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);
43 * Set func as the callback for each input Future and return a vector of
44 * Futures containing the results in the input order.
46 template <class It, class F,
47 class ItT = typename std::iterator_traits<It>::value_type,
49 = typename decltype(std::declval<ItT>().then(std::declval<F>()))::value_type>
50 std::vector<Future<Result>> map(It first, It last, F func);
52 // Sugar for the most common case
53 template <class Collection, class F>
54 auto map(Collection&& c, F&& func)
55 -> decltype(map(c.begin(), c.end(), func)) {
56 return map(c.begin(), c.end(), std::forward<F>(func));
59 } // namespace futures
62 Make a completed Future by moving in a value. e.g.
65 auto f = makeFuture(std::move(foo));
69 auto f = makeFuture<string>("foo");
72 Future<typename std::decay<T>::type> makeFuture(T&& t);
74 /** Make a completed void Future. */
75 Future<void> makeFuture();
77 /** Make a completed Future by executing a function. If the function throws
78 we capture the exception, otherwise we capture the result. */
82 typename std::enable_if<
83 !std::is_reference<F>::value, bool>::type sdf = false)
84 -> Future<decltype(func())>;
89 -> Future<decltype(func())>;
91 /// Make a failed Future from an exception_ptr.
92 /// Because the Future's type cannot be inferred you have to specify it, e.g.
94 /// auto f = makeFuture<string>(std::current_exception());
96 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
98 /// Make a failed Future from an exception_wrapper.
100 Future<T> makeFuture(exception_wrapper ew);
102 /** Make a Future from an exception type E that can be passed to
103 std::make_exception_ptr(). */
104 template <class T, class E>
105 typename std::enable_if<std::is_base_of<std::exception, E>::value,
107 makeFuture(E const& e);
109 /** Make a Future out of a Try */
111 Future<T> makeFuture(Try<T>&& t);
114 * Return a new Future that will call back on the given Executor.
115 * This is just syntactic sugar for makeFuture().via(executor)
117 * @param executor the Executor to call back on
118 * @param priority optionally, the priority to add with. Defaults to 0 which
119 * represents medium priority.
121 * @returns a void Future that will call back on the given executor
123 inline Future<void> via(
125 int8_t priority = Executor::MID_PRI);
127 /// Execute a function via the given executor and return a future.
128 /// This is semantically equivalent to via(executor).then(func), but
129 /// easier to read and slightly more efficient.
130 template <class Func>
131 auto via(Executor*, Func func)
132 -> Future<typename isFuture<decltype(func())>::Inner>;
134 /** When all the input Futures complete, the returned Future will complete.
135 Errors do not cause early termination; this Future will always succeed
136 after all its Futures have finished (whether successfully or with an
139 The Futures are moved in, so your copies are invalid. If you need to
140 chain further from these Futures, use the variant with an output iterator.
142 This function is thread-safe for Futures running on different threads. But
143 if you are doing anything non-trivial after, you will probably want to
144 follow with `via(executor)` because it will complete in whichever thread the
145 last Future completes in.
147 The return type for Future<T> input is a Future<std::vector<Try<T>>>
149 template <class InputIterator>
150 Future<std::vector<Try<
151 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
152 collectAll(InputIterator first, InputIterator last);
154 /// Sugar for the most common case
155 template <class Collection>
156 auto collectAll(Collection&& c) -> decltype(collectAll(c.begin(), c.end())) {
157 return collectAll(c.begin(), c.end());
160 /// This version takes a varying number of Futures instead of an iterator.
161 /// The return type for (Future<T1>, Future<T2>, ...) input
162 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
163 /// The Futures are moved in, so your copies are invalid.
164 template <typename... Fs>
165 typename detail::CollectAllVariadicContext<
166 typename std::decay<Fs>::type::value_type...>::type
167 collectAll(Fs&&... fs);
169 /// Like collectAll, but will short circuit on the first exception. Thus, the
170 /// type of the returned Future is std::vector<T> instead of
171 /// std::vector<Try<T>>
172 template <class InputIterator>
173 Future<typename detail::CollectContext<
174 typename std::iterator_traits<InputIterator>::value_type::value_type
176 collect(InputIterator first, InputIterator last);
178 /// Sugar for the most common case
179 template <class Collection>
180 auto collect(Collection&& c) -> decltype(collect(c.begin(), c.end())) {
181 return collect(c.begin(), c.end());
184 /// Like collectAll, but will short circuit on the first exception. Thus, the
185 /// type of the returned Future is std::tuple<T1, T2, ...> instead of
186 /// std::tuple<Try<T1>, Try<T2>, ...>
187 template <typename... Fs>
188 typename detail::CollectVariadicContext<
189 typename std::decay<Fs>::type::value_type...>::type
192 /** The result is a pair of the index of the first Future to complete and
193 the Try. If multiple Futures complete at the same time (or are already
194 complete when passed in), the "winner" is chosen non-deterministically.
196 This function is thread-safe for Futures running on different threads.
198 template <class InputIterator>
201 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
202 collectAny(InputIterator first, InputIterator last);
204 /// Sugar for the most common case
205 template <class Collection>
206 auto collectAny(Collection&& c) -> decltype(collectAny(c.begin(), c.end())) {
207 return collectAny(c.begin(), c.end());
210 /** when n Futures have completed, the Future completes with a vector of
211 the index and Try of those n Futures (the indices refer to the original
212 order, but the result vector will be in an arbitrary order)
216 template <class InputIterator>
217 Future<std::vector<std::pair<
219 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
220 collectN(InputIterator first, InputIterator last, size_t n);
222 /// Sugar for the most common case
223 template <class Collection>
224 auto collectN(Collection&& c, size_t n)
225 -> decltype(collectN(c.begin(), c.end(), n)) {
226 return collectN(c.begin(), c.end(), n);
229 /** window creates up to n Futures using the values
230 in the collection, and then another Future for each Future
233 this is basically a sliding window of Futures of size n
235 func must return a Future for each value in input
237 template <class Collection, class F,
238 class ItT = typename std::iterator_traits<
239 typename Collection::iterator>::value_type,
240 class Result = typename detail::resultOf<F, ItT&&>::value_type>
241 std::vector<Future<Result>>
242 window(Collection input, F func, size_t n);
244 template <typename F, typename T, typename ItT>
245 using MaybeTryArg = typename std::conditional<
246 detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;
248 template<typename F, typename T, typename Arg>
249 using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;
251 /** repeatedly calls func on every result, e.g.
252 reduce(reduce(reduce(T initial, result of first), result of second), ...)
254 The type of the final result is a Future of the type of the initial value.
256 Func can either return a T, or a Future<T>
258 func is called in order of the input, see unorderedReduce if that is not
261 template <class It, class T, class F>
262 Future<T> reduce(It first, It last, T&& initial, F&& func);
264 /// Sugar for the most common case
265 template <class Collection, class T, class F>
266 auto reduce(Collection&& c, T&& initial, F&& func)
267 -> decltype(reduce(c.begin(), c.end(), std::forward<T>(initial),
268 std::forward<F>(func))) {
272 std::forward<T>(initial),
273 std::forward<F>(func));
276 /** like reduce, but calls func on finished futures as they complete
277 does NOT keep the order of the input
279 template <class It, class T, class F,
280 class ItT = typename std::iterator_traits<It>::value_type::value_type,
281 class Arg = MaybeTryArg<F, T, ItT>>
282 Future<T> unorderedReduce(It first, It last, T initial, F func);
284 /// Sugar for the most common case
285 template <class Collection, class T, class F>
286 auto unorderedReduce(Collection&& c, T&& initial, F&& func)
287 -> decltype(unorderedReduce(c.begin(), c.end(), std::forward<T>(initial),
288 std::forward<F>(func))) {
289 return unorderedReduce(
292 std::forward<T>(initial),
293 std::forward<F>(func));