{s}:
120	      Url.absolute(s)
121	
122	# The defaults under the request headers, names lowercased; a request header replaces a default of its name.
123	# Host, Connection, Content-Length, and Transfer-Encoding are the client's own, so both sides drop them.
124	def merge(defaults: Map<&2, List<&2, String>>, headers: Map<&2, List<&2, String>>) -> Map<&2, List<&2, String>>:
125	  Http.req.put(Map.to_list(&2, List<&2, String>, headers), Http.req.put(Map.to_list(&2, List<&2, String>, defaults), Http.empty()))
126	
127	def jar.now(j: Maybe<&2, Http.Jar>) -> IO(Maybe<&2, Http.Jar>):
128	  match j:
129	    case None{}:
130	      IO.pure(Maybe<&2, Http.Jar>, None{})
131	    case Some{jar}:
132	      do IO<Maybe<&2, Http.Jar>>:
133	        now : Time.Instant <- Time.now()
134	        return Some{Http.jar.at(jar, now)}
135	
136	def once.final(+enc: String, x: Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar>) ->
137	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
138	  (p, r, j) = x
139	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
140	    d : Result<&1, &1, Http.Err, Http.Res> <- Http.fetch.final(enc, r)
141	    return (p, j, d)
142	
143	# One try: the redirect loop on the pool, with the jar, the certificate, and the CA file, then the body decoded.
144	def once(+ms: U32, +mode: Http.Mode, +cert: Maybe<&2, Http.Cert>, +ca: String, +enc: String, p: Http.Pool, j: Maybe<&2, Http.Jar>,
145	    +method: String, +u: Maybe<&2, Url.Abs>, +headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
146	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
147	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
148	    now : Maybe<&2, Http.Jar> <- jar.now(j)
149	    x : Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar> <-
150	      Http.pool.hops(mode, ms, Http.Hops{p, Http.fetch.start(method, headers, body, u, enc), cert, now, ca})
151	    once.final(enc, x)
152	
153	def hint.ms(m: Maybe<&2, U32>) -> Maybe<&2, Nat>:
154	  match m:
155	    case None{}:
156	      None{}
157	    case Some{ms}:
158	      Some{U32.to_nat(ms)}
159	
160	# A judge's verdict on one attempt. Ok: a success. Caller: the request is at fault (a 4xx), so it stops
161	# and the breaker does not count it. Target: the target failed in a way another try would repeat, so it
162	# stops and the breaker counts it. Again: another try may help; after is the Retry-After value, "" for none.
163	type Verdict is Data:
164	  VerdictOk{}
165	  VerdictCaller{}
166	  VerdictTarget{}
167	  VerdictAgain{after: String}
168	
169	# Ok and Caller do not count against the breaker; Target and Again do.
170	def counts(v: Verdict) -> Bool:
171	  match v:
172	    case VerdictOk{}:
173	      False{}
174	    case VerdictCaller{}:
175	      False{}
176	    case VerdictTarget{}:
177	      True{}
178	    case VerdictAgain{after}:
179	      True{}
180	
181	# Ok is Settled; Caller and Target are Failed. Again is Transient for an idempotent request, else Unsafe.
182	def outcome(+idem: Bool, v: Verdict, now: Time.Instant) -> Retry.Outcome:
183	  match v:
184	    case VerdictOk{}:
185	      Retry.Settled{}
186	    case VerdictCaller{}:
187	      Retry.Failed{}
188	    case VerdictTarget{}:
189	      Retry.Failed{}
190	    case VerdictAgain{+after}:
191	      Bool.pick(Retry.Outcome, idem, Retry.Transient{hint.ms(Http.retry.after(after, now))}, Retry.Unsafe{})
192	
193	def lift(-R: Type, r: Result<&1, &1, Http.Err, R>) -> Result<&1, &1, Err, R>:
194	  match r:
195	    case Fail{e}:
196	      Fail{ErrHttp{e}}
197	    case Done{res}:
198	      Done{res}
199	
200	# The live state of one request between attempts. R: a response or a stream. last: the result of the last
201	# attempt, None before the first.
202	type Step<-R: Type> is Type:
203	  Step{pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker, budget: Retry.Budget, body: Bytes.Bytes,
204	    last: Maybe<&1, Result<&1, &1, Err, R>>, plan: Retry.Plan}
205	
206	# The clock, read only when a decision depends on it; 0 otherwise, which no such decision reads.
207	def clock(need: Bool) -> IO(Nat):
208	  match need:
209	    case True{}:
210	      Time.mono.ms()
211	    case False{}:
212	      IO.pure(Nat, 0n)
213	
214	def breaker.on(br: Retry.Breaker) -> Bool:
215	  match br:
216	    case Retry.BreakerOff{}:
217	      False{}
218	    case Retry.BreakerOn{c, s}:
219	      True{}
220	
221	def transient(o: Retry.Outcome) -> Bool:
222	  match o:
223	    case Retry.Transient{after}:
224	      True{}
225	    case Retry.Settled{}:
226	      False{}
227	    case Retry.Unsafe{}:
228	      False{}
229	    case Retry.Failed{}:
230	      False{}
231	
232	def some(m: Maybe<&2, Nat>) -> Bool:
233	  match m:
234	    case None{}:
235	      False{}
236	    case Some{t}:
237	      True{}
238	
239	def deadline.some(b: Retry.Budget) -> Bool:
240	  Retry.Budget{l, d} = b
241	  some(d)
242	
243	# The wall clock dates a Retry-After, so it is read only for Again.
244	def outcome.io(+idem: Bool, v: Verdict) -> IO(Retry.Outcome):
245	  match v:
246	    case VerdictAgain{after}:
247	      do IO<Retry.Outcome>:
248	        wall : Time.Instant <- Time.now()
249	        return outcome(idem, VerdictAgain{after}, wall)
250	    case VerdictOk{}:
251	      IO.pure(Retry.Outcome, Retry.Settled{})
252	    case VerdictCaller{}:
253	      IO.pure(Retry.Outcome, Retry.Failed{})
254	    case VerdictTarget{}:
255	      IO.pure(Retry.Outcome, Retry.Failed{})
256	
257	# Jitter for the backoff, drawn only when a retry may follow.
258	def jitter.io(need: Bool) -> IO(Nat):
259	  match need:
260	    case True{}:
261	      do IO<Nat>:
262	        rnd : Result<&1, &1, U32 & String, U32> <- IO.random_u32()
263	        return U32.to_nat(Http.retry.jitter(rnd))
264	    case False{}:
265	      IO.pure(Nat, 0n)
266	
267	# hit: the breaker counts this attempt as a failure.
268	def settle.at(-R: Type, +now: Nat, +rnd: Nat, +p: Retry.Policy, +n: Nat, hit: Bool, +o: Retry.Outcome, br: Retry.Breaker, +b: Retry.Budget,
269	    body: Bytes.Bytes, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, R>) -> Step<R>:
270	  Step{pool, jar, Retry.Breaker.done(br, now, hit), b, body, Some{lift(R, r)}, Retry.plan(o, p, b, n, now, rnd)}
271	
272	# The attempt has ended: tell the breaker, and plan the next step.
273	def settle.outcome(-R: Type, +p: Retry.Policy, +n: Nat, hit: Bool, +o: Retry.Outcome, +br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
274	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, R>) -> IO(Step<R>):
275	  do IO<Step<R>>:
276	    now : Nat <- clock(Bool.or(breaker.on(br), transient(o)))
277	    rnd : Nat <- jitter.io(transient(o))
278	    return settle.at(R, now, rnd, p, n, hit, o, br, b, body, pool, jar, r)
279	
280	def settle.judged(-R: Type, +p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
281	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, j: Result<&1, &1, Http.Err, R> & Verdict) -> IO(Step<R>):
282	  (r, +v) = j
283	  do IO<Step<R>>:
284	    o : Retry.Outcome <- outcome.io(idem, v)
285	    settle.outcome(R, p, n, counts(v), o, br, b, body, pool, jar, r)
286	
287	# A status that Http.retry.judge does not retry: below 400 is Ok, a 4xx is Caller, and the rest is Target.
288	def judge.status(+s: U32) -> Verdict:
289	  Bool.pick(Verdict, U32.is_lt(s, 400), VerdictOk{}, Bool.pick(Verdict, U32.is_lt(s, 500), VerdictCaller{}, VerdictTarget{}))
290	
291	def judge.settled(r: Result<&1, &1, Http.Err, Http.Res>) -> Result<&1, &1, Http.Err, Http.Res> & Verdict:
292	  match r:
293	    case Fail{e}:
294	      (Fail{e}, VerdictTarget{})
295	    case Done{res}:
296	      Http.Res{+s, h, b} = res
297	      (Done{Http.Res{s, h, b}}, judge.status(s))
298	
299	def judge.of(x: Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>) -> Result<&1, &1, Http.Err, Http.Res> & Verdict:
300	  (r, m) = x
301	  match m:
302	    case Some{after}:
303	      (r, VerdictAgain{after})
304	    case None{}:
305	      judge.settled(r)
306	
307	# The default judge: Http.retry.judge, as if the request were idempotent, picks Again; outcome then applies idem.
308	def judge(r: Result<&1, &1, Http.Err, Http.Res>) -> Result<&1, &1, Http.Err, Http.Res> & Verdict:
309	  judge.of(Http.retry.judge(True{}, r))
310	
311	def settle(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
312	    +p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
313	    x: Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>) -> IO(Step<R>):
314	  (pool, jar, r) = x
315	  settle.judged(R, p, idem, n, br, b, body, pool, jar, judge(r))
316	
317	# The inputs of one attempt. u: the target, None when it is not absolute. headers: the merged headers.
318	type Call is Data:
319	  Call{enc: String, method: String, u: Maybe<&2, Url.Abs>, headers: Map<&2, List<&2, String>>}
320	
321	# One attempt that reads a whole response: the redirect loop on the pool, with the jar and the certificate.
322	def try.res(o: Opts, call: Call, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes) ->
323	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
324	  Opts{base, h, +ms, +mode, p, +cert, +ca} = o
325	  Call{+enc, +method, +u, +headers} = call
326	  once(ms, mode, cert, ca, enc, pool, jar, method, u, headers, body)
327	
328	# One attempt that opens a stream on the pool: the redirect loop with the mode, the jar, and the certificate,
329	# then the body decoded by enc.
330	def try.stream(o: Opts, call: Call, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes) ->
331	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>):
332	  Opts{base, h, +ms, mode, p, cert, ca} = o
333	  Call{+enc, method, u, headers} = call
334	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>>:
335	    now : Maybe<&2, Http.Jar> <- jar.now(jar)
336	    r : Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream> <-
337	      Http.stream.hops(mode, ms, (pool, Http.StreamHops{Http.stream.start(method, Http.accept.enc(headers, enc), body, u), cert, now, ca}))
338	    Http.stream.init.pool(enc, r)
339	
340	def close.res(r: Http.Res) -> IO(Unit):
341	  IO.pure(Unit, Unit{})
342	
343	def close.last(~R: Type, ~close: R -> IO(Unit), last: Maybe<&1, Result<&1, &1, Err, R>>) -> IO(Unit):
344	  match last:
345	    case None{}:
346	      IO.pure(Unit, Unit{})
347	    case Some{r}:
348	      match r:
349	        case Fail{e}:
350	          IO.pure(Unit, Unit{})
351	        case Done{x}:
352	          close(x)
353	
354	def attempt.run(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
355	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
356	    +o: Opts, +call: Call, +idem: Bool, +n: Nat, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, br: Retry.Breaker,
357	    b: Retry.Budget, r: Bytes.Bytes & Bytes.Bytes) -> IO(Step<R>):
358	  Opts{base, h, ms, mode, +p, cert, ca} = o
359	  (body, spare) = r
360	  do IO<Step<R>>:
361	    x : Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R> <- try(o, call, pool, jar, spare)
362	    settle(~R, ~judge, p, idem, n, br, b, body, x)
363	
364	def denied(-R: Type, why: Retry.Deny, last: Maybe<&1, Result<&1, &1, Err, R>>) -> Maybe<&1, Result<&1, &1, Err, R>>:
365	  match last:
366	    case None{}:
367	      Some{Fail{ErrDenied{why}}}
368	    case Some{r}:
369	      Some{r}
370	
371	# A refused stream closes only when the next attempt is admitted, so a denied attempt still returns it.
372	def attempt.gate(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
373	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
374	    ~close: R -> IO(Unit),
375	    x: Retry.Breaker & Retry.Budget & Retry.Gate, +o: Opts, +call: Call, +idem: Bool, +n: Nat, pool: Http.Pool,
376	    jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes, last: Maybe<&1, Result<&1, &1, Err, R>>) -> IO(Step<R>):
377	  (br, b, g) = x
378	  match g:
379	    case Retry.GateNo{why}:
380	      IO.pure(Step<R>, Step{pool, jar, br, b, body, denied(R, why, last), Retry.PlanStop{}})
381	    case Retry.GateGo{}:
382	      do IO<Step<R>>:
383	        close.last(~R, ~close, last)
384	        attempt.run(~R, ~judge, ~try, o, call, idem, n, pool, jar, br, b, Bytes.slice(body, 0, 4294967295))
385	
386	# Ask the gate; when it admits, make attempt n (0 first) of this layer. With no deadline and no breaker,
387	# admission does not read the clock.
388	def attempt(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
389	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
390	    ~close: R -> IO(Unit),
391	    +o: Opts, +call: Call, +idem: Bool, +n: Nat, s: Step<R>) -> IO(Step<R>):
392	  Step{pool, jar, +br, +b, body, last, plan} = s
393	  do IO<Step<R>>:
394	    now : Nat <- clock(Bool.or(breaker.on(br), deadline.some(b)))
395	    attempt.gate(~R, ~judge, ~try, ~close, Retry.admit(br, b, now), o, call, idem, n, pool, jar, body, last)
396	
397	def pause(ms: Nat) -> IO(Unit):
398	  match ms:
399	    case 0n:
400	      IO.pure(Unit, Unit{})
401	    case 1n+k:
402	      IO.sleep(U32.from_nat(1n+k))
403	
404	# left: attempts this layer may still make. n: attempts made so far. Each turn follows the plan of the last step.
405	def go(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
406	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
407	    ~close: R -> IO(Unit),
408	    left: Nat, +n: Nat, +o: Opts, +call: Call, +idem: Bool, s: Step<R>) -> IO(Step<R>):
409	  match left:
410	    case 0n:
411	      IO.pure(Step<R>, s)
412	    case 1n+rest:
413	      Step{pool, jar, br, b, body, last, plan} = s
414	      match plan:
415	        case Retry.PlanStop{}:
416	          IO.pure(Step<R>, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
417	        case Retry.PlanWait{ms}:
418	          do IO<Step<R>>:
419	            pause(ms)
420	            next : Step<R> <- attempt(~R, ~judge, ~try, ~close, o, call, idem, n, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
421	            go(~R, ~judge, ~try, ~close, rest, 1n+n, o, call, idem, next)
422	
423	def result.of(-R: Type, m: Maybe<&1, Result<&1, &1, Err, R>>) -> Result<&1, &1, Err, R>:
424	  match m:
425	    case None{}:
426	      Fail{ErrDenied{Retry.DenySpent{}}}
427	    case Some{r}:
428	      r
429	
430	def request.done(-R: Type, +o: Opts, s: Step<R>) -> Client & Retry.Budget & Result<&1, &1, Err, R>:
431	  Step{pool, jar, br, b, body, last, plan} = s
432	  (Client{o, pool, jar, br}, b, result.of(R, last))
433	
434	def attempts(p: Retry.Policy) -> Nat:
435	  Retry.Policy{a, base, cap, d} = p
436	  a
437	
438	# The budget of one plain request: the policy's attempts, and its deadline from now. The clock is read only for a deadline.
439	def budget(c: Client) -> IO(Client & Retry.Budget):
440	  Client{+o, pool, jar, br} = c
441	  Opts{base, h, ms, mode, Retry.Policy{+a, bs, cap, +d}, cert, ca} = o
442	  do IO<Client & Retry.Budget>:
443	    now : Nat <- clock(some(d))
444	    return (Client{o, pool, jar, br}, Retry.Budget.new(a, now, d))
445	
446	def run(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Verdict,
447	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
448	    ~close: R -> IO(Unit),
449	    c: Client, b: Retry.Budget, +idem: Bool, enc: String, method: String, url: String,
450	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, R>):
451	  Client{+o, pool, jar, br} = c
452	  Opts{+base, +h, ms, mode, +p, cert, ca} = o
453	  +call = {Call{enc, method, target.abs(target(base, url)), merge(h, headers)} : Call}
454	  do IO<Client & Retry.Budget & Result<&1, &1, Err, R>>:
455	    s : Step<R> <- go(~R, ~judge, ~try, ~close, attempts(p), 0n, o, call, idem, Step{pool, jar, br, b, body, None{}, Retry.PlanWait{0n}})
456	    return request.done(R, o, s)
457	
458	# One request inside a caller's retry layer, with every choice explicit. Every attempt takes from b, the
459	# caller's shared budget, and obeys its deadline; the client's own policy caps the attempts of this call.
460	# The budget left comes back beside the result: pass it to the next call, so nested layers never reset or
461	# multiply it. The client's own deadline does not apply here; b's does.
462	# judge: the Verdict on each attempt; Hairpin.judge is the default. idem: the request is safe to repeat, so
463	# Again is retried; else the breaker counts it and it never repeats. enc: the Accept-Encoding codings.
464	def request.as(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Verdict,
465	    c: Client, b: Retry.Budget, +idem: Bool, +enc: String, +method: String, url: String,
466	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
467	  run(~Http.Res, ~judge, ~try.res, ~close.res, c, b, idem, enc, method, url, headers, body)
468	
469	# A streamed request under the same budget, deadline, backoff, and breaker as request.as. The stream opens on
470	# the client's pool. It follows the client's redirect mode, presents its certificate, and sends and stores
471	# cookies through its jar, as request.as does. judge reads the status and headers through Http.stream.res.
472	# A refused stream closes before the next attempt; the last one comes back open, so the caller can read its
473	# body. Close it with stream.close to give its connection back to the pool.
474	def open.as(~judge: Result<&1, &1, Http.Err, Http.Stream> -> Result<&1, &1, Http.Err, Http.Stream> & Verdict,
475	    c: Client, b: Retry.Budget, +idem: Bool, +enc: String, +method: String, url: String,
476	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Stream>):
477	  run(~Http.Stream, ~judge, ~try.stream, ~Http.stream.close, c, b, idem, enc, method, url, headers, body)
478	
479	# request.as with judge, the idempotency of method, and every coding Http decodes.
480	def request.in(c: Client, b: Retry.Budget, +method: String, url: String, headers: Map<&2, List<&2, String>>,
481	    body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
482	  do IO<Client & Retry.Budget & Result<&1, &1, Err, Http.Res>>:
483	    enc : String <- Http.codings()
484	    request.as(~judge, c, b, Http.pool.idempotent(method), enc, method, url, headers, body)
485	
486	def request.drop(x: Client & Retry.Budget & Result<&1, &1, Err, Http.Res>) -> Client & Result<&1, &1, Err, Http.Res>:
487	  (c, b, r) = x
488	  (c, r)
489	
490	def request.budget(+method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes,
491	    x: Client & Retry.Budget) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
492	  (c, b) = x
493	  request.in(c, b, method, url, headers, body)
494	
495	# url resolves against the base; headers go over the defaults. Redirects follow the client's mode, at most 20.
496	# Each request gets a fresh budget from budget(c). Before each attempt the deadline, the
497	# budget, and the breaker must admit it. GET, HEAD, OPTIONS, TRACE, PUT, and DELETE try again after a connect
498	# error, a timeout, or 408, 429, 500, 502, 503, or 504, as Http.retry.judge says, on the same pool.
499	# When the attempts run out, the last result comes back; when none started, ErrDenied says why.
500	def request(c: Client, +method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
501	    IO(Client & Result<&1, &1, Err, Http.Res>):
502	  do IO<Client & Result<&1, &1, Err, Http.Res>>:
503	    y : Client & Retry.Budget <- budget(c)
504	    x : Client & Retry.Budget & Result<&1, &1, Err, Http.Res> <- request.budget(method, url, headers, body, y)
505	    return request.drop(x)
506	
507	def get(c: Client, url: String) -> IO(Client & Result<&1, &1, Err, Http.Res>):
508	  request(c, "GET", url, Http.empty(), Bytes.new(0))
509	
510	# POST v as compact JSON, with content-type: application/json.
511	def post.json(c: Client, url: String, v: Json.Val) -> IO(Client & Result<&1, &1, Err, Http.Res>):
512	  request(c, "POST", url, Http.set(Http.empty(), "content-type", "application/json"), Json.encode.bytes(v))