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.
18 #include <folly/futures/Future.h>
22 /// This namespace is for utility functions that would usually be static
23 /// members of Future, except they don't make sense there because they don't
24 /// depend on the template type (rather, on the type of their arguments in
25 /// some cases). This is the least-bad naming scheme we could think of. Some
26 /// of the functions herein have really-likely-to-collide names, like "map"
29 /// Returns a Future that will complete after the specified duration. The
30 /// Duration typedef of a `std::chrono` duration type indicates the
31 /// resolution you can expect to be meaningful (milliseconds at the time of
32 /// writing). Normally you wouldn't need to specify a Timekeeper, we will
33 /// use the global futures timekeeper (we run a thread whose job it is to
34 /// keep time for futures timeouts) but we provide the option for power
37 /// The Timekeeper thread will be lazily created the first time it is
38 /// needed. If your program never uses any timeouts or other time-based
39 /// Futures you will pay no Timekeeper thread overhead.
40 Future<void> sleep(Duration, Timekeeper* = nullptr);
42 /// Create a Future chain from a sequence of callbacks. i.e.
44 /// f.then(a).then(b).then(c);
46 /// where f is a Future<A> and the result of the chain is a Future<Z>
49 /// f.then(chain<A,Z>(a, b, c));
50 // If anyone figures how to get chain to deduce A and Z, I'll buy you a drink.
51 template <class A, class Z, class... Callbacks>
52 std::function<Future<Z>(Try<A>)>
53 chain(Callbacks... fns);
56 * Set func as the callback for each input Future and return a vector of
57 * Futures containing the results in the input order.
59 template <class It, class F,
60 class ItT = typename std::iterator_traits<It>::value_type,
61 class Result = decltype(std::declval<ItT>().then(std::declval<F>()))>
62 std::vector<Future<Result>> map(It first, It last, F func);
64 // Sugar for the most common case
65 template <class Collection, class F>
66 auto map(Collection&& c, F&& func)
67 -> decltype(map(c.begin(), c.end(), func)) {
68 return map(c.begin(), c.end(), std::forward<F>(func));
74 Make a completed Future by moving in a value. e.g.
77 auto f = makeFuture(std::move(foo));
81 auto f = makeFuture<string>("foo");
84 Future<typename std::decay<T>::type> makeFuture(T&& t);
86 /** Make a completed void Future. */
87 Future<void> makeFuture();
89 /** Make a completed Future by executing a function. If the function throws
90 we capture the exception, otherwise we capture the result. */
94 typename std::enable_if<
95 !std::is_reference<F>::value, bool>::type sdf = false)
96 -> Future<decltype(func())>;
101 -> Future<decltype(func())>;
103 /// Make a failed Future from an exception_ptr.
104 /// Because the Future's type cannot be inferred you have to specify it, e.g.
106 /// auto f = makeFuture<string>(std::current_exception());
108 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
110 /// Make a failed Future from an exception_wrapper.
112 Future<T> makeFuture(exception_wrapper ew);
114 /** Make a Future from an exception type E that can be passed to
115 std::make_exception_ptr(). */
116 template <class T, class E>
117 typename std::enable_if<std::is_base_of<std::exception, E>::value,
119 makeFuture(E const& e);
121 /** Make a Future out of a Try */
123 Future<T> makeFuture(Try<T>&& t);
126 * Return a new Future that will call back on the given Executor.
127 * This is just syntactic sugar for makeFuture().via(executor)
129 * @param executor the Executor to call back on
131 * @returns a void Future that will call back on the given executor
133 template <typename Executor>
134 Future<void> via(Executor* executor);
136 /** When all the input Futures complete, the returned Future will complete.
137 Errors do not cause early termination; this Future will always succeed
138 after all its Futures have finished (whether successfully or with an
141 The Futures are moved in, so your copies are invalid. If you need to
142 chain further from these Futures, use the variant with an output iterator.
144 This function is thread-safe for Futures running on different threads. But
145 if you are doing anything non-trivial after, you will probably want to
146 follow with `via(executor)` because it will complete in whichever thread the
147 last Future completes in.
149 The return type for Future<T> input is a Future<std::vector<Try<T>>>
151 template <class InputIterator>
152 Future<std::vector<Try<
153 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
154 collectAll(InputIterator first, InputIterator last);
156 // Sugar for the most common case
157 template <class Collection>
158 auto collectAll(Collection&& c) -> decltype(collectAll(c.begin(), c.end())) {
159 return collectAll(c.begin(), c.end());
162 /// This version takes a varying number of Futures instead of an iterator.
163 /// The return type for (Future<T1>, Future<T2>, ...) input
164 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
165 /// The Futures are moved in, so your copies are invalid.
166 template <typename... Fs>
167 typename detail::VariadicContext<
168 typename std::decay<Fs>::type::value_type...>::type
169 collectAll(Fs&&... fs);
171 /// Like collectAll, but will short circuit on the first exception. Thus, the
172 /// type of the returned Future is std::vector<T> instead of
173 /// std::vector<Try<T>>
174 template <class InputIterator>
175 Future<typename detail::CollectContext<
176 typename std::iterator_traits<InputIterator>::value_type::value_type
178 collect(InputIterator first, InputIterator last);
180 // Sugar for the most common case
181 template <class Collection>
182 auto collect(Collection&& c) -> decltype(collect(c.begin(), c.end())) {
183 return collect(c.begin(), c.end());
186 /** The result is a pair of the index of the first Future to complete and
187 the Try. If multiple Futures complete at the same time (or are already
188 complete when passed in), the "winner" is chosen non-deterministically.
190 This function is thread-safe for Futures running on different threads.
192 template <class InputIterator>
195 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
196 collectAny(InputIterator first, InputIterator last);
198 // Sugar for the most common case
199 template <class Collection>
200 auto collectAny(Collection&& c) -> decltype(collectAny(c.begin(), c.end())) {
201 return collectAny(c.begin(), c.end());
204 /** when n Futures have completed, the Future completes with a vector of
205 the index and Try of those n Futures (the indices refer to the original
206 order, but the result vector will be in an arbitrary order)
210 template <class InputIterator>
211 Future<std::vector<std::pair<
213 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
214 collectN(InputIterator first, InputIterator last, size_t n);
216 // Sugar for the most common case
217 template <class Collection>
218 auto collectN(Collection&& c, size_t n)
219 -> decltype(collectN(c.begin(), c.end(), n)) {
220 return collectN(c.begin(), c.end(), n);
223 template <typename F, typename T, typename ItT>
224 using MaybeTryArg = typename std::conditional<
225 detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;
227 template<typename F, typename T, typename Arg>
228 using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;
230 /** repeatedly calls func on every result, e.g.
231 reduce(reduce(reduce(T initial, result of first), result of second), ...)
233 The type of the final result is a Future of the type of the initial value.
235 Func can either return a T, or a Future<T>
237 template <class It, class T, class F,
238 class ItT = typename std::iterator_traits<It>::value_type::value_type,
239 class Arg = MaybeTryArg<F, T, ItT>>
240 typename std::enable_if<!isFutureResult<F, T, Arg>::value, Future<T>>::type
241 reduce(It first, It last, T initial, F func);
243 template <class It, class T, class F,
244 class ItT = typename std::iterator_traits<It>::value_type::value_type,
245 class Arg = MaybeTryArg<F, T, ItT>>
246 typename std::enable_if<isFutureResult<F, T, Arg>::value, Future<T>>::type
247 reduce(It first, It last, T initial, F func);