2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
23 #include <type_traits>
26 #include <folly/Optional.h>
27 #include <folly/MoveWrapper.h>
28 #include <folly/futures/Deprecated.h>
29 #include <folly/futures/DrivableExecutor.h>
30 #include <folly/futures/Promise.h>
31 #include <folly/futures/Try.h>
32 #include <folly/futures/FutureException.h>
33 #include <folly/futures/detail/Types.h>
37 template <class> struct Promise;
40 struct isFuture : std::false_type {
45 struct isFuture<Future<T>> : std::true_type {
50 struct isTry : std::false_type {};
53 struct isTry<Try<T>> : std::true_type {};
57 template <class> struct Core;
58 template <class...> struct VariadicContext;
59 template <class> struct CollectContext;
61 template<typename F, typename... Args>
62 using resultOf = decltype(std::declval<F>()(std::declval<Args>()...));
64 template <typename...>
67 template <typename Arg, typename... Args>
68 struct ArgType<Arg, Args...> {
74 typedef void FirstArg;
77 template <bool isTry, typename F, typename... Args>
79 typedef resultOf<F, Args...> Result;
82 template<typename F, typename... Args>
85 typename = detail::resultOf<T, Args...>>
86 static constexpr std::true_type
87 check(std::nullptr_t) { return std::true_type{}; };
90 static constexpr std::false_type
91 check(...) { return std::false_type{}; };
93 typedef decltype(check<F>(nullptr)) type;
94 static constexpr bool value = type::value;
97 template<typename T, typename F>
98 struct callableResult {
99 typedef typename std::conditional<
100 callableWith<F>::value,
101 detail::argResult<false, F>,
102 typename std::conditional<
103 callableWith<F, T&&>::value,
104 detail::argResult<false, F, T&&>,
105 typename std::conditional<
106 callableWith<F, T&>::value,
107 detail::argResult<false, F, T&>,
108 typename std::conditional<
109 callableWith<F, Try<T>&&>::value,
110 detail::argResult<true, F, Try<T>&&>,
111 detail::argResult<true, F, Try<T>&>>::type>::type>::type>::type Arg;
112 typedef isFuture<typename Arg::Result> ReturnsFuture;
113 typedef Future<typename ReturnsFuture::Inner> Return;
117 struct callableResult<void, F> {
118 typedef typename std::conditional<
119 callableWith<F>::value,
120 detail::argResult<false, F>,
121 typename std::conditional<
122 callableWith<F, Try<void>&&>::value,
123 detail::argResult<true, F, Try<void>&&>,
124 detail::argResult<true, F, Try<void>&>>::type>::type Arg;
125 typedef isFuture<typename Arg::Result> ReturnsFuture;
126 typedef Future<typename ReturnsFuture::Inner> Return;
129 template <typename L>
130 struct Extract : Extract<decltype(&L::operator())> { };
132 template <typename Class, typename R, typename... Args>
133 struct Extract<R(Class::*)(Args...) const> {
134 typedef isFuture<R> ReturnsFuture;
135 typedef Future<typename ReturnsFuture::Inner> Return;
136 typedef typename ReturnsFuture::Inner RawReturn;
137 typedef typename ArgType<Args...>::FirstArg FirstArg;
140 template <typename Class, typename R, typename... Args>
141 struct Extract<R(Class::*)(Args...)> {
142 typedef isFuture<R> ReturnsFuture;
143 typedef Future<typename ReturnsFuture::Inner> Return;
144 typedef typename ReturnsFuture::Inner RawReturn;
145 typedef typename ArgType<Args...>::FirstArg FirstArg;
152 /// This namespace is for utility functions that would usually be static
153 /// members of Future, except they don't make sense there because they don't
154 /// depend on the template type (rather, on the type of their arguments in
155 /// some cases). This is the least-bad naming scheme we could think of. Some
156 /// of the functions herein have really-likely-to-collide names, like "map"
159 /// Returns a Future that will complete after the specified duration. The
160 /// Duration typedef of a `std::chrono` duration type indicates the
161 /// resolution you can expect to be meaningful (milliseconds at the time of
162 /// writing). Normally you wouldn't need to specify a Timekeeper, we will
163 /// use the global futures timekeeper (we run a thread whose job it is to
164 /// keep time for futures timeouts) but we provide the option for power
167 /// The Timekeeper thread will be lazily created the first time it is
168 /// needed. If your program never uses any timeouts or other time-based
169 /// Futures you will pay no Timekeeper thread overhead.
170 Future<void> sleep(Duration, Timekeeper* = nullptr);
172 /// Create a Future chain from a sequence of callbacks. i.e.
174 /// f.then(a).then(b).then(c);
176 /// where f is a Future<A> and the result of the chain is a Future<Z>
179 /// f.then(chain<A,Z>(a, b, c));
180 // If anyone figures how to get chain to deduce A and Z, I'll buy you a drink.
181 template <class A, class Z, class... Callbacks>
182 std::function<Future<Z>(Try<A>)>
183 chain(Callbacks... fns);
186 * Set func as the callback for each input Future and return a vector of
187 * Futures containing the results in the input order.
189 template <class It, class F,
190 class ItT = typename std::iterator_traits<It>::value_type,
191 class Result = decltype(std::declval<ItT>().then(std::declval<F>()))>
192 std::vector<Future<Result>> map(It first, It last, F func);
199 typedef T value_type;
202 Future(Future const&) = delete;
203 Future& operator=(Future const&) = delete;
206 Future(Future&&) noexcept;
207 Future& operator=(Future&&) noexcept;
210 template <class F = T>
212 Future(const typename std::enable_if<!std::is_void<F>::value, F>::type& val);
214 template <class F = T>
216 Future(typename std::enable_if<!std::is_void<F>::value, F>::type&& val);
218 template <class F = T,
219 typename std::enable_if<std::is_void<F>::value, int>::type = 0>
224 /** Return the reference to result. Should not be called if !isReady().
225 Will rethrow the exception if an exception has been
228 typename std::add_lvalue_reference<T>::type
230 typename std::add_lvalue_reference<const T>::type
233 /// Returns an inactive Future which will call back on the other side of
234 /// executor (when it is activated).
236 /// NB remember that Futures activate when they destruct. This is good,
237 /// it means that this will work:
239 /// f.via(e).then(a).then(b);
241 /// a and b will execute in the same context (the far side of e), because
242 /// the Future (temporary variable) created by via(e) does not call back
243 /// until it destructs, which is after then(a) and then(b) have been wired
246 /// But this is still racy:
248 /// f = f.via(e).then(a);
250 // The ref-qualifier allows for `this` to be moved out so we
251 // don't get access-after-free situations in chaining.
252 // https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/
253 template <typename Executor>
254 Future<T> via(Executor* executor) &&;
256 /// This variant creates a new future, where the ref-qualifier && version
257 /// moves `this` out. This one is less efficient but avoids confusing users
258 /// when "return f.via(x);" fails.
259 template <typename Executor>
260 Future<T> via(Executor* executor) &;
262 /** True when the result (or exception) is ready. */
263 bool isReady() const;
265 /** A reference to the Try of the value */
268 /// If the promise has been fulfilled, return an Optional with the Try<T>.
269 /// Otherwise return an empty Optional.
270 /// Note that this moves the Try<T> out.
271 Optional<Try<T>> poll();
273 /// Block until the future is fulfilled. Returns the value (moved out), or
274 /// throws the exception. The future must not already have a callback.
277 /// Block until the future is fulfilled, or until timed out. Returns the
278 /// value (moved out), or throws the exception (which might be a TimedOut
282 /// Call e->drive() repeatedly until the future is fulfilled. Examples
283 /// of DrivableExecutor include EventBase and ManualExecutor. Returns the
284 /// value (moved out), or throws the exception.
285 T getVia(DrivableExecutor* e);
287 /// Unwraps the case of a Future<Future<T>> instance, and returns a simple
288 /// Future<T> instance.
289 template <class F = T>
290 typename std::enable_if<isFuture<F>::value,
291 Future<typename isFuture<T>::Inner>>::type
294 /** When this Future has completed, execute func which is a function that
304 Func shall return either another Future or a value.
306 A Future for the return type of func is returned.
308 Future<string> f2 = f1.then([](Try<T>&&) { return string("foo"); });
310 The Future given to the functor is ready, and the functor may call
311 value(), which may rethrow if this has captured an exception. If func
312 throws, the exception will be captured in the Future that is returned.
314 /* TODO n3428 and other async frameworks have something like then(scheduler,
315 Future), we might want to support a similar API which could be
316 implemented a little more efficiently than
317 f.via(executor).then(callback) */
318 template <typename F, typename R = detail::callableResult<T, F>>
319 typename R::Return then(F func) {
320 typedef typename R::Arg Arguments;
321 return thenImplementation<F, R>(std::move(func), Arguments());
324 /// Variant where func is an member function
326 /// struct Worker { R doWork(Try<T>); }
329 /// Future<R> f2 = f1.then(&Worker::doWork, w);
331 /// This is just sugar for
333 /// f1.then(std::bind(&Worker::doWork, w));
334 template <typename R, typename Caller, typename... Args>
335 Future<typename isFuture<R>::Inner>
336 then(R(Caller::*func)(Args...), Caller *instance);
338 /// Convenience method for ignoring the value and creating a Future<void>.
339 /// Exceptions still propagate.
342 /// Set an error callback for this Future. The callback should take a single
343 /// argument of the type that you want to catch, and should return a value of
344 /// the same type as this Future, or a Future of that type (see overload
345 /// below). For instance,
349 /// throw std::runtime_error("oh no!");
352 /// .onError([] (std::runtime_error& e) {
353 /// LOG(INFO) << "std::runtime_error: " << e.what();
354 /// return -1; // or makeFuture<int>(-1)
357 typename std::enable_if<
358 !detail::callableWith<F, exception_wrapper>::value &&
359 !detail::Extract<F>::ReturnsFuture::value,
363 /// Overload of onError where the error callback returns a Future<T>
365 typename std::enable_if<
366 !detail::callableWith<F, exception_wrapper>::value &&
367 detail::Extract<F>::ReturnsFuture::value,
371 /// Overload of onError that takes exception_wrapper and returns Future<T>
373 typename std::enable_if<
374 detail::callableWith<F, exception_wrapper>::value &&
375 detail::Extract<F>::ReturnsFuture::value,
379 /// Overload of onError that takes exception_wrapper and returns T
381 typename std::enable_if<
382 detail::callableWith<F, exception_wrapper>::value &&
383 !detail::Extract<F>::ReturnsFuture::value,
387 /// func is like std::function<void()> and is executed unconditionally, and
388 /// the value/exception is passed through to the resulting Future.
389 /// func shouldn't throw, but if it does it will be captured and propagated,
390 /// and discard any value/exception that this Future has obtained.
392 Future<T> ensure(F func);
394 /// Like onError, but for timeouts. example:
396 /// Future<int> f = makeFuture<int>(42)
397 /// .delayed(long_time)
398 /// .onTimeout(short_time,
399 /// []() -> int{ return -1; });
403 /// Future<int> f = makeFuture<int>(42)
404 /// .delayed(long_time)
405 /// .onTimeout(short_time,
406 /// []() { return makeFuture<int>(some_exception); });
408 Future<T> onTimeout(Duration, F&& func, Timekeeper* = nullptr);
410 /// This is not the method you're looking for.
412 /// This needs to be public because it's used by make* and when*, and it's
413 /// not worth listing all those and their fancy template signatures as
414 /// friends. But it's not for public consumption.
416 void setCallback_(F&& func);
418 /// A Future's callback is executed when all three of these conditions have
419 /// become true: it has a value (set by the Promise), it has a callback (set
420 /// by then), and it is active (active by default).
422 /// Inactive Futures will activate upon destruction.
423 Future<T>& activate() & {
427 Future<T>& deactivate() & {
431 Future<T> activate() && {
433 return std::move(*this);
435 Future<T> deactivate() && {
437 return std::move(*this);
441 return core_->isActive();
445 void raise(E&& exception) {
446 raise(make_exception_wrapper<typename std::remove_reference<E>::type>(
447 std::move(exception)));
450 /// Raise an interrupt. If the promise holder has an interrupt
451 /// handler it will be called and potentially stop asynchronous work from
452 /// being done. This is advisory only - a promise holder may not set an
453 /// interrupt handler, or may do anything including ignore. But, if you know
454 /// your future supports this the most likely result is stopping or
455 /// preventing the asynchronous operation (if in time), and the promise
456 /// holder setting an exception on the future. (That may happen
457 /// asynchronously, of course.)
458 void raise(exception_wrapper interrupt);
461 raise(FutureCancellation());
464 /// Throw TimedOut if this Future does not complete within the given
465 /// duration from now. The optional Timeekeeper is as with futures::sleep().
466 Future<T> within(Duration, Timekeeper* = nullptr);
468 /// Throw the given exception if this Future does not complete within the
469 /// given duration from now. The optional Timeekeeper is as with
470 /// futures::sleep().
472 Future<T> within(Duration, E exception, Timekeeper* = nullptr);
474 /// Delay the completion of this Future for at least this duration from
475 /// now. The optional Timekeeper is as with futures::sleep().
476 Future<T> delayed(Duration, Timekeeper* = nullptr);
478 /// Block until this Future is complete. Returns a reference to this Future.
481 /// Overload of wait() for rvalue Futures
482 Future<T>&& wait() &&;
484 /// Block until this Future is complete or until the given Duration passes.
485 /// Returns a reference to this Future
486 Future<T>& wait(Duration) &;
488 /// Overload of wait(Duration) for rvalue Futures
489 Future<T>&& wait(Duration) &&;
491 /// Call e->drive() repeatedly until the future is fulfilled. Examples
492 /// of DrivableExecutor include EventBase and ManualExecutor. Returns a
493 /// reference to this Future so that you can chain calls if desired.
494 /// value (moved out), or throws the exception.
495 Future<T>& waitVia(DrivableExecutor* e) &;
497 /// Overload of waitVia() for rvalue Futures
498 Future<T>&& waitVia(DrivableExecutor* e) &&;
500 /// If the value in this Future is equal to the given Future, when they have
501 /// both completed, the value of the resulting Future<bool> will be true. It
502 /// will be false otherwise (including when one or both Futures have an
504 Future<bool> willEqual(Future<T>&);
506 /// predicate behaves like std::function<bool(T const&)>
507 /// If the predicate does not obtain with the value, the result
508 /// is a folly::PredicateDoesNotObtain exception
510 Future<T> filter(F predicate);
513 typedef detail::Core<T>* corePtr;
515 // shared core state object
519 Future(corePtr obj) : core_(obj) {}
523 void throwIfInvalid() const;
525 friend class Promise<T>;
526 template <class> friend class Future;
528 // Variant: returns a value
529 // e.g. f.then([](Try<T> t){ return t.value(); });
530 template <typename F, typename R, bool isTry, typename... Args>
531 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
532 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
534 // Variant: returns a Future
535 // e.g. f.then([](Try<T> t){ return makeFuture<T>(t); });
536 template <typename F, typename R, bool isTry, typename... Args>
537 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
538 thenImplementation(F func, detail::argResult<isTry, F, Args...>);
540 Executor* getExecutor() { return core_->getExecutor(); }
541 void setExecutor(Executor* x) { core_->setExecutor(x); }
545 Make a completed Future by moving in a value. e.g.
548 auto f = makeFuture(std::move(foo));
552 auto f = makeFuture<string>("foo");
555 Future<typename std::decay<T>::type> makeFuture(T&& t);
557 /** Make a completed void Future. */
558 Future<void> makeFuture();
560 /** Make a completed Future by executing a function. If the function throws
561 we capture the exception, otherwise we capture the result. */
565 typename std::enable_if<
566 !std::is_reference<F>::value, bool>::type sdf = false)
567 -> Future<decltype(func())>;
572 -> Future<decltype(func())>;
574 /// Make a failed Future from an exception_ptr.
575 /// Because the Future's type cannot be inferred you have to specify it, e.g.
577 /// auto f = makeFuture<string>(std::current_exception());
579 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
581 /// Make a failed Future from an exception_wrapper.
583 Future<T> makeFuture(exception_wrapper ew);
585 /** Make a Future from an exception type E that can be passed to
586 std::make_exception_ptr(). */
587 template <class T, class E>
588 typename std::enable_if<std::is_base_of<std::exception, E>::value,
590 makeFuture(E const& e);
592 /** Make a Future out of a Try */
594 Future<T> makeFuture(Try<T>&& t);
597 * Return a new Future that will call back on the given Executor.
598 * This is just syntactic sugar for makeFuture().via(executor)
600 * @param executor the Executor to call back on
602 * @returns a void Future that will call back on the given executor
604 template <typename Executor>
605 Future<void> via(Executor* executor);
607 /** When all the input Futures complete, the returned Future will complete.
608 Errors do not cause early termination; this Future will always succeed
609 after all its Futures have finished (whether successfully or with an
612 The Futures are moved in, so your copies are invalid. If you need to
613 chain further from these Futures, use the variant with an output iterator.
615 This function is thread-safe for Futures running on different threads. But
616 if you are doing anything non-trivial after, you will probably want to
617 follow with `via(executor)` because it will complete in whichever thread the
618 last Future completes in.
620 The return type for Future<T> input is a Future<std::vector<Try<T>>>
622 template <class InputIterator>
623 Future<std::vector<Try<
624 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
625 whenAll(InputIterator first, InputIterator last);
627 /// This version takes a varying number of Futures instead of an iterator.
628 /// The return type for (Future<T1>, Future<T2>, ...) input
629 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
630 /// The Futures are moved in, so your copies are invalid.
631 template <typename... Fs>
632 typename detail::VariadicContext<
633 typename std::decay<Fs>::type::value_type...>::type
636 /// Like whenAll, but will short circuit on the first exception. Thus, the
637 /// type of the returned Future is std::vector<T> instead of
638 /// std::vector<Try<T>>
639 template <class InputIterator>
640 Future<typename detail::CollectContext<
641 typename std::iterator_traits<InputIterator>::value_type::value_type
643 collect(InputIterator first, InputIterator last);
645 /** The result is a pair of the index of the first Future to complete and
646 the Try. If multiple Futures complete at the same time (or are already
647 complete when passed in), the "winner" is chosen non-deterministically.
649 This function is thread-safe for Futures running on different threads.
651 template <class InputIterator>
654 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
655 whenAny(InputIterator first, InputIterator last);
657 /** when n Futures have completed, the Future completes with a vector of
658 the index and Try of those n Futures (the indices refer to the original
659 order, but the result vector will be in an arbitrary order)
663 template <class InputIterator>
664 Future<std::vector<std::pair<
666 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
667 whenN(InputIterator first, InputIterator last, size_t n);
669 template <typename F, typename T, typename ItT>
670 using MaybeTryArg = typename std::conditional<
671 detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;
673 template<typename F, typename T, typename Arg>
674 using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;
676 /** repeatedly calls func on every result, e.g.
677 reduce(reduce(reduce(T initial, result of first), result of second), ...)
679 The type of the final result is a Future of the type of the initial value.
681 Func can either return a T, or a Future<T>
683 template <class It, class T, class F,
684 class ItT = typename std::iterator_traits<It>::value_type::value_type,
685 class Arg = MaybeTryArg<F, T, ItT>>
686 typename std::enable_if<!isFutureResult<F, T, Arg>::value, Future<T>>::type
687 reduce(It first, It last, T initial, F func);
689 template <class It, class T, class F,
690 class ItT = typename std::iterator_traits<It>::value_type::value_type,
691 class Arg = MaybeTryArg<F, T, ItT>>
692 typename std::enable_if<isFutureResult<F, T, Arg>::value, Future<T>>::type
693 reduce(It first, It last, T initial, F func);
697 #include <folly/futures/Future-inl.h>