/*
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <algorithm>
#include <atomic>
#include <cassert>
#include <chrono>
#include <thread>
#include <utility>

#include <folly/Optional.h>
#include <folly/Traits.h>
#include <folly/container/Foreach.h>
#include <folly/detail/AsyncTrace.h>
#include <folly/executors/ExecutorWithPriority.h>
#include <folly/executors/GlobalExecutor.h>
#include <folly/executors/InlineExecutor.h>
#include <folly/executors/QueuedImmediateExecutor.h>
#include <folly/futures/detail/Core.h>
#include <folly/lang/Pretty.h>

#if defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
#include <folly/synchronization/Baton.h>
#endif

namespace folly {

class Timekeeper;

namespace futures {
namespace detail {
#if defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
typedef folly::Baton<> FutureBatonType;
#else
typedef folly::fibers::Baton FutureBatonType;
#endif
} // namespace detail
} // namespace futures

namespace detail {
std::shared_ptr<Timekeeper> getTimekeeperSingleton();
} // namespace detail

namespace futures {
namespace detail {

// InvokeResultWrapper and wrapInvoke enable wrapping a result value in its
// nearest Future-type counterpart capable of also carrying an exception.
// e.g.
//    (semi)Future<T> -> (semi)Future<T>           (no change)
//    Try<T>          -> Try<T>                    (no change)
//    void            -> Try<folly::Unit>
//    T               -> Try<T>
template <typename T>
struct InvokeResultWrapperBase {
  template <typename F>
  static T wrapResult(F fn) {
    return T(fn());
  }
  static T wrapException(exception_wrapper&& e) { return T(std::move(e)); }
};
template <typename T>
struct InvokeResultWrapper : InvokeResultWrapperBase<Try<T>> {};
template <typename T>
struct InvokeResultWrapper<Try<T>> : InvokeResultWrapperBase<Try<T>> {};
template <typename T>
struct InvokeResultWrapper<SemiFuture<T>>
    : InvokeResultWrapperBase<SemiFuture<T>> {};
template <typename T>
struct InvokeResultWrapper<Future<T>> : InvokeResultWrapperBase<Future<T>> {};
template <>
struct InvokeResultWrapper<void> : InvokeResultWrapperBase<Try<Unit>> {
  template <typename F>
  static Try<Unit> wrapResult(F fn) {
    fn();
    return Try<Unit>(unit);
  }
};

template <typename T, typename F>
auto wrapInvoke(folly::Try<T>&& t, F&& f) {
  auto fn = [&]() {
    return static_cast<F&&>(f)(
        t.template get<
            false,
            typename futures::detail::valueCallableResult<T, F>::FirstArg>());
  };
  using FnResult = decltype(fn());
  using Wrapper = InvokeResultWrapper<FnResult>;
  if (t.hasException()) {
    return Wrapper::wrapException(std::move(t).exception());
  }
  return Wrapper::wrapResult(fn);
}

//  Guarantees that the stored functor is destructed before the stored promise
//  may be fulfilled. Assumes the stored functor to be noexcept-destructible.
template <typename T, typename F>
class CoreCallbackState {
  using DF = std::decay_t<F>;

 public:
  CoreCallbackState(Promise<T>&& promise, F&& func) noexcept(
      noexcept(DF(std::declval<F&&>())))
      : func_(static_cast<F&&>(func)), promise_(std::move(promise)) {
    assert(before_barrier());
  }

  CoreCallbackState(CoreCallbackState&& that) noexcept(
      noexcept(DF(std::declval<F&&>()))) {
    if (that.before_barrier()) {
      new (&func_) DF(static_cast<F&&>(that.func_));
      promise_ = that.stealPromise();
    }
  }

  CoreCallbackState& operator=(CoreCallbackState&&) = delete;

  ~CoreCallbackState() {
    if (before_barrier()) {
      stealPromise();
    }
  }

  template <typename... Args>
  auto invoke(Args&&... args) noexcept(
      noexcept(std::declval<F&&>()(std::declval<Args&&>()...))) {
    assert(before_barrier());
    return static_cast<F&&>(func_)(static_cast<Args&&>(args)...);
  }

  template <typename... Args>
  auto tryInvoke(Args&&... args) noexcept {
    return makeTryWith([&] { return invoke(static_cast<Args&&>(args)...); });
  }

  void setTry(Executor::KeepAlive<>&& keepAlive, Try<T>&& t) {
    stealPromise().setTry(std::move(keepAlive), std::move(t));
  }

  void setException(Executor::KeepAlive<>&& keepAlive, exception_wrapper&& ew) {
    setTry(std::move(keepAlive), Try<T>(std::move(ew)));
  }

  Promise<T> stealPromise() noexcept {
    assert(before_barrier());
    func_.~DF();
    return std::move(promise_);
  }

 private:
  bool before_barrier() const noexcept { return !promise_.isFulfilled(); }

  union {
    DF func_;
  };
  Promise<T> promise_{Promise<T>::makeEmpty()};
};

template <typename T, typename F>
auto makeCoreCallbackState(Promise<T>&& p, F&& f) noexcept(
    noexcept(CoreCallbackState<T, F>(
        std::declval<Promise<T>&&>(), std::declval<F&&>()))) {
  return CoreCallbackState<T, F>(std::move(p), static_cast<F&&>(f));
}

template <typename T, typename R, typename... Args>
auto makeCoreCallbackState(Promise<T>&& p, R (&f)(Args...)) noexcept {
  return CoreCallbackState<T, R (*)(Args...)>(std::move(p), &f);
}

template <class T>
FutureBase<T>::FutureBase(SemiFuture<T>&& other) noexcept : core_(other.core_) {
  other.core_ = nullptr;
}

template <class T>
FutureBase<T>::FutureBase(Future<T>&& other) noexcept : core_(other.core_) {
  other.core_ = nullptr;
}

template <class T>
template <class T2, typename>
FutureBase<T>::FutureBase(T2&& val)
    : core_(Core::make(Try<T>(static_cast<T2&&>(val)))) {}

template <class T>
template <typename T2>
FutureBase<T>::FutureBase(
    typename std::enable_if<std::is_same<Unit, T2>::value>::type*)
    : core_(Core::make(Try<T>(T()))) {}

template <class T>
void FutureBase<T>::assign(FutureBase<T>&& other) noexcept {
  detach();
  core_ = std::exchange(other.core_, nullptr);
}

template <class T>
FutureBase<T>::~FutureBase() {
  detach();
}

template <class T>
T& FutureBase<T>::value() & {
  return result().value();
}

template <class T>
T const& FutureBase<T>::value() const& {
  return result().value();
}

template <class T>
T&& FutureBase<T>::value() && {
  return std::move(result().value());
}

template <class T>
T const&& FutureBase<T>::value() const&& {
  return std::move(result().value());
}

template <class T>
Try<T>& FutureBase<T>::result() & {
  return getCoreTryChecked();
}

template <class T>
Try<T> const& FutureBase<T>::result() const& {
  return getCoreTryChecked();
}

template <class T>
Try<T>&& FutureBase<T>::result() && {
  return std::move(getCoreTryChecked());
}

template <class T>
Try<T> const&& FutureBase<T>::result() const&& {
  return std::move(getCoreTryChecked());
}

template <class T>
bool FutureBase<T>::isReady() const {
  return getCore().hasResult();
}

template <class T>
bool FutureBase<T>::hasValue() const {
  return result().hasValue();
}

template <class T>
bool FutureBase<T>::hasException() const {
  return result().hasException();
}

template <class T>
void FutureBase<T>::detach() {
  if (core_) {
    core_->detachFuture();
    core_ = nullptr;
  }
}

template <class T>
void FutureBase<T>::throwIfInvalid() const {
  if (!core_) {
    throw_exception<FutureInvalid>();
  }
}

template <class T>
void FutureBase<T>::throwIfContinued() const {
  if (!core_ || core_->hasCallback()) {
    throw_exception<FutureAlreadyContinued>();
  }
}

template <class T>
Optional<Try<T>> FutureBase<T>::poll() {
  auto& core = getCore();
  return core.hasResult() ? Optional<Try<T>>(std::move(core.getTry()))
                          : Optional<Try<T>>();
}

template <class T>
void FutureBase<T>::raise(exception_wrapper exception) {
  getCore().raise(std::move(exception));
}

template <class T>
template <class F>
void FutureBase<T>::setCallback_(
    F&& func, futures::detail::InlineContinuation allowInline) {
  throwIfContinued();
  getCore().setCallback(
      static_cast<F&&>(func), RequestContext::saveContext(), allowInline);
}

template <class T>
FutureBase<T>::FutureBase(futures::detail::EmptyConstruct) noexcept
    : core_(nullptr) {}

// MSVC 2017 Update 7 released with a bug that causes issues expanding to an
// empty parameter pack when invoking a templated member function. It should
// be fixed for MSVC 2017 Update 8.
// TODO: Remove.
namespace detail_msvc_15_7_workaround {
template <typename R, std::size_t S>
using IfArgsSizeIs = std::enable_if_t<R::Arg::ArgsSize::value == S, int>;
template <typename R, typename State, typename T, IfArgsSizeIs<R, 0> = 0>
decltype(auto) invoke(
    R, State& state, Executor::KeepAlive<>&&, Try<T>&& /* t */) {
  return state.invoke();
}
template <typename R, typename State, typename T, IfArgsSizeIs<R, 2> = 0>
decltype(auto) invoke(R, State& state, Executor::KeepAlive<>&& ka, Try<T>&& t) {
  using Arg1 = typename R::Arg::ArgList::Tail::FirstArg;
  return state.invoke(
      std::move(ka), std::move(t).template get<R::Arg::isTry(), Arg1>());
}
template <typename R, typename State, typename T, IfArgsSizeIs<R, 0> = 0>
decltype(auto) tryInvoke(
    R, State& state, Executor::KeepAlive<>&&, Try<T>&& /* t */) {
  return state.tryInvoke();
}
template <typename R, typename State, typename T, IfArgsSizeIs<R, 2> = 0>
decltype(auto) tryInvoke(
    R, State& state, Executor::KeepAlive<>&& ka, Try<T>&& t) {
  using Arg1 = typename R::Arg::ArgList::Tail::FirstArg;
  return state.tryInvoke(
      std::move(ka), std::move(t).template get<R::Arg::isTry(), Arg1>());
}
} // namespace detail_msvc_15_7_workaround

// then

// Variant: returns a value
// e.g. f.then([](Try<T>&& t){ return t.value(); });
template <class T>
template <typename F, typename R>
typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
FutureBase<T>::thenImplementation(
    F&& func, R, futures::detail::InlineContinuation allowInline) {
  static_assert(R::Arg::ArgsSize::value == 2, "Then must take two arguments");
  typedef typename R::ReturnsFuture::Inner B;

  Promise<B> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());

  // grab the Future now before we lose our handle on the Promise
  auto sf = p.getSemiFuture();
  sf.setExecutor(folly::Executor::KeepAlive<>{this->getExecutor()});
  auto f = Future<B>(sf.core_);
  sf.core_ = nullptr;

  this->setCallback_(
      [state = futures::detail::makeCoreCallbackState(
           std::move(p), static_cast<F&&>(func))](
          Executor::KeepAlive<>&& ka, Try<T>&& t) mutable {
        if (!R::Arg::isTry() && t.hasException()) {
          state.setException(std::move(ka), std::move(t.exception()));
        } else {
          auto propagateKA = ka.copy();
          state.setTry(std::move(propagateKA), makeTryWith([&] {
                         return detail_msvc_15_7_workaround::invoke(
                             R{}, state, std::move(ka), std::move(t));
                       }));
        }
      },
      allowInline);
  return f;
}

// Pass through a simple future as it needs no deferral adaptation
template <class T>
Future<T> chainExecutor(Executor::KeepAlive<>, Future<T>&& f) {
  return std::move(f);
}

// Correctly chain a SemiFuture for deferral
template <class T>
Future<T> chainExecutor(Executor::KeepAlive<> e, SemiFuture<T>&& f) {
  if (!e) {
    e = folly::getKeepAliveToken(InlineExecutor::instance());
  }
  return std::move(f).via(e);
}

// Variant: returns a Future
// e.g. f.then([](T&& t){ return makeFuture<T>(t); });
template <class T>
template <typename F, typename R>
typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
FutureBase<T>::thenImplementation(
    F&& func, R, futures::detail::InlineContinuation allowInline) {
  static_assert(R::Arg::ArgsSize::value == 2, "Then must take two arguments");
  typedef typename R::ReturnsFuture::Inner B;

  Promise<B> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());

  // grab the Future now before we lose our handle on the Promise
  auto sf = p.getSemiFuture();
  auto e = getKeepAliveToken(this->getExecutor());
  sf.setExecutor(std::move(e));
  auto f = Future<B>(sf.core_);
  sf.core_ = nullptr;

  this->setCallback_(
      [state = futures::detail::makeCoreCallbackState(
           std::move(p), static_cast<F&&>(func))](
          Executor::KeepAlive<>&& ka, Try<T>&& t) mutable {
        if (!R::Arg::isTry() && t.hasException()) {
          state.setException(std::move(ka), std::move(t.exception()));
        } else {
          // Ensure that if function returned a SemiFuture we correctly chain
          // potential deferral.
          auto tf2 = detail_msvc_15_7_workaround::tryInvoke(
              R{}, state, ka.copy(), std::move(t));
          if (tf2.hasException()) {
            state.setException(std::move(ka), std::move(tf2.exception()));
          } else {
            auto statePromise = state.stealPromise();
            auto tf3 = chainExecutor(std::move(ka), *std::move(tf2));
            std::exchange(statePromise.core_, nullptr)
                ->setProxy(std::exchange(tf3.core_, nullptr));
          }
        }
      },
      allowInline);

  return f;
}

class WaitExecutor final : public folly::Executor {
 public:
  void add(Func func) override {
    bool empty;
    {
      auto wQueue = queue_.wlock();
      if (wQueue->detached) {
        return;
      }
      empty = wQueue->funcs.empty();
      wQueue->funcs.push_back(std::move(func));
    }
    if (empty) {
      baton_.post();
    }
  }

  void drive() {
    baton_.wait();

#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
    fibers::runInMainContext([&]() {
#endif
      baton_.reset();
      auto funcs = std::move(queue_.wlock()->funcs);
      for (auto& func : funcs) {
        std::exchange(func, nullptr)();
      }
#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
    });
#endif
  }

  using Clock = std::chrono::steady_clock;

  bool driveUntil(Clock::time_point deadline) {
    if (!baton_.try_wait_until(deadline)) {
      return false;
    }
#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
    return fibers::runInMainContext([&]() {
#endif
      baton_.reset();
      auto funcs = std::move(queue_.wlock()->funcs);
      for (auto& func : funcs) {
        std::exchange(func, nullptr)();
      }
      return true;
#if !defined(FOLLY_DISABLE_FUTURE_FIBERS_BATON)
    });
#endif
  }

  void detach() {
    // Make sure we don't hold the lock while destroying funcs.
    [&] {
      auto wQueue = queue_.wlock();
      wQueue->detached = true;
      return std::move(wQueue->funcs);
    }();
  }

  static KeepAlive<WaitExecutor> create() {
    return makeKeepAlive<WaitExecutor>(new WaitExecutor());
  }

 private:
  WaitExecutor() {}

  bool keepAliveAcquire() noexcept override {
    auto keepAliveCount =
        keepAliveCount_.fetch_add(1, std::memory_order_relaxed);
    DCHECK(keepAliveCount > 0);
    return true;
  }

  void keepAliveRelease() noexcept override {
    auto keepAliveCount =
        keepAliveCount_.fetch_sub(1, std::memory_order_acq_rel);
    DCHECK(keepAliveCount > 0);
    if (keepAliveCount == 1) {
      delete this;
    }
  }

  struct Queue {
    std::vector<Func> funcs;
    bool detached{false};
  };

  folly::Synchronized<Queue> queue_;
  FutureBatonType baton_;

  std::atomic<ssize_t> keepAliveCount_{1};
};

// Vector-like structure to play with window,
// which otherwise expects a vector of size `times`,
// which would be expensive with large `times` sizes.
struct WindowFakeVector {
  using iterator = std::vector<size_t>::iterator;

  WindowFakeVector(size_t size) : size_(size) {}

  size_t operator[](const size_t index) const { return index; }
  size_t size() const { return size_; }

 private:
  size_t size_;
};
} // namespace detail
} // namespace futures

template <class T>
SemiFuture<typename std::decay<T>::type> makeSemiFuture(T&& t) {
  return makeSemiFuture(Try<typename std::decay<T>::type>(static_cast<T&&>(t)));
}

// makeSemiFutureWith(SemiFuture<T>()) -> SemiFuture<T>
template <class F>
typename std::enable_if<
    isFutureOrSemiFuture<invoke_result_t<F>>::value,
    SemiFuture<typename invoke_result_t<F>::value_type>>::type
makeSemiFutureWith(F&& func) {
  using InnerType = typename isFutureOrSemiFuture<invoke_result_t<F>>::Inner;
  try {
    return static_cast<F&&>(func)();
  } catch (...) {
    return makeSemiFuture<InnerType>(
        exception_wrapper(std::current_exception()));
  }
}

// makeSemiFutureWith(T()) -> SemiFuture<T>
// makeSemiFutureWith(void()) -> SemiFuture<Unit>
template <class F>
typename std::enable_if<
    !(isFutureOrSemiFuture<invoke_result_t<F>>::value),
    SemiFuture<lift_unit_t<invoke_result_t<F>>>>::type
makeSemiFutureWith(F&& func) {
  using LiftedResult = lift_unit_t<invoke_result_t<F>>;
  return makeSemiFuture<LiftedResult>(
      makeTryWith([&func]() mutable { return static_cast<F&&>(func)(); }));
}

template <class T>
SemiFuture<T> makeSemiFuture(std::exception_ptr const& e) {
  return makeSemiFuture(Try<T>(e));
}

template <class T>
SemiFuture<T> makeSemiFuture(exception_wrapper ew) {
  return makeSemiFuture(Try<T>(std::move(ew)));
}

template <class T, class E>
typename std::
    enable_if<std::is_base_of<std::exception, E>::value, SemiFuture<T>>::type
    makeSemiFuture(E const& e) {
  return makeSemiFuture(Try<T>(make_exception_wrapper<E>(e)));
}

template <class T>
SemiFuture<T> makeSemiFuture(Try<T> t) {
  return SemiFuture<T>(SemiFuture<T>::Core::make(std::move(t)));
}

// This must be defined after the constructors to avoid a bug in MSVC
// https://connect.microsoft.com/VisualStudio/feedback/details/3142777/out-of-line-constructor-definition-after-implicit-reference-causes-incorrect-c2244
inline SemiFuture<Unit> makeSemiFuture() {
  return makeSemiFuture(Unit{});
}

template <class T>
SemiFuture<T> SemiFuture<T>::makeEmpty() {
  return SemiFuture<T>(futures::detail::EmptyConstruct{});
}

template <class T>
futures::detail::DeferredWrapper SemiFuture<T>::stealDeferredExecutor() {
  return this->getCore().stealDeferredExecutor();
}

template <class T>
void SemiFuture<T>::releaseDeferredExecutor(Core* core) {
  if (!core || core->hasCallback()) {
    return;
  }
  auto executor = core->stealDeferredExecutor();
  async_tracing::logSemiFutureDiscard(
      executor.get() ? async_tracing::DiscardHasDeferred::DEFERRED_EXECUTOR
                     : async_tracing::DiscardHasDeferred::NO_EXECUTOR);
  if (executor) {
    executor.get()->detach();
  }
}

template <class T>
SemiFuture<T>::~SemiFuture() {
  releaseDeferredExecutor(this->core_);
}

template <class T>
SemiFuture<T>::SemiFuture(SemiFuture<T>&& other) noexcept
    : futures::detail::FutureBase<T>(std::move(other)) {}

template <class T>
SemiFuture<T>::SemiFuture(Future<T>&& other) noexcept
    : futures::detail::FutureBase<T>(std::move(other)) {
  // SemiFuture should not have an executor on construction
  if (this->core_) {
    this->setExecutor(futures::detail::KeepAliveOrDeferred{});
  }
}

template <class T>
SemiFuture<T>& SemiFuture<T>::operator=(SemiFuture<T>&& other) noexcept {
  releaseDeferredExecutor(this->core_);
  this->assign(std::move(other));
  return *this;
}

template <class T>
SemiFuture<T>& SemiFuture<T>::operator=(Future<T>&& other) noexcept {
  releaseDeferredExecutor(this->core_);
  this->assign(std::move(other));
  // SemiFuture should not have an executor on construction
  if (this->core_) {
    this->setExecutor(Executor::KeepAlive<>{});
  }
  return *this;
}

template <class T>
Future<T> SemiFuture<T>::via(Executor::KeepAlive<> executor) && {
  folly::async_tracing::logSemiFutureVia(this->getExecutor(), executor.get());

  if (!executor) {
    throw_exception<FutureNoExecutor>();
  }

  if (auto deferredExecutor = this->getDeferredExecutor()) {
    deferredExecutor->setExecutor(executor.copy());
  }

  auto newFuture = Future<T>(this->core_);
  this->core_ = nullptr;
  newFuture.setExecutor(std::move(executor));

  return newFuture;
}

template <class T>
Future<T> SemiFuture<T>::via(
    Executor::KeepAlive<> executor, int8_t priority) && {
  return std::move(*this).via(
      ExecutorWithPriority::create(std::move(executor), priority));
}

template <class T>
Future<T> SemiFuture<T>::toUnsafeFuture() && {
  return std::move(*this).via(&InlineExecutor::instance());
}

template <class T>
template <typename F>
SemiFuture<typename futures::detail::tryCallableResult<T, F>::value_type>
SemiFuture<T>::defer(F&& func) && {
  auto deferredExecutorPtr = this->getDeferredExecutor();
  futures::detail::KeepAliveOrDeferred deferredExecutor = [&]() {
    if (deferredExecutorPtr) {
      return futures::detail::KeepAliveOrDeferred{deferredExecutorPtr->copy()};
    } else {
      auto newDeferredExecutor = futures::detail::KeepAliveOrDeferred(
          futures::detail::DeferredExecutor::create());
      this->setExecutor(newDeferredExecutor.copy());
      return newDeferredExecutor;
    }
  }();

  auto sf = Future<T>(this->core_).thenTryInline(static_cast<F&&>(func)).semi();
  this->core_ = nullptr;
  // Carry deferred executor through chain as constructor from Future will
  // nullify it
  sf.setExecutor(std::move(deferredExecutor));
  return sf;
}

template <class T>
template <typename F>
SemiFuture<
    typename futures::detail::tryExecutorCallableResult<T, F>::value_type>
SemiFuture<T>::deferExTry(F&& func) && {
  auto deferredExecutorPtr = this->getDeferredExecutor();
  futures::detail::DeferredWrapper deferredExecutor = [&]() mutable {
    if (deferredExecutorPtr) {
      return deferredExecutorPtr->copy();
    } else {
      auto newDeferredExecutor = futures::detail::DeferredExecutor::create();
      this->setExecutor(
          futures::detail::KeepAliveOrDeferred{newDeferredExecutor->copy()});
      return newDeferredExecutor;
    }
  }();

  auto sf = Future<T>(this->core_)
                .thenExTryInline([func = static_cast<F&&>(func)](
                                     folly::Executor::KeepAlive<>&& keepAlive,
                                     folly::Try<T>&& val) mutable {
                  return static_cast<F&&>(func)(
                      std::move(keepAlive), static_cast<decltype(val)>(val));
                })
                .semi();
  this->core_ = nullptr;
  // Carry deferred executor through chain as constructor from Future will
  // nullify it
  sf.setExecutor(
      futures::detail::KeepAliveOrDeferred{std::move(deferredExecutor)});
  return sf;
}

template <class T>
template <typename F>
SemiFuture<typename futures::detail::valueCallableResult<T, F>::value_type>
SemiFuture<T>::deferValue(F&& func) && {
  return std::move(*this).defer(
      [f = static_cast<F&&>(func)](folly::Try<T>&& t) mutable {
        return futures::detail::wrapInvoke(std::move(t), static_cast<F&&>(f));
      });
}

template <class T>
template <typename F>
SemiFuture<
    typename futures::detail::valueExecutorCallableResult<T, F>::value_type>
SemiFuture<T>::deferExValue(F&& func) && {
  return std::move(*this).deferExTry(
      [f = static_cast<F&&>(func)](
          folly::Executor::KeepAlive<> ka, folly::Try<T>&& t) mutable {
        return static_cast<F&&>(f)(
            ka,
            t.template get<
                false,
                typename futures::detail::valueExecutorCallableResult<T, F>::
                    ValueArg>());
      });
}

template <class T>
template <class ExceptionType, class F>
SemiFuture<T> SemiFuture<T>::deferError(tag_t<ExceptionType>, F&& func) && {
  return std::move(*this).defer(
      [func = static_cast<F&&>(func)](Try<T>&& t) mutable {
        if (auto e = t.template tryGetExceptionObject<ExceptionType>()) {
          return makeSemiFutureWith(
              [&]() mutable { return static_cast<F&&>(func)(*e); });
        } else {
          return makeSemiFuture<T>(std::move(t));
        }
      });
}

template <class T>
template <class F>
SemiFuture<T> SemiFuture<T>::deferError(F&& func) && {
  return std::move(*this).defer(
      [func = static_cast<F&&>(func)](Try<T> t) mutable {
        if (t.hasException()) {
          return makeSemiFutureWith([&]() mutable {
            return static_cast<F&&>(func)(std::move(t.exception()));
          });
        } else {
          return makeSemiFuture<T>(std::move(t));
        }
      });
}

template <class T>
SemiFuture<Unit> SemiFuture<T>::unit() && {
  return std::move(*this).deferValue([](T&&) {});
}

template <typename T>
SemiFuture<T> SemiFuture<T>::delayed(HighResDuration dur, Timekeeper* tk) && {
  return collectAll(*this, futures::sleep(dur, tk))
      .deferValue([](std::tuple<Try<T>, Try<Unit>> tup) {
        Try<T>& t = std::get<0>(tup);
        return makeFuture<T>(std::move(t));
      });
}

template <class T>
Future<T> Future<T>::makeEmpty() {
  return Future<T>(futures::detail::EmptyConstruct{});
}

template <class T>
Future<T>::Future(Future<T>&& other) noexcept
    : futures::detail::FutureBase<T>(std::move(other)) {}

template <class T>
Future<T>& Future<T>::operator=(Future<T>&& other) noexcept {
  this->assign(std::move(other));
  return *this;
}

// unwrap

template <class T>
template <class F>
typename std::
    enable_if<isFuture<F>::value, Future<typename isFuture<T>::Inner>>::type
    Future<T>::unwrap() && {
  return std::move(*this).thenValue(
      [](Future<typename isFuture<T>::Inner> internal_future) {
        return internal_future;
      });
}

template <class T>
Future<T> Future<T>::via(Executor::KeepAlive<> executor) && {
  folly::async_tracing::logFutureVia(this->getExecutor(), executor.get());

  this->setExecutor(std::move(executor));

  auto newFuture = Future<T>(this->core_);
  this->core_ = nullptr;
  return newFuture;
}

template <class T>
Future<T> Future<T>::via(Executor::KeepAlive<> executor, int8_t priority) && {
  return std::move(*this).via(
      ExecutorWithPriority::create(std::move(executor), priority));
}

template <class T>
Future<T> Future<T>::via(Executor::KeepAlive<> executor) & {
  folly::async_tracing::logFutureVia(this->getExecutor(), executor.get());

  this->throwIfInvalid();
  Promise<T> p;
  auto sf = p.getSemiFuture();
  auto func = [p = std::move(p)](Executor::KeepAlive<>&&, Try<T>&& t) mutable {
    p.setTry(std::move(t));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(func)>;
  this->thenImplementation(
      std::move(func), R{}, futures::detail::InlineContinuation::forbid);
  // Construct future from semifuture manually because this may not have
  // an executor set due to legacy code. This means we can bypass the executor
  // check in SemiFuture::via
  auto f = Future<T>(sf.core_);
  sf.core_ = nullptr;
  return std::move(f).via(std::move(executor));
}

template <class T>
Future<T> Future<T>::via(Executor::KeepAlive<> executor, int8_t priority) & {
  return this->via(ExecutorWithPriority::create(std::move(executor), priority));
}

template <typename T>
template <typename R, typename Caller, typename... Args>
Future<typename isFuture<R>::Inner> Future<T>::then(
    R (Caller::*func)(Args...), Caller* instance) && {
  using FirstArg =
      remove_cvref_t<typename futures::detail::ArgType<Args...>::FirstArg>;

  return std::move(*this).thenTry([instance, func](Try<T>&& t) {
    return (instance->*func)(t.template get<isTry<FirstArg>::value, Args>()...);
  });
}

template <class T>
template <typename F>
Future<typename futures::detail::tryCallableResult<T, F>::value_type>
Future<T>::thenTry(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        folly::Executor::KeepAlive<>&&,
                        folly::Try<T>&& t) mutable {
    return static_cast<F&&>(f)(std::move(t));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::forbid);
}

template <class T>
template <typename F>
Future<typename futures::detail::tryCallableResult<T, F>::value_type>
Future<T>::thenTryInline(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        folly::Executor::KeepAlive<>&&,
                        folly::Try<T>&& t) mutable {
    return static_cast<F&&>(f)(std::move(t));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::permit);
}

template <class T>
template <typename F>
Future<typename futures::detail::tryExecutorCallableResult<T, F>::value_type>
Future<T>::thenExTry(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&& ka, folly::Try<T>&& t) mutable {
    // Enforce that executor cannot be null
    DCHECK(ka);
    return static_cast<F&&>(f)(std::move(ka), std::move(t));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::forbid);
}

template <class T>
template <typename F>
Future<typename futures::detail::tryExecutorCallableResult<T, F>::value_type>
Future<T>::thenExTryInline(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&& ka, folly::Try<T>&& t) mutable {
    // Enforce that executor cannot be null
    DCHECK(ka);
    return static_cast<F&&>(f)(std::move(ka), std::move(t));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::permit);
}

template <class T>
template <typename F>
Future<typename futures::detail::valueCallableResult<T, F>::value_type>
Future<T>::thenValue(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&&, folly::Try<T>&& t) mutable {
    return futures::detail::wrapInvoke(std::move(t), static_cast<F&&>(f));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::forbid);
}

template <class T>
template <typename F>
Future<typename futures::detail::valueCallableResult<T, F>::value_type>
Future<T>::thenValueInline(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&&, folly::Try<T>&& t) mutable {
    return futures::detail::wrapInvoke(std::move(t), static_cast<F&&>(f));
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::permit);
}

template <class T>
template <typename F>
Future<typename futures::detail::valueExecutorCallableResult<T, F>::value_type>
Future<T>::thenExValue(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&& ka, folly::Try<T>&& t) mutable {
    // Enforce that executor cannot be null
    DCHECK(ka);
    return static_cast<F&&>(f)(
        std::move(ka),
        t.template get<
            false,
            typename futures::detail::valueExecutorCallableResult<T, F>::
                ValueArg>());
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::forbid);
}

template <class T>
template <typename F>
Future<typename futures::detail::valueExecutorCallableResult<T, F>::value_type>
Future<T>::thenExValueInline(F&& func) && {
  auto lambdaFunc = [f = static_cast<F&&>(func)](
                        Executor::KeepAlive<>&& ka, folly::Try<T>&& t) mutable {
    // Enforce that executor cannot be null
    DCHECK(ka);
    return static_cast<F&&>(f)(
        std::move(ka),
        t.template get<
            false,
            typename futures::detail::valueExecutorCallableResult<T, F>::
                ValueArg>());
  };
  using R = futures::detail::tryExecutorCallableResult<T, decltype(lambdaFunc)>;
  return this->thenImplementation(
      std::move(lambdaFunc), R{}, futures::detail::InlineContinuation::permit);
}

template <class T>
template <class ExceptionType, class F>
typename std::enable_if<
    isFutureOrSemiFuture<invoke_result_t<F, ExceptionType>>::value,
    Future<T>>::type
Future<T>::thenError(tag_t<ExceptionType>, F&& func) && {
  Promise<T> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());
  auto sf = p.getSemiFuture();
  auto* ePtr = this->getExecutor();
  auto e = folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());

  this->setCallback_([state = futures::detail::makeCoreCallbackState(
                          std::move(p), static_cast<F&&>(func))](
                         Executor::KeepAlive<>&& ka, Try<T>&& t) mutable {
    if (auto ex = t.template tryGetExceptionObject<
                  std::remove_reference_t<ExceptionType>>()) {
      auto tf2 = state.tryInvoke(std::move(*ex));
      if (tf2.hasException()) {
        state.setException(std::move(ka), std::move(tf2.exception()));
      } else {
        tf2->setCallback_(
            [p = state.stealPromise()](
                Executor::KeepAlive<>&& innerKA, Try<T>&& t3) mutable {
              p.setTry(std::move(innerKA), std::move(t3));
            });
      }
    } else {
      state.setTry(std::move(ka), std::move(t));
    }
  });

  return std::move(sf).via(std::move(e));
}

template <class T>
template <class ExceptionType, class F>
typename std::enable_if<
    !isFutureOrSemiFuture<invoke_result_t<F, ExceptionType>>::value,
    Future<T>>::type
Future<T>::thenError(tag_t<ExceptionType>, F&& func) && {
  Promise<T> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());
  auto sf = p.getSemiFuture();
  auto* ePtr = this->getExecutor();
  auto e = folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());

  this->setCallback_([state = futures::detail::makeCoreCallbackState(
                          std::move(p), static_cast<F&&>(func))](
                         Executor::KeepAlive<>&& ka, Try<T>&& t) mutable {
    if (auto ex = t.template tryGetExceptionObject<
                  std::remove_reference_t<ExceptionType>>()) {
      state.setTry(std::move(ka), makeTryWith([&] {
                     return state.invoke(std::move(*ex));
                   }));
    } else {
      state.setTry(std::move(ka), std::move(t));
    }
  });

  return std::move(sf).via(std::move(e));
}

template <class T>
template <class F>
typename std::enable_if<
    isFutureOrSemiFuture<invoke_result_t<F, exception_wrapper>>::value,
    Future<T>>::type
Future<T>::thenError(F&& func) && {
  auto* ePtr = this->getExecutor();
  auto e = folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());

  Promise<T> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());
  auto sf = p.getSemiFuture();
  this->setCallback_([state = futures::detail::makeCoreCallbackState(
                          std::move(p), static_cast<F&&>(func))](
                         Executor::KeepAlive<>&& ka, Try<T> t) mutable {
    if (t.hasException()) {
      auto tf2 = state.tryInvoke(std::move(t.exception()));
      if (tf2.hasException()) {
        state.setException(std::move(ka), std::move(tf2.exception()));
      } else {
        tf2->setCallback_(
            [p = state.stealPromise()](
                Executor::KeepAlive<>&& innerKA, Try<T>&& t3) mutable {
              p.setTry(std::move(innerKA), std::move(t3));
            });
      }
    } else {
      state.setTry(std::move(ka), std::move(t));
    }
  });

  return std::move(sf).via(std::move(e));
}

template <class T>
template <class F>
typename std::enable_if<
    !isFutureOrSemiFuture<invoke_result_t<F, exception_wrapper>>::value,
    Future<T>>::type
Future<T>::thenError(F&& func) && {
  auto* ePtr = this->getExecutor();
  auto e = folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());

  Promise<T> p;
  p.core_->initCopyInterruptHandlerFrom(this->getCore());
  auto sf = p.getSemiFuture();
  this->setCallback_([state = futures::detail::makeCoreCallbackState(
                          std::move(p), static_cast<F&&>(func))](
                         Executor::KeepAlive<>&& ka, Try<T>&& t) mutable {
    if (t.hasException()) {
      state.setTry(std::move(ka), makeTryWith([&] {
                     return state.invoke(std::move(t.exception()));
                   }));
    } else {
      state.setTry(std::move(ka), std::move(t));
    }
  });

  return std::move(sf).via(std::move(e));
}

template <class T>
Future<Unit> Future<T>::then() && {
  return std::move(*this).thenValue([](T&&) {});
}

template <class T>
template <class F>
Future<T> Future<T>::ensure(F&& func) && {
  return std::move(*this).thenTry(
      [funcw = static_cast<F&&>(func)](Try<T>&& t) mutable {
        static_cast<F&&>(funcw)();
        return makeFuture(std::move(t));
      });
}

template <class T>
template <class F>
Future<T> Future<T>::onTimeout(
    HighResDuration dur, F&& func, Timekeeper* tk) && {
  return std::move(*this).within(dur, tk).thenError(
      tag_t<FutureTimeout>{},
      [funcw = static_cast<F&&>(func)](auto const&) mutable {
        return static_cast<F&&>(funcw)();
      });
}

template <class Func>
auto via(Executor::KeepAlive<> x, Func&& func) -> Future<
    typename isFutureOrSemiFuture<decltype(std::declval<Func>()())>::Inner> {
  return via(std::move(x))
      .thenValue([f = static_cast<Func&&>(func)](auto&&) mutable {
        return static_cast<Func&&>(f)();
      });
}

// makeFuture

template <class T>
Future<typename std::decay<T>::type> makeFuture(T&& t) {
  return makeFuture(Try<typename std::decay<T>::type>(static_cast<T&&>(t)));
}

inline Future<Unit> makeFuture() {
  return makeFuture(Unit{});
}

// makeFutureWith(Future<T>()) -> Future<T>
template <class F>
typename std::
    enable_if<isFuture<invoke_result_t<F>>::value, invoke_result_t<F>>::type
    makeFutureWith(F&& func) {
  using InnerType = typename isFuture<invoke_result_t<F>>::Inner;
  try {
    return static_cast<F&&>(func)();
  } catch (...) {
    return makeFuture<InnerType>(exception_wrapper(std::current_exception()));
  }
}

// makeFutureWith(T()) -> Future<T>
// makeFutureWith(void()) -> Future<Unit>
template <class F>
typename std::enable_if<
    !(isFuture<invoke_result_t<F>>::value),
    Future<lift_unit_t<invoke_result_t<F>>>>::type
makeFutureWith(F&& func) {
  using LiftedResult = lift_unit_t<invoke_result_t<F>>;
  return makeFuture<LiftedResult>(
      makeTryWith([&func]() mutable { return static_cast<F&&>(func)(); }));
}

template <class T>
Future<T> makeFuture(std::exception_ptr const& e) {
  return makeFuture(Try<T>(e));
}

template <class T>
Future<T> makeFuture(exception_wrapper ew) {
  return makeFuture(Try<T>(std::move(ew)));
}

template <class T, class E>
typename std::enable_if<std::is_base_of<std::exception, E>::value, Future<T>>::
    type
    makeFuture(E const& e) {
  return makeFuture(Try<T>(make_exception_wrapper<E>(e)));
}

template <class T>
Future<T> makeFuture(Try<T> t) {
  return Future<T>(Future<T>::Core::make(std::move(t)));
}

// via
Future<Unit> via(Executor::KeepAlive<> executor) {
  return makeFuture().via(std::move(executor));
}

Future<Unit> via(Executor::KeepAlive<> executor, int8_t priority) {
  return makeFuture().via(std::move(executor), priority);
}

namespace futures {
namespace detail {

template <typename V, typename... Fs, std::size_t... Is>
FOLLY_ERASE void foreach_(std::index_sequence<Is...>, V&& v, Fs&&... fs) {
  using _ = int[];
  void(_{0, (void(v(index_constant<Is>{}, static_cast<Fs&&>(fs))), 0)...});
}
template <typename V, typename... Fs>
FOLLY_ERASE void foreach(V&& v, Fs&&... fs) {
  using _ = std::index_sequence_for<Fs...>;
  foreach_(_{}, static_cast<V&&>(v), static_cast<Fs&&>(fs)...);
}

template <typename T>
futures::detail::DeferredExecutor* getDeferredExecutor(SemiFuture<T>& future) {
  return future.getDeferredExecutor();
}

template <typename T>
futures::detail::DeferredWrapper stealDeferredExecutor(SemiFuture<T>& future) {
  return future.stealDeferredExecutor();
}

template <typename T>
futures::detail::DeferredWrapper stealDeferredExecutor(Future<T>&) {
  return {};
}

template <typename... Ts>
void stealDeferredExecutorsVariadic(
    std::vector<futures::detail::DeferredWrapper>& executors, Ts&... ts) {
  foreach(
      [&](auto, auto& future) {
        if (auto executor = stealDeferredExecutor(future)) {
          executors.push_back(std::move(executor));
        }
      },
      ts...);
}

template <class InputIterator>
void stealDeferredExecutors(
    std::vector<futures::detail::DeferredWrapper>& executors,
    InputIterator first,
    InputIterator last) {
  for (auto it = first; it != last; ++it) {
    if (auto executor = stealDeferredExecutor(*it)) {
      executors.push_back(std::move(executor));
    }
  }
}
} // namespace detail
} // namespace futures

// collectAll (variadic)

template <typename... Fs>
SemiFuture<std::tuple<Try<typename remove_cvref_t<Fs>::value_type>...>>
collectAll(Fs&&... fs) {
  using Result = std::tuple<Try<typename remove_cvref_t<Fs>::value_type>...>;
  struct Context {
    ~Context() { p.setValue(std::move(results)); }
    Promise<Result> p;
    Result results;
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutorsVariadic(executors, fs...);

  auto ctx = std::make_shared<Context>();
  futures::detail::foreach(
      [&](auto i, auto&& f) {
        f.setCallback_([i, ctx](auto&&, auto&& t) {
          std::get<i.value>(ctx->results) = std::move(t);
        });
      },
      static_cast<Fs&&>(fs)...);

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    auto work = [](Try<typename decltype(future)::value_type>&& t) {
      return std::move(t).value();
    };
    future = std::move(future).defer(work);
    auto deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

template <typename... Fs>
Future<std::tuple<Try<typename remove_cvref_t<Fs>::value_type>...>>
collectAllUnsafe(Fs&&... fs) {
  return collectAll(static_cast<Fs&&>(fs)...).toUnsafeFuture();
}

// collectAll (iterator)

template <class InputIterator>
SemiFuture<std::vector<
    Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
collectAll(InputIterator first, InputIterator last) {
  using F = typename std::iterator_traits<InputIterator>::value_type;
  using T = typename F::value_type;

  struct Context {
    explicit Context(size_t n) : results(n), count(n) {}
    ~Context() {
      futures::detail::setTry(
          p, std::move(ka), Try<std::vector<Try<T>>>(std::move(results)));
    }
    Promise<std::vector<Try<T>>> p;
    Executor::KeepAlive<> ka;
    std::vector<Try<T>> results;
    std::atomic<size_t> count;
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  auto ctx = std::make_shared<Context>(size_t(std::distance(first, last)));

  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_(
        [i, ctx](Executor::KeepAlive<>&& ka, Try<T>&& t) {
          ctx->results[i] = std::move(t);
          if (ctx->count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
            ctx->ka = std::move(ka);
          }
        },
        futures::detail::InlineContinuation::permit);
  }

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    future = std::move(future).defer(
        [](Try<typename decltype(future)::value_type>&& t) {
          return std::move(t).value();
        });
    auto deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

template <class InputIterator>
Future<std::vector<
    Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
collectAllUnsafe(InputIterator first, InputIterator last) {
  return collectAll(first, last).toUnsafeFuture();
}

// collect (iterator)

template <class InputIterator>
SemiFuture<std::vector<
    typename std::iterator_traits<InputIterator>::value_type::value_type>>
collect(InputIterator first, InputIterator last) {
  using F = typename std::iterator_traits<InputIterator>::value_type;
  using T = typename F::value_type;

  struct Context {
    explicit Context(size_t n) : result(n) { finalResult.reserve(n); }
    ~Context() {
      if (!threw.load(std::memory_order_relaxed)) {
        // map Optional<T> -> T
        for (auto& value : result) {
          // if any of the input futures were off the end of a weakRef(), the
          // logic added in setCallback_ will not execute as an executor
          // weakRef() drops all callbacks added silently without executing them
          if (!value.has_value()) {
            p.setException(BrokenPromise{pretty_name<std::vector<T>>()});
            return;
          }

          finalResult.push_back(std::move(value.value()));
        }

        p.setValue(std::move(finalResult));
      }
    }
    Promise<std::vector<T>> p;
    std::vector<Optional<T>> result;
    std::vector<T> finalResult;
    std::atomic<bool> threw{false};
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  auto ctx = std::make_shared<Context>(std::distance(first, last));
  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_([i, ctx](Executor::KeepAlive<>&&, Try<T>&& t) {
      if (t.hasException()) {
        if (!ctx->threw.exchange(true, std::memory_order_relaxed)) {
          ctx->p.setException(std::move(t.exception()));
        }
      } else if (!ctx->threw.load(std::memory_order_relaxed)) {
        ctx->result[i] = std::move(t.value());
      }
    });
  }

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    auto work = [](Try<typename decltype(future)::value_type>&& t) {
      return std::move(t).value();
    };
    future = std::move(future).defer(work);
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

template <class InputIterator>
Future<std::vector<
    typename std::iterator_traits<InputIterator>::value_type::value_type>>
collectUnsafe(InputIterator first, InputIterator last) {
  return collect(first, last).toUnsafeFuture();
}

// collect (variadic)

template <typename... Fs>
SemiFuture<std::tuple<typename remove_cvref_t<Fs>::value_type...>> collect(
    Fs&&... fs) {
  using Result = std::tuple<typename remove_cvref_t<Fs>::value_type...>;
  struct Context {
    ~Context() {
      if (!threw.load(std::memory_order_relaxed)) {
        // if any of the input futures were off the end of a weakRef(), the
        // logic added in setCallback_ will not execute as an executor
        // weakRef() drops all callbacks added silently without executing them
        auto brokenPromise = false;
        folly::for_each(results, [&](auto& result) {
          if (!result.hasValue() && !result.hasException()) {
            brokenPromise = true;
          }
        });

        if (brokenPromise) {
          p.setException(BrokenPromise{pretty_name<Result>()});
        } else {
          p.setValue(unwrapTryTuple(std::move(results)));
        }
      }
    }
    Promise<Result> p;
    std::tuple<Try<typename remove_cvref_t<Fs>::value_type>...> results;
    std::atomic<bool> threw{false};
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutorsVariadic(executors, fs...);

  auto ctx = std::make_shared<Context>();
  futures::detail::foreach(
      [&](auto i, auto&& f) {
        f.setCallback_([i, ctx](Executor::KeepAlive<>&&, auto&& t) {
          if (t.hasException()) {
            if (!ctx->threw.exchange(true, std::memory_order_relaxed)) {
              ctx->p.setException(std::move(t.exception()));
            }
          } else if (!ctx->threw.load(std::memory_order_relaxed)) {
            std::get<i.value>(ctx->results) = std::move(t);
          }
        });
      },
      static_cast<Fs&&>(fs)...);

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    auto work = [](Try<typename decltype(future)::value_type>&& t) {
      return std::move(t).value();
    };
    future = std::move(future).defer(work);
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

template <typename... Fs>
Future<std::tuple<typename remove_cvref_t<Fs>::value_type...>> collectUnsafe(
    Fs&&... fs) {
  return collect(static_cast<Fs&&>(fs)...).toUnsafeFuture();
}

// collectAny (iterator)

template <class InputIterator>
SemiFuture<std::pair<
    size_t,
    Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
collectAny(InputIterator first, InputIterator last) {
  using F = typename std::iterator_traits<InputIterator>::value_type;
  using T = typename F::value_type;

  struct Context {
    Promise<std::pair<size_t, Try<T>>> p;
    std::atomic<bool> done{false};
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  auto ctx = std::make_shared<Context>();
  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_([i, ctx](Executor::KeepAlive<>&&, Try<T>&& t) {
      if (!ctx->done.exchange(true, std::memory_order_relaxed)) {
        ctx->p.setValue(std::make_pair(i, std::move(t)));
      }
    });
  }
  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    future = std::move(future).defer(
        [](Try<typename decltype(future)::value_type>&& t) {
          return std::move(t).value();
        });
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

// collectAnyWithoutException (iterator)

template <class InputIterator>
SemiFuture<std::pair<
    size_t,
    typename std::iterator_traits<InputIterator>::value_type::value_type>>
collectAnyWithoutException(InputIterator first, InputIterator last) {
  using F = typename std::iterator_traits<InputIterator>::value_type;
  using T = typename F::value_type;

  struct Context {
    Context(size_t n) : nTotal(n) {}
    Promise<std::pair<size_t, T>> p;
    std::atomic<bool> done{false};
    std::atomic<size_t> nFulfilled{0};
    size_t nTotal;
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  auto ctx = std::make_shared<Context>(size_t(std::distance(first, last)));
  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_([i, ctx](Executor::KeepAlive<>&&, Try<T>&& t) {
      if (!t.hasException() &&
          !ctx->done.exchange(true, std::memory_order_relaxed)) {
        ctx->p.setValue(std::make_pair(i, std::move(t.value())));
      } else if (
          ctx->nFulfilled.fetch_add(1, std::memory_order_relaxed) + 1 ==
          ctx->nTotal) {
        ctx->p.setException(t.exception());
      }
    });
  }

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    future = std::move(future).defer(
        [](Try<typename decltype(future)::value_type>&& t) {
          return std::move(t).value();
        });
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

// collectN (iterator)

template <class InputIterator>
SemiFuture<std::vector<std::pair<
    size_t,
    Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
collectN(InputIterator first, InputIterator last, size_t n) {
  using F = typename std::iterator_traits<InputIterator>::value_type;
  using T = typename F::value_type;
  using Result = std::vector<std::pair<size_t, Try<T>>>;

  struct Context {
    explicit Context(size_t numFutures, size_t min_)
        : v(numFutures), min(min_) {}

    std::vector<Optional<Try<T>>> v;
    size_t min;
    std::atomic<size_t> completed = {0}; // # input futures completed
    std::atomic<size_t> stored = {0}; // # output values stored
    Promise<Result> p;
  };

  assert(n > 0);
  assert(std::distance(first, last) >= 0);

  if (size_t(std::distance(first, last)) < n) {
    return SemiFuture<Result>(
        exception_wrapper(std::runtime_error("Not enough futures")));
  }

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  // for each completed Future, increase count and add to vector, until we
  // have n completed futures at which point we fulfil our Promise with the
  // vector
  auto ctx = std::make_shared<Context>(size_t(std::distance(first, last)), n);
  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_([i, ctx](Executor::KeepAlive<>&&, Try<T>&& t) {
      // relaxed because this guards control but does not guard data
      auto const c = 1 + ctx->completed.fetch_add(1, std::memory_order_relaxed);
      if (c > ctx->min) {
        return;
      }
      ctx->v[i] = std::move(t);

      // release because the stored values in all threads must be visible below
      // acquire because no stored value is permitted to be fetched early
      auto const s = 1 + ctx->stored.fetch_add(1, std::memory_order_acq_rel);
      if (s < ctx->min) {
        return;
      }
      Result result;
      result.reserve(ctx->completed.load());
      for (size_t j = 0; j < ctx->v.size(); ++j) {
        auto& entry = ctx->v[j];
        if (entry.hasValue()) {
          result.emplace_back(j, std::move(entry).value());
        }
      }
      ctx->p.setTry(Try<Result>(std::move(result)));
    });
  }

  auto future = ctx->p.getSemiFuture();
  if (!executors.empty()) {
    future = std::move(future).defer(
        [](Try<typename decltype(future)::value_type>&& t) {
          return std::move(t).value();
        });
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

// reduce (iterator)

template <class It, class T, class F>
Future<T> reduce(It first, It last, T&& initial, F&& func) {
  if (first == last) {
    return makeFuture(static_cast<T&&>(initial));
  }

  typedef typename std::iterator_traits<It>::value_type::value_type ItT;
  typedef typename std::
      conditional<is_invocable<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type
          Arg;
  typedef isTry<Arg> IsTry;

  auto sfunc = std::make_shared<std::decay_t<F>>(static_cast<F&&>(func));

  auto f = std::move(*first).thenTry(
      [initial = static_cast<T&&>(initial), sfunc](Try<ItT>&& head) mutable {
        return (*sfunc)(
            std::move(initial), head.template get<IsTry::value, Arg&&>());
      });

  for (++first; first != last; ++first) {
    f = collectAllUnsafe(f, *first).thenValue(
        [sfunc](std::tuple<Try<T>, Try<ItT>>&& t) {
          return (*sfunc)(
              std::move(std::get<0>(t).value()),
              // Either return a ItT&& or a Try<ItT>&& depending
              // on the type of the argument of func.
              std::get<1>(t).template get<IsTry::value, Arg&&>());
        });
  }

  return f;
}

// window (collection)

template <class Collection, class F, class ItT, class Result>
std::vector<Future<Result>> window(Collection input, F func, size_t n) {
  // Use global QueuedImmediateExecutor singleton to avoid stack overflow.
  auto executor = &QueuedImmediateExecutor::instance();
  return window(executor, std::move(input), std::move(func), n);
}

template <class F>
auto window(size_t times, F func, size_t n)
    -> std::vector<invoke_result_t<F, size_t>> {
  return window(futures::detail::WindowFakeVector(times), std::move(func), n);
}

template <class Collection, class F, class ItT, class Result>
std::vector<Future<Result>> window(
    Executor::KeepAlive<> executor, Collection input, F func, size_t n) {
  struct WindowContext {
    WindowContext(
        Executor::KeepAlive<> executor_, Collection&& input_, F&& func_)
        : executor(std::move(executor_)),
          input(std::move(input_)),
          promises(input.size()),
          func(std::move(func_)) {}
    std::atomic<size_t> i{0};
    Executor::KeepAlive<> executor;
    Collection input;
    std::vector<Promise<Result>> promises;
    F func;

    static void spawn(std::shared_ptr<WindowContext> ctx) {
      size_t i = ctx->i.fetch_add(1, std::memory_order_relaxed);
      if (i < ctx->input.size()) {
        auto fut = makeSemiFutureWith([&] {
                     return ctx->func(std::move(ctx->input[i]));
                   }).via(ctx->executor.get());

        fut.setCallback_([ctx = std::move(ctx), i](
                             Executor::KeepAlive<>&&, Try<Result>&& t) mutable {
          ctx->promises[i].setTry(std::move(t));
          // Chain another future onto this one
          spawn(std::move(ctx));
        });
      }
    }
  };

  auto max = std::min(n, input.size());

  auto ctx = std::make_shared<WindowContext>(
      executor.copy(), std::move(input), std::move(func));

  // Start the first n Futures
  for (size_t i = 0; i < max; ++i) {
    executor->add([ctx]() mutable { WindowContext::spawn(std::move(ctx)); });
  }

  std::vector<Future<Result>> futures;
  futures.reserve(ctx->promises.size());
  for (auto& promise : ctx->promises) {
    futures.emplace_back(promise.getSemiFuture().via(executor.copy()));
  }

  return futures;
}

// reduce

template <class T>
template <class I, class F>
Future<I> Future<T>::reduce(I&& initial, F&& func) && {
  return std::move(*this).thenValue(
      [minitial = static_cast<I&&>(initial),
       mfunc = static_cast<F&&>(func)](T&& vals) mutable {
        auto ret = std::move(minitial);
        for (auto& val : vals) {
          ret = mfunc(std::move(ret), std::move(val));
        }
        return ret;
      });
}

// unorderedReduce (iterator)

template <class It, class T, class F>
SemiFuture<T> unorderedReduceSemiFuture(It first, It last, T initial, F func) {
  using ItF = typename std::iterator_traits<It>::value_type;
  using ItT = typename ItF::value_type;
  using Arg = MaybeTryArg<F, T, ItT>;

  if (first == last) {
    return makeFuture(std::move(initial));
  }

  typedef isTry<Arg> IsTry;

  struct Context {
    Context(T&& memo, F&& fn, size_t n)
        : lock_(),
          memo_(makeFuture<T>(std::move(memo))),
          func_(std::move(fn)),
          numThens_(0),
          numFutures_(n),
          promise_() {}

    folly::MicroSpinLock lock_; // protects memo_ and numThens_
    Future<T> memo_;
    F func_;
    size_t numThens_; // how many Futures completed and called .then()
    size_t numFutures_; // how many Futures in total
    Promise<T> promise_;
  };

  struct Fulfill {
    void operator()(Promise<T>&& p, T&& v) const { p.setValue(std::move(v)); }
    void operator()(Promise<T>&& p, Future<T>&& f) const {
      f.setCallback_(
          [p = std::move(p)](Executor::KeepAlive<>&&, Try<T>&& t) mutable {
            p.setTry(std::move(t));
          });
    }
  };

  std::vector<futures::detail::DeferredWrapper> executors;
  futures::detail::stealDeferredExecutors(executors, first, last);

  auto ctx = std::make_shared<Context>(
      std::move(initial), std::move(func), std::distance(first, last));
  for (size_t i = 0; first != last; ++first, ++i) {
    first->setCallback_([i, ctx](Executor::KeepAlive<>&&, Try<ItT>&& t) {
      (void)i;
      // Futures can be completed in any order, simultaneously.
      // To make this non-blocking, we create a new Future chain in
      // the order of completion to reduce the values.
      // The spinlock just protects chaining a new Future, not actually
      // executing the reduce, which should be really fast.
      Promise<T> p;
      auto f = p.getFuture();
      {
        folly::MSLGuard lock(ctx->lock_);
        f = std::exchange(ctx->memo_, std::move(f));
        if (++ctx->numThens_ == ctx->numFutures_) {
          // After reducing the value of the last Future, fulfill the Promise
          ctx->memo_.setCallback_([ctx](Executor::KeepAlive<>&&, Try<T>&& t2) {
            ctx->promise_.setValue(std::move(t2));
          });
        }
      }
      f.setCallback_([ctx, mp = std::move(p), mt = std::move(t)](
                         Executor::KeepAlive<>&&, Try<T>&& v) mutable {
        if (v.hasValue()) {
          exception_wrapper ew;
          try {
            Fulfill{}(
                std::move(mp),
                ctx->func_(
                    std::move(v.value()),
                    mt.template get<IsTry::value, Arg&&>()));
          } catch (...) {
            ew = exception_wrapper{std::current_exception()};
          }
          if (ew) {
            mp.setException(std::move(ew));
          }
        } else {
          mp.setTry(std::move(v));
        }
      });
    });
  }

  auto future = ctx->promise_.getSemiFuture();
  if (!executors.empty()) {
    future = std::move(future).defer(
        [](Try<typename decltype(future)::value_type>&& t) {
          return std::move(t).value();
        });
    const auto& deferredExecutor = futures::detail::getDeferredExecutor(future);
    deferredExecutor->setNestedExecutors(std::move(executors));
  }
  return future;
}

template <class It, class T, class F>
Future<T> unorderedReduce(It first, It last, T initial, F func) {
  return unorderedReduceSemiFuture(
             first, last, std::move(initial), std::move(func))
      .via(&InlineExecutor::instance());
}

// within

template <class T>
Future<T> Future<T>::within(HighResDuration dur, Timekeeper* tk) && {
  return std::move(*this).within(dur, FutureTimeout(), tk);
}

template <class T>
template <class E>
Future<T> Future<T>::within(HighResDuration dur, E e, Timekeeper* tk) && {
  if (this->isReady()) {
    return std::move(*this);
  }

  auto* ePtr = this->getExecutor();
  auto exe =
      folly::getKeepAliveToken(ePtr ? *ePtr : InlineExecutor::instance());
  return std::move(*this).semi().within(dur, e, tk).via(std::move(exe));
}

template <class T>
template <typename E>
SemiFuture<T> SemiFuture<T>::within(
    HighResDuration dur, E e, Timekeeper* tk) && {
  if (this->isReady()) {
    return std::move(*this);
  }

  struct Context {
    explicit Context(E ex) : exception(std::move(ex)) {}
    E exception;
    SemiFuture<Unit> thisFuture{SemiFuture<Unit>::makeEmpty()};
    SemiFuture<Unit> afterFuture{SemiFuture<Unit>::makeEmpty()};
    Promise<T> promise;
    std::atomic<bool> token{false};
  };

  std::shared_ptr<Timekeeper> tks;
  if (LIKELY(!tk)) {
    tks = folly::detail::getTimekeeperSingleton();
    tk = tks.get();
  }

  if (UNLIKELY(!tk)) {
    return makeSemiFuture<T>(FutureNoTimekeeper());
  }

  auto ctx = std::make_shared<Context>(std::move(e));

  ctx->thisFuture = std::move(*this).defer([ctx](Try<T>&& t) {
    if (!ctx->token.exchange(true, std::memory_order_relaxed)) {
      ctx->promise.setTry(std::move(t));
      ctx->afterFuture.cancel();
    }
  });

  // Have time keeper use a weak ptr to hold ctx,
  // so that ctx can be deallocated as soon as the future job finished.
  ctx->afterFuture =
      tk->after(dur).defer([weakCtx = to_weak_ptr(ctx)](Try<Unit>&& t) mutable {
        if (t.hasException() &&
            t.exception().is_compatible_with<FutureCancellation>()) {
          // This got cancelled by thisFuture so we can just return.
          return;
        }

        auto lockedCtx = weakCtx.lock();
        if (!lockedCtx) {
          // ctx already released. "this" completed first, cancel "after"
          return;
        }
        // "after" completed first, cancel "this"
        lockedCtx->thisFuture.raise(FutureTimeout());
        if (!lockedCtx->token.exchange(true, std::memory_order_relaxed)) {
          if (t.hasException()) {
            lockedCtx->promise.setException(std::move(t.exception()));
          } else {
            lockedCtx->promise.setException(std::move(lockedCtx->exception));
          }
        }
      });

  // Properly propagate interrupt values through futures chained after within()
  ctx->promise.setInterruptHandler(
      [weakCtx = to_weak_ptr(ctx)](const exception_wrapper& ex) {
        if (auto lockedCtx = weakCtx.lock()) {
          lockedCtx->thisFuture.raise(ex);
        }
      });

  // Construct the future to return, create a fresh DeferredExecutor and
  // nest the other two inside it, in case they already carry nested executors.
  auto fut = ctx->promise.getSemiFuture();
  auto newDeferredExecutor = futures::detail::KeepAliveOrDeferred(
      futures::detail::DeferredExecutor::create());
  fut.setExecutor(std::move(newDeferredExecutor));

  std::vector<folly::futures::detail::DeferredWrapper> nestedExecutors;
  nestedExecutors.emplace_back(ctx->thisFuture.stealDeferredExecutor());
  nestedExecutors.emplace_back(ctx->afterFuture.stealDeferredExecutor());
  // Set trivial callbacks to treat the futures as consumed
  ctx->thisFuture.setCallback_([](Executor::KeepAlive<>&&, Try<Unit>&&) {});
  ctx->afterFuture.setCallback_([](Executor::KeepAlive<>&&, Try<Unit>&&) {});
  futures::detail::getDeferredExecutor(fut)->setNestedExecutors(
      std::move(nestedExecutors));
  return fut;
}

// delayed

template <class T>
Future<T> Future<T>::delayed(HighResDuration dur, Timekeeper* tk) && {
  auto e = this->getExecutor();
  return collectAll(*this, futures::sleep(dur, tk))
      .via(e ? e : &InlineExecutor::instance())
      .thenValue([](std::tuple<Try<T>, Try<Unit>>&& tup) {
        return makeFuture<T>(std::get<0>(std::move(tup)));
      });
}

namespace futures {
namespace detail {

template <class FutureType, typename T = typename FutureType::value_type>
void waitImpl(FutureType& f) {
  if (std::is_base_of<Future<T>, FutureType>::value) {
    f = std::move(f).via(&InlineExecutor::instance());
  }
  // short-circuit if there's nothing to do
  if (f.isReady()) {
    return;
  }

  Promise<T> promise;
  auto ret = convertFuture(promise.getSemiFuture(), f);
  FutureBatonType baton;
  f.setCallback_([&baton, promise = std::move(promise)](
                     Executor::KeepAlive<>&&, Try<T>&& t) mutable {
    promise.setTry(std::move(t));
    baton.post();
  });
  f = std::move(ret);
  baton.wait();
  assert(f.isReady());
}

template <class T>
Future<T> convertFuture(SemiFuture<T>&& sf, const Future<T>& f) {
  // Carry executor from f, inserting an inline executor if it did not have one
  auto* exe = f.getExecutor();
  auto newFut = std::move(sf).via(exe ? exe : &InlineExecutor::instance());
  newFut.core_->initCopyInterruptHandlerFrom(*f.core_);
  return newFut;
}

template <class T>
SemiFuture<T> convertFuture(SemiFuture<T>&& sf, const SemiFuture<T>&) {
  return std::move(sf);
}

template <class FutureType, typename T = typename FutureType::value_type>
void waitImpl(FutureType& f, HighResDuration dur) {
  if (std::is_base_of<Future<T>, FutureType>::value) {
    f = std::move(f).via(&InlineExecutor::instance());
  }
  // short-circuit if there's nothing to do
  if (f.isReady()) {
    return;
  }

  Promise<T> promise;
  auto ret = convertFuture(promise.getSemiFuture(), f);
  auto baton = std::make_shared<FutureBatonType>();
  f.setCallback_([baton, promise = std::move(promise)](
                     Executor::KeepAlive<>&&, Try<T>&& t) mutable {
    promise.setTry(std::move(t));
    baton->post();
  });
  f = std::move(ret);
  if (baton->try_wait_for(dur)) {
    assert(f.isReady());
  }
}

template <class T>
void waitViaImpl(Future<T>& f, DrivableExecutor* e) {
  // Set callback so to ensure that the via executor has something on it
  // so that once the preceding future triggers this callback, drive will
  // always have a callback to satisfy it
  if (f.isReady()) {
    return;
  }
  f = std::move(f).via(e).thenTry([](Try<T>&& t) { return std::move(t); });
  while (!f.isReady()) {
    e->drive();
  }
  assert(f.isReady());
  f = std::move(f).via(&InlineExecutor::instance());
}

template <class T, typename Rep, typename Period>
void waitViaImpl(
    Future<T>& f,
    TimedDrivableExecutor* e,
    const std::chrono::duration<Rep, Period>& timeout) {
  // Set callback so to ensure that the via executor has something on it
  // so that once the preceding future triggers this callback, drive will
  // always have a callback to satisfy it
  if (f.isReady()) {
    return;
  }
  // Chain operations, ensuring that the executor is kept alive for the duration
  f = std::move(f).via(e).thenValue(
      [keepAlive = getKeepAliveToken(e)](T&& t) { return std::move(t); });
  auto now = std::chrono::steady_clock::now();
  auto deadline = now + timeout;
  while (!f.isReady() && (now < deadline)) {
    e->try_drive_until(deadline);
    now = std::chrono::steady_clock::now();
  }
  assert(f.isReady() || (now >= deadline));
  if (f.isReady()) {
    f = std::move(f).via(&InlineExecutor::instance());
  }
}

} // namespace detail
} // namespace futures

template <class T>
SemiFuture<T>& SemiFuture<T>::wait() & {
  if (auto deferredExecutor = this->getDeferredExecutor()) {
    // Make sure that the last callback in the future chain will be run on the
    // WaitExecutor.
    Promise<T> promise;
    auto ret = promise.getSemiFuture();
    setCallback_(
        [p = std::move(promise)](Executor::KeepAlive<>&&, auto&& r) mutable {
          p.setTry(std::move(r));
        });
    auto waitExecutor = futures::detail::WaitExecutor::create();
    deferredExecutor->setExecutor(waitExecutor.copy());
    while (!ret.isReady()) {
      waitExecutor->drive();
    }
    waitExecutor->detach();
    this->detach();
    *this = std::move(ret);
  } else {
    futures::detail::waitImpl(*this);
  }
  return *this;
}

template <class T>
SemiFuture<T>&& SemiFuture<T>::wait() && {
  return std::move(wait());
}

template <class T>
SemiFuture<T>& SemiFuture<T>::wait(HighResDuration dur) & {
  if (auto deferredExecutor = this->getDeferredExecutor()) {
    // Make sure that the last callback in the future chain will be run on the
    // WaitExecutor.
    Promise<T> promise;
    auto ret = promise.getSemiFuture();
    setCallback_(
        [p = std::move(promise)](Executor::KeepAlive<>&&, auto&& r) mutable {
          p.setTry(std::move(r));
        });
    auto waitExecutor = futures::detail::WaitExecutor::create();
    auto deadline = futures::detail::WaitExecutor::Clock::now() + dur;
    deferredExecutor->setExecutor(waitExecutor.copy());
    while (!ret.isReady()) {
      if (!waitExecutor->driveUntil(deadline)) {
        break;
      }
    }
    waitExecutor->detach();
    this->detach();
    *this = std::move(ret);
  } else {
    futures::detail::waitImpl(*this, dur);
  }
  return *this;
}

template <class T>
bool SemiFuture<T>::wait(HighResDuration dur) && {
  auto future = std::move(*this);
  future.wait(dur);
  return future.isReady();
}

template <class T>
T SemiFuture<T>::get() && {
  return std::move(*this).getTry().value();
}

template <class T>
T SemiFuture<T>::get(HighResDuration dur) && {
  return std::move(*this).getTry(dur).value();
}

template <class T>
Try<T> SemiFuture<T>::getTry() && {
  wait();
  auto future = folly::Future<T>(this->core_);
  this->core_ = nullptr;
  return std::move(std::move(future).result());
}

template <class T>
Try<T> SemiFuture<T>::getTry(HighResDuration dur) && {
  wait(dur);
  auto future = folly::Future<T>(this->core_);
  this->core_ = nullptr;

  if (!future.isReady()) {
    throw_exception<FutureTimeout>();
  }
  return std::move(std::move(future).result());
}

template <class T>
Future<T>& Future<T>::wait() & {
  futures::detail::waitImpl(*this);
  return *this;
}

template <class T>
Future<T>&& Future<T>::wait() && {
  futures::detail::waitImpl(*this);
  return std::move(*this);
}

template <class T>
Future<T>& Future<T>::wait(HighResDuration dur) & {
  futures::detail::waitImpl(*this, dur);
  return *this;
}

template <class T>
Future<T>&& Future<T>::wait(HighResDuration dur) && {
  futures::detail::waitImpl(*this, dur);
  return std::move(*this);
}

template <class T>
Future<T>& Future<T>::waitVia(DrivableExecutor* e) & {
  futures::detail::waitViaImpl(*this, e);
  return *this;
}

template <class T>
Future<T>&& Future<T>::waitVia(DrivableExecutor* e) && {
  futures::detail::waitViaImpl(*this, e);
  return std::move(*this);
}

template <class T>
Future<T>& Future<T>::waitVia(TimedDrivableExecutor* e, HighResDuration dur) & {
  futures::detail::waitViaImpl(*this, e, dur);
  return *this;
}

template <class T>
Future<T>&& Future<T>::waitVia(
    TimedDrivableExecutor* e, HighResDuration dur) && {
  futures::detail::waitViaImpl(*this, e, dur);
  return std::move(*this);
}

template <class T>
T Future<T>::get() && {
  return std::move(*this).getTry().value();
}

template <class T>
T Future<T>::get(HighResDuration dur) && {
  return std::move(*this).getTry(dur).value();
}

template <class T>
Try<T> Future<T>::getTry() && {
  return std::move(*this).semi().getTry();
}

template <class T>
Try<T> Future<T>::getTry(HighResDuration dur) && {
  return std::move(*this).semi().getTry(dur);
}

template <class T>
T Future<T>::getVia(DrivableExecutor* e) && {
  return std::move(waitVia(e).value());
}

template <class T>
T Future<T>::getVia(TimedDrivableExecutor* e, HighResDuration dur) && {
  waitVia(e, dur);
  if (!this->isReady()) {
    throw_exception<FutureTimeout>();
  }
  return std::move(value());
}

template <class T>
Try<T> Future<T>::getTryVia(DrivableExecutor* e) && {
  return std::move(waitVia(e).result());
}

template <class T>
Try<T> Future<T>::getTryVia(TimedDrivableExecutor* e, HighResDuration dur) && {
  waitVia(e, dur);
  if (!this->isReady()) {
    throw_exception<FutureTimeout>();
  }
  return std::move(result());
}

namespace futures {
namespace detail {
template <class T>
struct TryEquals {
  static bool equals(const Try<T>& t1, const Try<T>& t2) {
    return t1.value() == t2.value();
  }
};
} // namespace detail
} // namespace futures

template <class T>
Future<bool> Future<T>::willEqual(Future<T>& f) {
  return collectAllUnsafe(*this, f).thenValue(
      [](const std::tuple<Try<T>, Try<T>>& t) {
        if (std::get<0>(t).hasValue() && std::get<1>(t).hasValue()) {
          return futures::detail::TryEquals<T>::equals(
              std::get<0>(t), std::get<1>(t));
        } else {
          return false;
        }
      });
}

template <class T>
template <class F>
Future<T> Future<T>::filter(F&& predicate) && {
  return std::move(*this).thenValue([p = static_cast<F&&>(predicate)](T val) {
    T const& valConstRef = val;
    if (!p(valConstRef)) {
      throw_exception<FuturePredicateDoesNotObtain>();
    }
    return val;
  });
}

template <class F>
auto when(bool p, F&& thunk)
    -> decltype(std::declval<invoke_result_t<F>>().unit()) {
  return p ? static_cast<F&&>(thunk)().unit() : makeFuture();
}

template <class P, class F>
typename std::
    enable_if<isSemiFuture<invoke_result_t<F>>::value, SemiFuture<Unit>>::type
    whileDo(P&& predicate, F&& thunk) {
  if (predicate()) {
    auto future = thunk();
    return std::move(future).deferExValue(
        [predicate = static_cast<P&&>(predicate),
         thunk = static_cast<F&&>(thunk)](auto&& ex, auto&&) mutable {
          return whileDo(static_cast<P&&>(predicate), static_cast<F&&>(thunk))
              .via(std::move(ex));
        });
  }
  return makeSemiFuture();
}

template <class P, class F>
typename std::enable_if<isFuture<invoke_result_t<F>>::value, Future<Unit>>::type
whileDo(P&& predicate, F&& thunk) {
  if (predicate()) {
    auto future = thunk();
    return std::move(future).thenValue(
        [predicate = static_cast<P&&>(predicate),
         thunk = static_cast<F&&>(thunk)](auto&&) mutable {
          return whileDo(static_cast<P&&>(predicate), static_cast<F&&>(thunk));
        });
  }
  return makeFuture();
}

template <class F>
auto times(const int n, F&& thunk) {
  return folly::whileDo(
      [n, count = std::make_unique<std::atomic<int>>(0)]() mutable {
        return count->fetch_add(1, std::memory_order_relaxed) < n;
      },
      static_cast<F&&>(thunk));
}

namespace futures {
template <class It, class F, class ItT, class Tag, class Result>
std::vector<Future<Result>> mapValue(It first, It last, F func) {
  std::vector<Future<Result>> results;
  results.reserve(std::distance(first, last));
  for (auto it = first; it != last; it++) {
    results.push_back(std::move(*it).thenValue(func));
  }
  return results;
}

template <class It, class F, class ItT, class Tag, class Result>
std::vector<Future<Result>> mapTry(It first, It last, F func, int) {
  std::vector<Future<Result>> results;
  results.reserve(std::distance(first, last));
  for (auto it = first; it != last; it++) {
    results.push_back(std::move(*it).thenTry(func));
  }
  return results;
}

template <class It, class F, class ItT, class Tag, class Result>
std::vector<Future<Result>> mapValue(
    Executor& exec, It first, It last, F func) {
  std::vector<Future<Result>> results;
  results.reserve(std::distance(first, last));
  for (auto it = first; it != last; it++) {
    results.push_back(std::move(*it).via(&exec).thenValue(func));
  }
  return results;
}

template <class It, class F, class ItT, class Tag, class Result>
std::vector<Future<Result>> mapTry(
    Executor& exec, It first, It last, F func, int) {
  std::vector<Future<Result>> results;
  results.reserve(std::distance(first, last));
  for (auto it = first; it != last; it++) {
    results.push_back(std::move(*it).via(&exec).thenTry(func));
  }
  return results;
}

template <typename F, class Ensure>
auto ensure(F&& f, Ensure&& ensure) {
  return makeSemiFuture()
      .deferValue([f = static_cast<F&&>(f)](auto) mutable { return f(); })
      .defer([ensure = static_cast<Ensure&&>(ensure)](auto resultTry) mutable {
        ensure();
        return std::move(resultTry).value();
      });
}

template <class T>
void detachOn(folly::Executor::KeepAlive<> exec, folly::SemiFuture<T>&& fut) {
  std::move(fut).via(exec).detach();
}

template <class T>
void detachOnGlobalCPUExecutor(folly::SemiFuture<T>&& fut) {
  detachOn(folly::getGlobalCPUExecutor(), std::move(fut));
}

template <class T>
void maybeDetachOnGlobalExecutorAfter(
    HighResDuration dur, folly::SemiFuture<T>&& fut) {
  sleep(dur).toUnsafeFuture().thenValue([fut = std::move(fut)](auto&&) mutable {
    if (auto ptr = folly::detail::tryGetImmutableCPUPtr()) {
      detachOn(folly::getKeepAliveToken(ptr.get()), std::move(fut));
    }
  });
}

template <class T>
void detachWithoutExecutor(folly::SemiFuture<T>&& fut) {
  auto executor = futures::detail::stealDeferredExecutor(fut);
  // Fail if we try to detach a SemiFuture with deferred work
  DCHECK(executor.get() == nullptr);
  if (executor) {
    executor.get()->detach();
  }
}

} // namespace futures

template <class Clock>
SemiFuture<Unit> Timekeeper::at(std::chrono::time_point<Clock> when) {
  auto now = Clock::now();

  if (when <= now) {
    return makeSemiFuture();
  }

  return after(std::chrono::duration_cast<HighResDuration>(when - now));
}

#if FOLLY_USE_EXTERN_FUTURE_UNIT
// limited to the instances unconditionally forced by the futures library
namespace futures {
namespace detail {
extern template class FutureBase<Unit>;
} // namespace detail
} // namespace futures
extern template class Future<Unit>;
extern template class SemiFuture<Unit>;
#endif
} // namespace folly
