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<Unit> 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<Unit> 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<!std::is_reference<F>::value, bool>::type sdf)
83 -> Future<typename Unit::Lift<decltype(func())>::type>;
86 auto makeFutureWith(F const& func)
87 -> Future<typename Unit::Lift<decltype(func())>::type>;
89 /// Make a failed Future from an exception_ptr.
90 /// Because the Future's type cannot be inferred you have to specify it, e.g.
92 /// auto f = makeFuture<string>(std::current_exception());
94 Future<T> makeFuture(std::exception_ptr const& e) DEPRECATED;
96 /// Make a failed Future from an exception_wrapper.
98 Future<T> makeFuture(exception_wrapper ew);
100 /** Make a Future from an exception type E that can be passed to
101 std::make_exception_ptr(). */
102 template <class T, class E>
103 typename std::enable_if<std::is_base_of<std::exception, E>::value,
105 makeFuture(E const& e);
107 /** Make a Future out of a Try */
109 Future<T> makeFuture(Try<T>&& t);
112 * Return a new Future that will call back on the given Executor.
113 * This is just syntactic sugar for makeFuture().via(executor)
115 * @param executor the Executor to call back on
116 * @param priority optionally, the priority to add with. Defaults to 0 which
117 * represents medium priority.
119 * @returns a void Future that will call back on the given executor
121 inline Future<Unit> via(
123 int8_t priority = Executor::MID_PRI);
125 /// Execute a function via the given executor and return a future.
126 /// This is semantically equivalent to via(executor).then(func), but
127 /// easier to read and slightly more efficient.
128 template <class Func>
129 auto via(Executor*, Func func)
130 -> Future<typename isFuture<decltype(func())>::Inner>;
132 /** When all the input Futures complete, the returned Future will complete.
133 Errors do not cause early termination; this Future will always succeed
134 after all its Futures have finished (whether successfully or with an
137 The Futures are moved in, so your copies are invalid. If you need to
138 chain further from these Futures, use the variant with an output iterator.
140 This function is thread-safe for Futures running on different threads. But
141 if you are doing anything non-trivial after, you will probably want to
142 follow with `via(executor)` because it will complete in whichever thread the
143 last Future completes in.
145 The return type for Future<T> input is a Future<std::vector<Try<T>>>
147 template <class InputIterator>
148 Future<std::vector<Try<
149 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
150 collectAll(InputIterator first, InputIterator last);
152 /// Sugar for the most common case
153 template <class Collection>
154 auto collectAll(Collection&& c) -> decltype(collectAll(c.begin(), c.end())) {
155 return collectAll(c.begin(), c.end());
158 /// This version takes a varying number of Futures instead of an iterator.
159 /// The return type for (Future<T1>, Future<T2>, ...) input
160 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
161 /// The Futures are moved in, so your copies are invalid.
162 template <typename... Fs>
163 typename detail::CollectAllVariadicContext<
164 typename std::decay<Fs>::type::value_type...>::type
165 collectAll(Fs&&... fs);
167 /// Like collectAll, but will short circuit on the first exception. Thus, the
168 /// type of the returned Future is std::vector<T> instead of
169 /// std::vector<Try<T>>
170 template <class InputIterator>
171 Future<typename detail::CollectContext<
172 typename std::iterator_traits<InputIterator>::value_type::value_type
174 collect(InputIterator first, InputIterator last);
176 /// Sugar for the most common case
177 template <class Collection>
178 auto collect(Collection&& c) -> decltype(collect(c.begin(), c.end())) {
179 return collect(c.begin(), c.end());
182 /// Like collectAll, but will short circuit on the first exception. Thus, the
183 /// type of the returned Future is std::tuple<T1, T2, ...> instead of
184 /// std::tuple<Try<T1>, Try<T2>, ...>
185 template <typename... Fs>
186 typename detail::CollectVariadicContext<
187 typename std::decay<Fs>::type::value_type...>::type
190 /** The result is a pair of the index of the first Future to complete and
191 the Try. If multiple Futures complete at the same time (or are already
192 complete when passed in), the "winner" is chosen non-deterministically.
194 This function is thread-safe for Futures running on different threads.
196 template <class InputIterator>
199 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
200 collectAny(InputIterator first, InputIterator last);
202 /// Sugar for the most common case
203 template <class Collection>
204 auto collectAny(Collection&& c) -> decltype(collectAny(c.begin(), c.end())) {
205 return collectAny(c.begin(), c.end());
208 /** when n Futures have completed, the Future completes with a vector of
209 the index and Try of those n Futures (the indices refer to the original
210 order, but the result vector will be in an arbitrary order)
214 template <class InputIterator>
215 Future<std::vector<std::pair<
217 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
218 collectN(InputIterator first, InputIterator last, size_t n);
220 /// Sugar for the most common case
221 template <class Collection>
222 auto collectN(Collection&& c, size_t n)
223 -> decltype(collectN(c.begin(), c.end(), n)) {
224 return collectN(c.begin(), c.end(), n);
227 /** window creates up to n Futures using the values
228 in the collection, and then another Future for each Future
231 this is basically a sliding window of Futures of size n
233 func must return a Future for each value in input
235 template <class Collection, class F,
236 class ItT = typename std::iterator_traits<
237 typename Collection::iterator>::value_type,
238 class Result = typename detail::resultOf<F, ItT&&>::value_type>
239 std::vector<Future<Result>>
240 window(Collection input, F func, size_t n);
242 template <typename F, typename T, typename ItT>
243 using MaybeTryArg = typename std::conditional<
244 detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;
246 template<typename F, typename T, typename Arg>
247 using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;
249 /** repeatedly calls func on every result, e.g.
250 reduce(reduce(reduce(T initial, result of first), result of second), ...)
252 The type of the final result is a Future of the type of the initial value.
254 Func can either return a T, or a Future<T>
256 func is called in order of the input, see unorderedReduce if that is not
259 template <class It, class T, class F>
260 Future<T> reduce(It first, It last, T&& initial, F&& func);
262 /// Sugar for the most common case
263 template <class Collection, class T, class F>
264 auto reduce(Collection&& c, T&& initial, F&& func)
265 -> decltype(reduce(c.begin(), c.end(), std::forward<T>(initial),
266 std::forward<F>(func))) {
270 std::forward<T>(initial),
271 std::forward<F>(func));
274 /** like reduce, but calls func on finished futures as they complete
275 does NOT keep the order of the input
277 template <class It, class T, class F,
278 class ItT = typename std::iterator_traits<It>::value_type::value_type,
279 class Arg = MaybeTryArg<F, T, ItT>>
280 Future<T> unorderedReduce(It first, It last, T initial, F func);
282 /// Sugar for the most common case
283 template <class Collection, class T, class F>
284 auto unorderedReduce(Collection&& c, T&& initial, F&& func)
285 -> decltype(unorderedReduce(c.begin(), c.end(), std::forward<T>(initial),
286 std::forward<F>(func))) {
287 return unorderedReduce(
290 std::forward<T>(initial),
291 std::forward<F>(func));