+/** like reduce, but calls func on finished futures as they complete
+ does NOT keep the order of the input
+ */
+template <class It, class T, class F,
+ class ItT = typename std::iterator_traits<It>::value_type::value_type,
+ class Arg = MaybeTryArg<F, T, ItT>>
+Future<T> unorderedReduce(It first, It last, T initial, F func);
+
+/// Sugar for the most common case
+template <class Collection, class T, class F>
+auto unorderedReduce(Collection&& c, T&& initial, F&& func)
+ -> decltype(unorderedReduce(c.begin(), c.end(), std::forward<T>(initial),
+ std::forward<F>(func))) {
+ return unorderedReduce(
+ c.begin(),
+ c.end(),
+ std::forward<T>(initial),
+ std::forward<F>(func));
+}
+
+namespace futures {
+
+/**
+ * retrying
+ *
+ * Given a policy and a future-factory, creates futures according to the
+ * policy.
+ *
+ * The policy must be moveable - retrying will move it a lot - and callable of
+ * either of the two forms:
+ * - Future<bool>(size_t, exception_wrapper)
+ * - bool(size_t, exception_wrapper)
+ * Internally, the latter is transformed into the former in the obvious way.
+ * The first parameter is the attempt number of the next prospective attempt;
+ * the second parameter is the most recent exception. The policy returns a
+ * Future<bool> which, when completed with true, indicates that a retry is
+ * desired.
+ *
+ * We provide a few generic policies:
+ * - Basic
+ * - CappedJitteredexponentialBackoff
+ *
+ * Custom policies may use the most recent try number and exception to decide
+ * whether to retry and optionally to do something interesting like delay
+ * before the retry. Users may pass inline lambda expressions as policies, or
+ * may define their own data types meeting the above requirements. Users are
+ * responsible for managing the lifetimes of anything pointed to or referred to
+ * from inside the policy.
+ *
+ * For example, one custom policy may try up to k times, but only if the most
+ * recent exception is one of a few types or has one of a few error codes
+ * indicating that the failure was transitory.
+ *
+ * Cancellation is not supported.
+ *
+ * If both FF and Policy inline executes, then it is possible to hit a stack
+ * overflow due to the recursive nature of the retry implementation
+ */
+template <class Policy, class FF>
+typename std::result_of<FF(size_t)>::type
+retrying(Policy&& p, FF&& ff);
+
+/**
+ * generic retrying policies
+ */
+
+inline
+std::function<bool(size_t, const exception_wrapper&)>
+retryingPolicyBasic(
+ size_t max_tries);
+
+template <class Policy, class URNG>
+std::function<Future<bool>(size_t, const exception_wrapper&)>
+retryingPolicyCappedJitteredExponentialBackoff(
+ size_t max_tries,
+ Duration backoff_min,
+ Duration backoff_max,
+ double jitter_param,
+ URNG&& rng,
+ Policy&& p);
+
+inline
+std::function<Future<bool>(size_t, const exception_wrapper&)>
+retryingPolicyCappedJitteredExponentialBackoff(
+ size_t max_tries,
+ Duration backoff_min,
+ Duration backoff_max,
+ double jitter_param);
+
+}
+
+} // namespace