X-Git-Url: http://plrg.eecs.uci.edu/git/?p=folly.git;a=blobdiff_plain;f=folly%2Ffutures%2FFuture-inl.h;h=56b98410abcfe59d24cfcdbe89caecf08a0d8ba2;hp=c4f9ca2f307ab6b49e6c1286f1947346d72867f4;hb=ff18deaf720fbe59551a7ff275b09003a61c4351;hpb=4762a35e70fb08622baed5d30157c99c215f4f15

diff --git a/folly/futures/Future-inl.h b/folly/futures/Future-inl.h
index c4f9ca2f..56b98410 100644
--- a/folly/futures/Future-inl.h
+++ b/folly/futures/Future-inl.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 Facebook, Inc.
+ * Copyright 2017-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -13,72 +13,230 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-
 #pragma once
 
 #include <algorithm>
 #include <cassert>
 #include <chrono>
-#include <random>
 #include <thread>
-#include <folly/Baton.h>
+
 #include <folly/Optional.h>
-#include <folly/Random.h>
-#include <folly/Traits.h>
-#include <folly/futures/detail/Core.h>
+#include <folly/executors/InlineExecutor.h>
 #include <folly/futures/Timekeeper.h>
+#include <folly/futures/detail/Core.h>
+#include <folly/synchronization/Baton.h>
 
+#ifndef FOLLY_FUTURE_USING_FIBER
 #if FOLLY_MOBILE || defined(__APPLE__)
 #define FOLLY_FUTURE_USING_FIBER 0
 #else
 #define FOLLY_FUTURE_USING_FIBER 1
 #include <folly/fibers/Baton.h>
 #endif
+#endif
 
 namespace folly {
 
 class Timekeeper;
 
+namespace futures {
 namespace detail {
 #if FOLLY_FUTURE_USING_FIBER
 typedef folly::fibers::Baton FutureBatonType;
 #else
 typedef folly::Baton<> FutureBatonType;
 #endif
-}
+} // namespace detail
+} // namespace futures
 
 namespace detail {
-  std::shared_ptr<Timekeeper> getTimekeeperSingleton();
+std::shared_ptr<Timekeeper> getTimekeeperSingleton();
+} // namespace detail
+
+namespace futures {
+namespace detail {
+//  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 {
+ public:
+  template <typename FF>
+  CoreCallbackState(Promise<T>&& promise, FF&& func) noexcept(
+      noexcept(F(std::declval<FF>())))
+      : func_(std::forward<FF>(func)), promise_(std::move(promise)) {
+    assert(before_barrier());
+  }
+
+  CoreCallbackState(CoreCallbackState&& that) noexcept(
+      noexcept(F(std::declval<F>()))) {
+    if (that.before_barrier()) {
+      new (&func_) F(std::move(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 std::move(func_)(std::forward<Args>(args)...);
+  }
+
+  template <typename... Args>
+  auto tryInvoke(Args&&... args) noexcept {
+    return makeTryWith([&] { return invoke(std::forward<Args>(args)...); });
+  }
+
+  void setTry(Try<T>&& t) {
+    stealPromise().setTry(std::move(t));
+  }
+
+  void setException(exception_wrapper&& ew) {
+    stealPromise().setException(std::move(ew));
+  }
+
+  Promise<T> stealPromise() noexcept {
+    assert(before_barrier());
+    func_.~F();
+    return std::move(promise_);
+  }
+
+ private:
+  bool before_barrier() const noexcept {
+    return !promise_.isFulfilled();
+  }
+
+  union {
+    F func_;
+  };
+  Promise<T> promise_{Promise<T>::makeEmpty()};
+};
+
+template <typename T, typename F>
+inline auto makeCoreCallbackState(Promise<T>&& p, F&& f) noexcept(
+    noexcept(CoreCallbackState<T, _t<std::decay<F>>>(
+        std::declval<Promise<T>&&>(),
+        std::declval<F&&>()))) {
+  return CoreCallbackState<T, _t<std::decay<F>>>(
+      std::move(p), std::forward<F>(f));
 }
 
 template <class T>
-Future<T>::Future(Future<T>&& other) noexcept : core_(other.core_) {
+FutureBase<T>::FutureBase(SemiFuture<T>&& other) noexcept : core_(other.core_) {
   other.core_ = nullptr;
 }
 
 template <class T>
-Future<T>& Future<T>::operator=(Future<T>&& other) noexcept {
-  std::swap(core_, other.core_);
-  return *this;
+FutureBase<T>::FutureBase(Future<T>&& other) noexcept : core_(other.core_) {
+  other.core_ = nullptr;
 }
 
 template <class T>
 template <class T2, typename>
-Future<T>::Future(T2&& val)
-    : core_(new detail::Core<T>(Try<T>(std::forward<T2>(val)))) {}
+FutureBase<T>::FutureBase(T2&& val)
+    : core_(new futures::detail::Core<T>(Try<T>(std::forward<T2>(val)))) {}
 
 template <class T>
 template <typename T2>
-Future<T>::Future(typename std::enable_if<std::is_same<Unit, T2>::value>::type*)
-    : core_(new detail::Core<T>(Try<T>(T()))) {}
+FutureBase<T>::FutureBase(
+    typename std::enable_if<std::is_same<Unit, T2>::value>::type*)
+    : core_(new futures::detail::Core<T>(Try<T>(T()))) {}
+
+template <class T>
+template <
+    class... Args,
+    typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
+        type>
+FutureBase<T>::FutureBase(in_place_t, Args&&... args)
+    : core_(
+          new futures::detail::Core<T>(in_place, std::forward<Args>(args)...)) {
+}
+
+template <class T>
+template <class FutureType>
+void FutureBase<T>::assign(FutureType& other) noexcept {
+  std::swap(core_, other.core_);
+}
 
 template <class T>
-Future<T>::~Future() {
+FutureBase<T>::~FutureBase() {
   detach();
 }
 
 template <class T>
-void Future<T>::detach() {
+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() & {
+  throwIfInvalid();
+
+  return core_->getTry();
+}
+
+template <class T>
+Try<T> const& FutureBase<T>::result() const& {
+  throwIfInvalid();
+
+  return core_->getTry();
+}
+
+template <class T>
+Try<T>&& FutureBase<T>::result() && {
+  throwIfInvalid();
+
+  return std::move(core_->getTry());
+}
+
+template <class T>
+Try<T> const&& FutureBase<T>::result() const&& {
+  throwIfInvalid();
+
+  return std::move(core_->getTry());
+}
+
+template <class T>
+bool FutureBase<T>::isReady() const {
+  throwIfInvalid();
+  return core_->ready();
+}
+
+template <class T>
+bool FutureBase<T>::hasValue() {
+  return core_->getTry().hasValue();
+}
+
+template <class T>
+bool FutureBase<T>::hasException() {
+  return core_->getTry().hasException();
+}
+
+template <class T>
+void FutureBase<T>::detach() {
   if (core_) {
     core_->detachFuture();
     core_ = nullptr;
@@ -86,30 +244,37 @@ void Future<T>::detach() {
 }
 
 template <class T>
-void Future<T>::throwIfInvalid() const {
-  if (!core_)
-    throw NoState();
+void FutureBase<T>::throwIfInvalid() const {
+  if (!core_) {
+    throwNoState();
+  }
 }
 
 template <class T>
-template <class F>
-void Future<T>::setCallback_(F&& func) {
-  throwIfInvalid();
-  core_->setCallback(std::forward<F>(func));
+Optional<Try<T>> FutureBase<T>::poll() {
+  Optional<Try<T>> o;
+  if (core_->ready()) {
+    o = std::move(core_->getTry());
+  }
+  return o;
 }
 
-// unwrap
+template <class T>
+void FutureBase<T>::raise(exception_wrapper exception) {
+  core_->raise(std::move(exception));
+}
 
 template <class T>
 template <class F>
-typename std::enable_if<isFuture<F>::value,
-                        Future<typename isFuture<T>::Inner>>::type
-Future<T>::unwrap() {
-  return then([](Future<typename isFuture<T>::Inner> internal_future) {
-      return internal_future;
-  });
+void FutureBase<T>::setCallback_(F&& func) {
+  throwIfInvalid();
+  core_->setCallback(std::forward<F>(func));
 }
 
+template <class T>
+FutureBase<T>::FutureBase(futures::detail::EmptyConstruct) noexcept
+    : core_(nullptr) {}
+
 // then
 
 // Variant: returns a value
@@ -117,18 +282,20 @@ Future<T>::unwrap() {
 template <class T>
 template <typename F, typename R, bool isTry, typename... Args>
 typename std::enable_if<!R::ReturnsFuture::value, typename R::Return>::type
-Future<T>::thenImplementation(F&& func, detail::argResult<isTry, F, Args...>) {
+FutureBase<T>::thenImplementation(
+    F&& func,
+    futures::detail::argResult<isTry, F, Args...>) {
   static_assert(sizeof...(Args) <= 1, "Then must take zero/one argument");
   typedef typename R::ReturnsFuture::Inner B;
 
-  throwIfInvalid();
+  this->throwIfInvalid();
 
   Promise<B> p;
-  p.core_->setInterruptHandlerNoLock(core_->getInterruptHandler());
+  p.core_->setInterruptHandlerNoLock(this->core_->getInterruptHandler());
 
   // grab the Future now before we lose our handle on the Promise
   auto f = p.getFuture();
-  f.core_->setExecutorNoLock(getExecutor());
+  f.core_->setExecutorNoLock(this->getExecutor());
 
   /* This is a bit tricky.
 
@@ -149,27 +316,27 @@ Future<T>::thenImplementation(F&& func, detail::argResult<isTry, F, Args...>) {
      persist beyond the callback, if it gets moved), and so it is an
      optimization to just make it shared from the get-go.
 
-     Two subtle but important points about this design. detail::Core has no
-     back pointers to Future or Promise, so if Future or Promise get moved
-     (and they will be moved in performant code) we don't have to do
+     Two subtle but important points about this design. futures::detail::Core
+     has no back pointers to Future or Promise, so if Future or Promise get
+     moved (and they will be moved in performant code) we don't have to do
      anything fancy. And because we store the continuation in the
-     detail::Core, not in the Future, we can execute the continuation even
-     after the Future has gone out of scope. This is an intentional design
+     futures::detail::Core, not in the Future, we can execute the continuation
+     even after the Future has gone out of scope. This is an intentional design
      decision. It is likely we will want to be able to cancel a continuation
      in some circumstances, but I think it should be explicit not implicit
      in the destruction of the Future used to create it.
      */
-  setCallback_(
-      [ func = std::forward<F>(func), pm = std::move(p) ](Try<T> && t) mutable {
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T>&& t) mutable {
+
         if (!isTry && t.hasException()) {
-          pm.setException(std::move(t.exception()));
+          state.setException(std::move(t.exception()));
         } else {
-          pm.setWith([&]() {
-            return std::move(func)(t.template get<isTry, Args>()...);
-          });
+          state.setTry(makeTryWith(
+              [&] { return state.invoke(t.template get<isTry, Args>()...); }));
         }
       });
-
   return f;
 }
 
@@ -178,71 +345,311 @@ Future<T>::thenImplementation(F&& func, detail::argResult<isTry, F, Args...>) {
 template <class T>
 template <typename F, typename R, bool isTry, typename... Args>
 typename std::enable_if<R::ReturnsFuture::value, typename R::Return>::type
-Future<T>::thenImplementation(F&& func, detail::argResult<isTry, F, Args...>) {
+FutureBase<T>::thenImplementation(
+    F&& func,
+    futures::detail::argResult<isTry, F, Args...>) {
   static_assert(sizeof...(Args) <= 1, "Then must take zero/one argument");
   typedef typename R::ReturnsFuture::Inner B;
-
-  throwIfInvalid();
+  this->throwIfInvalid();
 
   Promise<B> p;
-  p.core_->setInterruptHandlerNoLock(core_->getInterruptHandler());
+  p.core_->setInterruptHandlerNoLock(this->core_->getInterruptHandler());
 
   // grab the Future now before we lose our handle on the Promise
   auto f = p.getFuture();
-  f.core_->setExecutorNoLock(getExecutor());
+  f.core_->setExecutorNoLock(this->getExecutor());
 
-  setCallback_([ func = std::forward<F>(func), pm = std::move(p) ](
-      Try<T> && t) mutable {
-    auto ew = [&] {
-      if (!isTry && t.hasException()) {
-        return std::move(t.exception());
-      } else {
-        try {
-          auto f2 = std::move(func)(t.template get<isTry, Args>()...);
-          // that didn't throw, now we can steal p
-          f2.setCallback_([p = std::move(pm)](Try<B> && b) mutable {
-            p.setTry(std::move(b));
-          });
-          return exception_wrapper();
-        } catch (const std::exception& e) {
-          return exception_wrapper(std::current_exception(), e);
-        } catch (...) {
-          return exception_wrapper(std::current_exception());
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T>&& t) mutable {
+        if (!isTry && t.hasException()) {
+          state.setException(std::move(t.exception()));
+        } else {
+          auto tf2 = state.tryInvoke(t.template get<isTry, Args>()...);
+          if (tf2.hasException()) {
+            state.setException(std::move(tf2.exception()));
+          } else {
+            tf2->setCallback_([p = state.stealPromise()](Try<B> && b) mutable {
+              p.setTry(std::move(b));
+            });
+          }
         }
-      }
-    }();
-    if (ew) {
-      pm.setException(std::move(ew));
+      });
+
+  return f;
+}
+} // namespace detail
+} // namespace futures
+
+template <class T>
+SemiFuture<typename std::decay<T>::type> makeSemiFuture(T&& t) {
+  return makeSemiFuture(Try<typename std::decay<T>::type>(std::forward<T>(t)));
+}
+
+// makeSemiFutureWith(SemiFuture<T>()) -> SemiFuture<T>
+template <class F>
+typename std::enable_if<
+    isSemiFuture<typename std::result_of<F()>::type>::value,
+    typename std::result_of<F()>::type>::type
+makeSemiFutureWith(F&& func) {
+  using InnerType =
+      typename isSemiFuture<typename std::result_of<F()>::type>::Inner;
+  try {
+    return std::forward<F>(func)();
+  } catch (std::exception& e) {
+    return makeSemiFuture<InnerType>(
+        exception_wrapper(std::current_exception(), e));
+  } catch (...) {
+    return makeSemiFuture<InnerType>(
+        exception_wrapper(std::current_exception()));
+  }
+}
+
+// makeSemiFutureWith(T()) -> SemiFuture<T>
+// makeSemiFutureWith(void()) -> SemiFuture<Unit>
+template <class F>
+typename std::enable_if<
+    !(isSemiFuture<typename std::result_of<F()>::type>::value),
+    SemiFuture<Unit::LiftT<typename std::result_of<F()>::type>>>::type
+makeSemiFutureWith(F&& func) {
+  using LiftedResult = Unit::LiftT<typename std::result_of<F()>::type>;
+  return makeSemiFuture<LiftedResult>(
+      makeTryWith([&func]() mutable { return std::forward<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>(new futures::detail::Core<T>(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>
+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(nullptr);
+  }
+}
+
+template <class T>
+SemiFuture<T>& SemiFuture<T>::operator=(SemiFuture<T>&& other) noexcept {
+  this->assign(other);
+  return *this;
+}
+
+template <class T>
+SemiFuture<T>& SemiFuture<T>::operator=(Future<T>&& other) noexcept {
+  this->assign(other);
+  // SemiFuture should not have an executor on construction
+  if (this->core_) {
+    this->setExecutor(nullptr);
+  }
+  return *this;
+}
+
+template <class T>
+void SemiFuture<T>::boost_() {
+  // If a SemiFuture has an executor it should be deferred, so boost it
+  if (auto e = this->getExecutor()) {
+    // We know in a SemiFuture that if we have an executor it should be
+    // DeferredExecutor. Verify this in debug mode.
+    DCHECK(nullptr != dynamic_cast<DeferredExecutor*>(e));
+
+    auto ka = static_cast<DeferredExecutor*>(e)->getKeepAliveToken();
+    static_cast<DeferredExecutor*>(e)->boost();
+  }
+}
+
+template <class T>
+inline Future<T> SemiFuture<T>::via(Executor* executor, int8_t priority) && {
+  throwIfInvalid();
+  if (!executor) {
+    throwNoExecutor();
+  }
+
+  // If current executor is deferred, boost block to ensure that work
+  // progresses and is run on the new executor.
+  auto oldExecutor = this->getExecutor();
+  if (oldExecutor && executor && (executor != oldExecutor)) {
+    // We know in a SemiFuture that if we have an executor it should be
+    // DeferredExecutor. Verify this in debug mode.
+    DCHECK(nullptr != dynamic_cast<DeferredExecutor*>(this->getExecutor()));
+    if (static_cast<DeferredExecutor*>(oldExecutor)) {
+      executor->add([oldExecutorKA = oldExecutor->getKeepAliveToken()]() {
+        static_cast<DeferredExecutor*>(oldExecutorKA.get())->boost();
+      });
     }
+  }
+
+  this->setExecutor(executor, priority);
+
+  auto newFuture = Future<T>(this->core_);
+  this->core_ = nullptr;
+  return newFuture;
+}
+
+template <class T>
+template <typename F>
+SemiFuture<typename futures::detail::callableResult<T, F>::Return::value_type>
+SemiFuture<T>::defer(F&& func) && {
+  // If we already have a deferred executor, use it, otherwise create one
+  auto defKeepAlive = this->getExecutor()
+      ? this->getExecutor()->getKeepAliveToken()
+      : DeferredExecutor::create();
+  auto e = defKeepAlive.get();
+  // We know in a SemiFuture that if we have an executor it should be
+  // DeferredExecutor (either it was that way before, or we just created it).
+  // Verify this in debug mode.
+  DCHECK(nullptr != dynamic_cast<DeferredExecutor*>(e));
+  // Convert to a folly::future with a deferred executor
+  // Will be low-cost if this is not a new executor as via optimises for that
+  // case
+  auto sf =
+      std::move(*this)
+          .via(e)
+          // Then add the work, with a wrapper function that captures the
+          // keepAlive so the executor is destroyed at the right time.
+          .then(
+              DeferredExecutor::wrap(std::move(defKeepAlive), std::move(func)))
+          // Finally, convert back o a folly::SemiFuture to hide the executor
+          .semi();
+  // Carry deferred executor through chain as constructor from Future will
+  // nullify it
+  sf.setExecutor(e);
+  return sf;
+}
+
+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(other);
+  return *this;
+}
+
+template <class T>
+template <
+    class T2,
+    typename std::enable_if<
+        !std::is_same<T, typename std::decay<T2>::type>::value &&
+            std::is_constructible<T, T2&&>::value &&
+            std::is_convertible<T2&&, T>::value,
+        int>::type>
+Future<T>::Future(Future<T2>&& other)
+    : Future(std::move(other).then([](T2&& v) { return T(std::move(v)); })) {}
+
+template <class T>
+template <
+    class T2,
+    typename std::enable_if<
+        !std::is_same<T, typename std::decay<T2>::type>::value &&
+            std::is_constructible<T, T2&&>::value &&
+            !std::is_convertible<T2&&, T>::value,
+        int>::type>
+Future<T>::Future(Future<T2>&& other)
+    : Future(std::move(other).then([](T2&& v) { return T(std::move(v)); })) {}
+
+template <class T>
+template <
+    class T2,
+    typename std::enable_if<
+        !std::is_same<T, typename std::decay<T2>::type>::value &&
+            std::is_constructible<T, T2&&>::value,
+        int>::type>
+Future<T>& Future<T>::operator=(Future<T2>&& other) {
+  return operator=(
+      std::move(other).then([](T2&& v) { return T(std::move(v)); }));
+}
+
+// unwrap
+
+template <class T>
+template <class F>
+typename std::
+    enable_if<isFuture<F>::value, Future<typename isFuture<T>::Inner>>::type
+    Future<T>::unwrap() {
+  return then([](Future<typename isFuture<T>::Inner> internal_future) {
+    return internal_future;
   });
+}
 
-  return f;
+template <class T>
+inline Future<T> Future<T>::via(Executor* executor, int8_t priority) && {
+  this->throwIfInvalid();
+
+  this->setExecutor(executor, priority);
+
+  auto newFuture = Future<T>(this->core_);
+  this->core_ = nullptr;
+  return newFuture;
+}
+
+template <class T>
+inline Future<T> Future<T>::via(Executor* executor, int8_t priority) & {
+  this->throwIfInvalid();
+  Promise<T> p;
+  auto f = p.getFuture();
+  auto func = [p = std::move(p)](Try<T>&& t) mutable {
+    p.setTry(std::move(t));
+  };
+  using R = futures::detail::callableResult<T, decltype(func)>;
+  this->template thenImplementation<decltype(func), R>(
+      std::move(func), typename R::Arg());
+  return std::move(f).via(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) {
-  typedef typename std::remove_cv<
-    typename std::remove_reference<
-      typename detail::ArgType<Args...>::FirstArg>::type>::type FirstArg;
+  typedef typename std::remove_cv<typename std::remove_reference<
+      typename futures::detail::ArgType<Args...>::FirstArg>::type>::type
+      FirstArg;
+
   return then([instance, func](Try<T>&& t){
     return (instance->*func)(t.template get<isTry<FirstArg>::value, Args>()...);
   });
 }
 
-template <class T>
-template <class Executor, class Arg, class... Args>
-auto Future<T>::then(Executor* x, Arg&& arg, Args&&... args)
-  -> decltype(this->then(std::forward<Arg>(arg),
-                         std::forward<Args>(args)...))
-{
-  auto oldX = getExecutor();
-  setExecutor(x);
-  return this->then(std::forward<Arg>(arg), std::forward<Args>(args)...).
-               via(oldX);
-}
-
 template <class T>
 Future<Unit> Future<T>::then() {
   return then([] () {});
@@ -252,25 +659,29 @@ Future<Unit> Future<T>::then() {
 template <class T>
 template <class F>
 typename std::enable_if<
-  !detail::callableWith<F, exception_wrapper>::value &&
-  !detail::Extract<F>::ReturnsFuture::value,
-  Future<T>>::type
+    !futures::detail::callableWith<F, exception_wrapper>::value &&
+        !futures::detail::callableWith<F, exception_wrapper&>::value &&
+        !futures::detail::Extract<F>::ReturnsFuture::value,
+    Future<T>>::type
 Future<T>::onError(F&& func) {
-  typedef typename detail::Extract<F>::FirstArg Exn;
+  typedef std::remove_reference_t<
+      typename futures::detail::Extract<F>::FirstArg>
+      Exn;
   static_assert(
-      std::is_same<typename detail::Extract<F>::RawReturn, T>::value,
+      std::is_same<typename futures::detail::Extract<F>::RawReturn, T>::value,
       "Return type of onError callback must be T or Future<T>");
 
   Promise<T> p;
-  p.core_->setInterruptHandlerNoLock(core_->getInterruptHandler());
+  p.core_->setInterruptHandlerNoLock(this->core_->getInterruptHandler());
   auto f = p.getFuture();
 
-  setCallback_(
-      [ func = std::forward<F>(func), pm = std::move(p) ](Try<T> && t) mutable {
-        if (!t.template withException<Exn>([&](Exn& e) {
-              pm.setWith([&] { return std::move(func)(e); });
-            })) {
-          pm.setTry(std::move(t));
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T>&& t) mutable {
+        if (auto e = t.template tryGetExceptionObject<Exn>()) {
+          state.setTry(makeTryWith([&] { return state.invoke(*e); }));
+        } else {
+          state.setTry(std::move(t));
         }
       });
 
@@ -281,41 +692,38 @@ Future<T>::onError(F&& func) {
 template <class T>
 template <class F>
 typename std::enable_if<
-  !detail::callableWith<F, exception_wrapper>::value &&
-  detail::Extract<F>::ReturnsFuture::value,
-  Future<T>>::type
+    !futures::detail::callableWith<F, exception_wrapper>::value &&
+        !futures::detail::callableWith<F, exception_wrapper&>::value &&
+        futures::detail::Extract<F>::ReturnsFuture::value,
+    Future<T>>::type
 Future<T>::onError(F&& func) {
   static_assert(
-      std::is_same<typename detail::Extract<F>::Return, Future<T>>::value,
+      std::is_same<typename futures::detail::Extract<F>::Return, Future<T>>::
+          value,
       "Return type of onError callback must be T or Future<T>");
-  typedef typename detail::Extract<F>::FirstArg Exn;
+  typedef std::remove_reference_t<
+      typename futures::detail::Extract<F>::FirstArg>
+      Exn;
 
   Promise<T> p;
   auto f = p.getFuture();
 
-  setCallback_([ pm = std::move(p), func = std::forward<F>(func) ](
-      Try<T> && t) mutable {
-    if (!t.template withException<Exn>([&](Exn& e) {
-          auto ew = [&] {
-            try {
-              auto f2 = std::move(func)(e);
-              f2.setCallback_([pm = std::move(pm)](Try<T> && t2) mutable {
-                pm.setTry(std::move(t2));
-              });
-              return exception_wrapper();
-            } catch (const std::exception& e2) {
-              return exception_wrapper(std::current_exception(), e2);
-            } catch (...) {
-              return exception_wrapper(std::current_exception());
-            }
-          }();
-          if (ew) {
-            pm.setException(std::move(ew));
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T>&& t) mutable {
+        if (auto e = t.template tryGetExceptionObject<Exn>()) {
+          auto tf2 = state.tryInvoke(*e);
+          if (tf2.hasException()) {
+            state.setException(std::move(tf2.exception()));
+          } else {
+            tf2->setCallback_([p = state.stealPromise()](Try<T> && t3) mutable {
+              p.setTry(std::move(t3));
+            });
           }
-        })) {
-      pm.setTry(std::move(t));
-    }
-  });
+        } else {
+          state.setTry(std::move(t));
+        }
+      });
 
   return f;
 }
@@ -324,7 +732,7 @@ template <class T>
 template <class F>
 Future<T> Future<T>::ensure(F&& func) {
   return this->then([funcw = std::forward<F>(func)](Try<T> && t) mutable {
-    funcw();
+    std::move(funcw)();
     return makeFuture(std::move(t));
   });
 }
@@ -333,42 +741,37 @@ template <class T>
 template <class F>
 Future<T> Future<T>::onTimeout(Duration dur, F&& func, Timekeeper* tk) {
   return within(dur, tk).onError([funcw = std::forward<F>(func)](
-      TimedOut const&) { return funcw(); });
+      TimedOut const&) { return std::move(funcw)(); });
 }
 
 template <class T>
 template <class F>
-typename std::enable_if<detail::callableWith<F, exception_wrapper>::value &&
-                            detail::Extract<F>::ReturnsFuture::value,
-                        Future<T>>::type
+typename std::enable_if<
+    futures::detail::callableWith<F, exception_wrapper>::value &&
+        futures::detail::Extract<F>::ReturnsFuture::value,
+    Future<T>>::type
 Future<T>::onError(F&& func) {
   static_assert(
-      std::is_same<typename detail::Extract<F>::Return, Future<T>>::value,
+      std::is_same<typename futures::detail::Extract<F>::Return, Future<T>>::
+          value,
       "Return type of onError callback must be T or Future<T>");
 
   Promise<T> p;
   auto f = p.getFuture();
-  setCallback_(
-      [ pm = std::move(p), func = std::forward<F>(func) ](Try<T> t) mutable {
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T> t) mutable {
         if (t.hasException()) {
-          auto ew = [&] {
-            try {
-              auto f2 = std::move(func)(std::move(t.exception()));
-              f2.setCallback_([pm = std::move(pm)](Try<T> t2) mutable {
-                pm.setTry(std::move(t2));
-              });
-              return exception_wrapper();
-            } catch (const std::exception& e2) {
-              return exception_wrapper(std::current_exception(), e2);
-            } catch (...) {
-              return exception_wrapper(std::current_exception());
-            }
-          }();
-          if (ew) {
-            pm.setException(std::move(ew));
+          auto tf2 = state.tryInvoke(std::move(t.exception()));
+          if (tf2.hasException()) {
+            state.setException(std::move(tf2.exception()));
+          } else {
+            tf2->setCallback_([p = state.stealPromise()](Try<T> && t3) mutable {
+              p.setTry(std::move(t3));
+            });
           }
         } else {
-          pm.setTry(std::move(t));
+          state.setTry(std::move(t));
         }
       });
 
@@ -379,111 +782,38 @@ Future<T>::onError(F&& func) {
 template <class T>
 template <class F>
 typename std::enable_if<
-  detail::callableWith<F, exception_wrapper>::value &&
-  !detail::Extract<F>::ReturnsFuture::value,
-  Future<T>>::type
+    futures::detail::callableWith<F, exception_wrapper>::value &&
+        !futures::detail::Extract<F>::ReturnsFuture::value,
+    Future<T>>::type
 Future<T>::onError(F&& func) {
   static_assert(
-      std::is_same<typename detail::Extract<F>::Return, Future<T>>::value,
+      std::is_same<typename futures::detail::Extract<F>::Return, Future<T>>::
+          value,
       "Return type of onError callback must be T or Future<T>");
 
   Promise<T> p;
   auto f = p.getFuture();
-  setCallback_(
-      [ pm = std::move(p), func = std::forward<F>(func) ](Try<T> t) mutable {
+  this->setCallback_(
+      [state = futures::detail::makeCoreCallbackState(
+           std::move(p), std::forward<F>(func))](Try<T>&& t) mutable {
         if (t.hasException()) {
-          pm.setWith([&] { return std::move(func)(std::move(t.exception())); });
+          state.setTry(makeTryWith(
+              [&] { return state.invoke(std::move(t.exception())); }));
         } else {
-          pm.setTry(std::move(t));
+          state.setTry(std::move(t));
         }
       });
 
   return f;
 }
 
-template <class T>
-typename std::add_lvalue_reference<T>::type Future<T>::value() {
-  throwIfInvalid();
-
-  return core_->getTry().value();
-}
-
-template <class T>
-typename std::add_lvalue_reference<const T>::type Future<T>::value() const {
-  throwIfInvalid();
-
-  return core_->getTry().value();
-}
-
-template <class T>
-Try<T>& Future<T>::getTry() {
-  throwIfInvalid();
-
-  return core_->getTry();
-}
-
-template <class T>
-Try<T>& Future<T>::getTryVia(DrivableExecutor* e) {
-  return waitVia(e).getTry();
-}
-
-template <class T>
-Optional<Try<T>> Future<T>::poll() {
-  Optional<Try<T>> o;
-  if (core_->ready()) {
-    o = std::move(core_->getTry());
-  }
-  return o;
-}
-
-template <class T>
-inline Future<T> Future<T>::via(Executor* executor, int8_t priority) && {
-  throwIfInvalid();
-
-  setExecutor(executor, priority);
-
-  return std::move(*this);
-}
-
-template <class T>
-inline Future<T> Future<T>::via(Executor* executor, int8_t priority) & {
-  throwIfInvalid();
-
-  Promise<T> p;
-  auto f = p.getFuture();
-  then([p = std::move(p)](Try<T> && t) mutable { p.setTry(std::move(t)); });
-  return std::move(f).via(executor, priority);
-}
-
 template <class Func>
 auto via(Executor* x, Func&& func)
-  -> Future<typename isFuture<decltype(func())>::Inner>
-{
+    -> Future<typename isFuture<decltype(std::declval<Func>()())>::Inner> {
   // TODO make this actually more performant. :-P #7260175
   return via(x).then(std::forward<Func>(func));
 }
 
-template <class T>
-bool Future<T>::isReady() const {
-  throwIfInvalid();
-  return core_->ready();
-}
-
-template <class T>
-bool Future<T>::hasValue() {
-  return getTry().hasValue();
-}
-
-template <class T>
-bool Future<T>::hasException() {
-  return getTry().hasException();
-}
-
-template <class T>
-void Future<T>::raise(exception_wrapper exception) {
-  core_->raise(std::move(exception));
-}
-
 // makeFuture
 
 template <class T>
@@ -491,8 +821,7 @@ Future<typename std::decay<T>::type> makeFuture(T&& t) {
   return makeFuture(Try<typename std::decay<T>::type>(std::forward<T>(t)));
 }
 
-inline // for multiple translation units
-Future<Unit> makeFuture() {
+inline Future<Unit> makeFuture() {
   return makeFuture(Unit{});
 }
 
@@ -504,7 +833,7 @@ makeFutureWith(F&& func) {
   using InnerType =
       typename isFuture<typename std::result_of<F()>::type>::Inner;
   try {
-    return func();
+    return std::forward<F>(func)();
   } catch (std::exception& e) {
     return makeFuture<InnerType>(
         exception_wrapper(std::current_exception(), e));
@@ -518,13 +847,11 @@ makeFutureWith(F&& func) {
 template <class F>
 typename std::enable_if<
     !(isFuture<typename std::result_of<F()>::type>::value),
-    Future<typename Unit::Lift<typename std::result_of<F()>::type>::type>>::type
+    Future<Unit::LiftT<typename std::result_of<F()>::type>>>::type
 makeFutureWith(F&& func) {
-  using LiftedResult =
-      typename Unit::Lift<typename std::result_of<F()>::type>::type;
-  return makeFuture<LiftedResult>(makeTryWith([&func]() mutable {
-    return func();
-  }));
+  using LiftedResult = Unit::LiftT<typename std::result_of<F()>::type>;
+  return makeFuture<LiftedResult>(
+      makeTryWith([&func]() mutable { return std::forward<F>(func)(); }));
 }
 
 template <class T>
@@ -546,7 +873,7 @@ makeFuture(E const& e) {
 
 template <class T>
 Future<T> makeFuture(Try<T>&& t) {
-  return Future<T>(new detail::Core<T>(std::move(t)));
+  return Future<T>(new futures::detail::Core<T>(std::move(t)));
 }
 
 // via
@@ -568,13 +895,13 @@ void mapSetCallback(InputIterator first, InputIterator last, F func) {
 // collectAll (variadic)
 
 template <typename... Fs>
-typename detail::CollectAllVariadicContext<
-  typename std::decay<Fs>::type::value_type...>::type
+typename futures::detail::CollectAllVariadicContext<
+    typename std::decay<Fs>::type::value_type...>::type
 collectAll(Fs&&... fs) {
-  auto ctx = std::make_shared<detail::CollectAllVariadicContext<
-    typename std::decay<Fs>::type::value_type...>>();
-  detail::collectVariadicHelper<detail::CollectAllVariadicContext>(
-    ctx, std::forward<typename std::decay<Fs>::type>(fs)...);
+  auto ctx = std::make_shared<futures::detail::CollectAllVariadicContext<
+      typename std::decay<Fs>::type::value_type...>>();
+  futures::detail::collectVariadicHelper<
+      futures::detail::CollectAllVariadicContext>(ctx, std::forward<Fs>(fs)...);
   return ctx->p.getFuture();
 }
 
@@ -607,6 +934,7 @@ collectAll(InputIterator first, InputIterator last) {
 
 // collect (iterator)
 
+namespace futures {
 namespace detail {
 
 template <typename T>
@@ -645,17 +973,18 @@ struct CollectContext {
   std::atomic<bool> threw {false};
 };
 
-}
+} // namespace detail
+} // namespace futures
 
 template <class InputIterator>
-Future<typename detail::CollectContext<
-  typename std::iterator_traits<InputIterator>::value_type::value_type>::Result>
+Future<typename futures::detail::CollectContext<typename std::iterator_traits<
+    InputIterator>::value_type::value_type>::Result>
 collect(InputIterator first, InputIterator last) {
   typedef
     typename std::iterator_traits<InputIterator>::value_type::value_type T;
 
-  auto ctx = std::make_shared<detail::CollectContext<T>>(
-    std::distance(first, last));
+  auto ctx = std::make_shared<futures::detail::CollectContext<T>>(
+      std::distance(first, last));
   mapSetCallback<T>(first, last, [ctx](size_t i, Try<T>&& t) {
     if (t.hasException()) {
        if (!ctx->threw.exchange(true)) {
@@ -671,13 +1000,13 @@ collect(InputIterator first, InputIterator last) {
 // collect (variadic)
 
 template <typename... Fs>
-typename detail::CollectVariadicContext<
-  typename std::decay<Fs>::type::value_type...>::type
+typename futures::detail::CollectVariadicContext<
+    typename std::decay<Fs>::type::value_type...>::type
 collect(Fs&&... fs) {
-  auto ctx = std::make_shared<detail::CollectVariadicContext<
-    typename std::decay<Fs>::type::value_type...>>();
-  detail::collectVariadicHelper<detail::CollectVariadicContext>(
-    ctx, std::forward<typename std::decay<Fs>::type>(fs)...);
+  auto ctx = std::make_shared<futures::detail::CollectVariadicContext<
+      typename std::decay<Fs>::type::value_type...>>();
+  futures::detail::collectVariadicHelper<
+      futures::detail::CollectVariadicContext>(ctx, std::forward<Fs>(fs)...);
   return ctx->p.getFuture();
 }
 
@@ -786,10 +1115,10 @@ Future<T> reduce(It first, It last, T&& initial, F&& func) {
   }
 
   typedef typename std::iterator_traits<It>::value_type::value_type ItT;
-  typedef
-      typename std::conditional<detail::callableWith<F, T&&, Try<ItT>&&>::value,
-                                Try<ItT>,
-                                ItT>::type Arg;
+  typedef typename std::conditional<
+      futures::detail::callableWith<F, T&&, Try<ItT>&&>::value,
+      Try<ItT>,
+      ItT>::type Arg;
   typedef isTry<Arg> IsTry;
 
   auto sfunc = std::make_shared<F>(std::move(func));
@@ -817,27 +1146,38 @@ Future<T> reduce(It first, It last, T&& initial, F&& func) {
 template <class Collection, class F, class ItT, class Result>
 std::vector<Future<Result>>
 window(Collection input, F func, size_t n) {
-  struct WindowContext {
-    WindowContext(Collection&& i, F&& fn)
-        : input_(std::move(i)), promises_(input_.size()),
-          func_(std::move(fn))
-      {}
-    std::atomic<size_t> i_ {0};
-    Collection input_;
-    std::vector<Promise<Result>> promises_;
-    F func_;
+  // Use global inline executor singleton
+  auto executor = &InlineExecutor::instance();
+  return window(executor, std::move(input), std::move(func), n);
+}
 
-    static inline void spawn(const std::shared_ptr<WindowContext>& ctx) {
-      size_t i = ctx->i_++;
-      if (i < ctx->input_.size()) {
-        // Using setCallback_ directly since we don't need the Future
-        ctx->func_(std::move(ctx->input_[i])).setCallback_(
-          // ctx is captured by value
-          [ctx, i](Try<Result>&& t) {
-            ctx->promises_[i].setTry(std::move(t));
+template <class Collection, class F, class ItT, class Result>
+std::vector<Future<Result>>
+window(Executor* executor, Collection input, F func, size_t n) {
+  struct WindowContext {
+    WindowContext(Executor* executor_, Collection&& input_, F&& func_)
+        : executor(executor_),
+          input(std::move(input_)),
+          promises(input.size()),
+          func(std::move(func_)) {}
+    std::atomic<size_t> i{0};
+    Executor* executor;
+    Collection input;
+    std::vector<Promise<Result>> promises;
+    F func;
+
+    static inline void spawn(std::shared_ptr<WindowContext> ctx) {
+      size_t i = ctx->i++;
+      if (i < ctx->input.size()) {
+        auto fut = ctx->func(std::move(ctx->input[i]));
+        fut.setCallback_([ctx = std::move(ctx), i](Try<Result>&& t) mutable {
+          const auto executor_ = ctx->executor;
+          executor_->add([ctx = std::move(ctx), i, t = std::move(t)]() mutable {
+            ctx->promises[i].setTry(std::move(t));
             // Chain another future onto this one
             spawn(std::move(ctx));
           });
+        });
       }
     }
   };
@@ -845,16 +1185,16 @@ window(Collection input, F func, size_t n) {
   auto max = std::min(n, input.size());
 
   auto ctx = std::make_shared<WindowContext>(
-    std::move(input), std::move(func));
+      executor, std::move(input), std::move(func));
 
+  // Start the first n Futures
   for (size_t i = 0; i < max; ++i) {
-    // Start the first n Futures
-    WindowContext::spawn(ctx);
+    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.reserve(ctx->promises.size());
+  for (auto& promise : ctx->promises) {
     futures.emplace_back(promise.getFuture());
   }
 
@@ -950,34 +1290,48 @@ Future<T> Future<T>::within(Duration dur, E e, Timekeeper* tk) {
     std::atomic<bool> token {false};
   };
 
+  if (this->isReady()) {
+    return std::move(*this);
+  }
+
   std::shared_ptr<Timekeeper> tks;
-  if (!tk) {
+  if (LIKELY(!tk)) {
     tks = folly::detail::getTimekeeperSingleton();
-    tk = DCHECK_NOTNULL(tks.get());
+    tk = tks.get();
+  }
+
+  if (UNLIKELY(!tk)) {
+    return makeFuture<T>(NoTimekeeper());
   }
 
   auto ctx = std::make_shared<Context>(std::move(e));
 
   ctx->thisFuture = this->then([ctx](Try<T>&& t) mutable {
-    // TODO: "this" completed first, cancel "after"
     if (ctx->token.exchange(true) == false) {
       ctx->promise.setTry(std::move(t));
     }
   });
 
-  tk->after(dur).then([ctx](Try<Unit> const& t) mutable {
+  // Have time keeper use a weak ptr to hold ctx,
+  // so that ctx can be deallocated as soon as the future job finished.
+  tk->after(dur).then([weakCtx = to_weak_ptr(ctx)](Try<Unit> const& t) mutable {
+    auto lockedCtx = weakCtx.lock();
+    if (!lockedCtx) {
+      // ctx already released. "this" completed first, cancel "after"
+      return;
+    }
     // "after" completed first, cancel "this"
-    ctx->thisFuture.raise(TimedOut());
-    if (ctx->token.exchange(true) == false) {
+    lockedCtx->thisFuture.raise(TimedOut());
+    if (lockedCtx->token.exchange(true) == false) {
       if (t.hasException()) {
-        ctx->promise.setException(std::move(t.exception()));
+        lockedCtx->promise.setException(std::move(t.exception()));
       } else {
-        ctx->promise.setException(std::move(ctx->exception));
+        lockedCtx->promise.setException(std::move(lockedCtx->exception));
       }
     }
   });
 
-  return ctx->promise.getFuture().via(getExecutor());
+  return ctx->promise.getFuture().via(this->getExecutor());
 }
 
 // delayed
@@ -985,27 +1339,39 @@ Future<T> Future<T>::within(Duration dur, E e, Timekeeper* tk) {
 template <class T>
 Future<T> Future<T>::delayed(Duration dur, Timekeeper* tk) {
   return collectAll(*this, futures::sleep(dur, tk))
-    .then([](std::tuple<Try<T>, Try<Unit>> tup) {
-      Try<T>& t = std::get<0>(tup);
-      return makeFuture<T>(std::move(t));
-    });
+      .then([](std::tuple<Try<T>, Try<Unit>> tup) {
+        Try<T>& t = std::get<0>(tup);
+        return makeFuture<T>(std::move(t));
+      });
 }
 
+namespace futures {
 namespace detail {
 
 template <class T>
-void waitImpl(Future<T>& f) {
+void doBoost(folly::Future<T>& /* usused */) {}
+
+template <class T>
+void doBoost(folly::SemiFuture<T>& f) {
+  f.boost_();
+}
+
+template <class FutureType, typename T = typename FutureType::value_type>
+void waitImpl(FutureType& f) {
   // short-circuit if there's nothing to do
-  if (f.isReady()) return;
+  if (f.isReady()) {
+    return;
+  }
 
   FutureBatonType baton;
   f.setCallback_([&](const Try<T>& /* t */) { baton.post(); });
+  doBoost(f);
   baton.wait();
   assert(f.isReady());
 }
 
-template <class T>
-void waitImpl(Future<T>& f, Duration dur) {
+template <class FutureType, typename T = typename FutureType::value_type>
+void waitImpl(FutureType& f, Duration dur) {
   // short-circuit if there's nothing to do
   if (f.isReady()) {
     return;
@@ -1014,12 +1380,13 @@ void waitImpl(Future<T>& f, Duration dur) {
   Promise<T> promise;
   auto ret = promise.getFuture();
   auto baton = std::make_shared<FutureBatonType>();
-  f.setCallback_([ baton, promise = std::move(promise) ](Try<T> && t) mutable {
+  f.setCallback_([baton, promise = std::move(promise)](Try<T>&& t) mutable {
     promise.setTry(std::move(t));
     baton->post();
   });
+  doBoost(f);
   f = std::move(ret);
-  if (baton->timed_wait(dur)) {
+  if (baton->try_wait_for(dur)) {
     assert(f.isReady());
   }
 }
@@ -1029,8 +1396,9 @@ 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())
+  if (f.isReady()) {
     return;
+  }
   f = f.via(e).then([](T&& t) { return std::move(t); });
   while (!f.isReady()) {
     e->drive();
@@ -1038,41 +1406,133 @@ void waitViaImpl(Future<T>& f, DrivableExecutor* e) {
   assert(f.isReady());
 }
 
-} // detail
+template <class T>
+void waitViaImpl(SemiFuture<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).then([](T&& t) { return std::move(t); });
+  while (!f.isReady()) {
+    e->drive();
+  }
+  assert(f.isReady());
+}
+
+} // namespace detail
+} // namespace futures
+
+template <class T>
+SemiFuture<T>& SemiFuture<T>::wait() & {
+  futures::detail::waitImpl(*this);
+  return *this;
+}
+
+template <class T>
+SemiFuture<T>&& SemiFuture<T>::wait() && {
+  futures::detail::waitImpl(*this);
+  return std::move(*this);
+}
+
+template <class T>
+SemiFuture<T>& SemiFuture<T>::wait(Duration dur) & {
+  futures::detail::waitImpl(*this, dur);
+  return *this;
+}
+
+template <class T>
+SemiFuture<T>&& SemiFuture<T>::wait(Duration dur) && {
+  futures::detail::waitImpl(*this, dur);
+  return std::move(*this);
+}
+
+template <class T>
+SemiFuture<T>& SemiFuture<T>::waitVia(DrivableExecutor* e) & {
+  futures::detail::waitViaImpl(*this, e);
+  return *this;
+}
+
+template <class T>
+SemiFuture<T>&& SemiFuture<T>::waitVia(DrivableExecutor* e) && {
+  futures::detail::waitViaImpl(*this, e);
+  return std::move(*this);
+}
+
+template <class T>
+T SemiFuture<T>::get() && {
+  return std::move(wait()).value();
+}
+
+template <class T>
+T SemiFuture<T>::get(Duration dur) && {
+  wait(dur);
+  if (this->isReady()) {
+    return std::move(this->value());
+  } else {
+    throwTimedOut();
+  }
+}
+
+template <class T>
+Try<T> SemiFuture<T>::getTry() && {
+  return std::move(wait()).result();
+}
+
+template <class T>
+Try<T> SemiFuture<T>::getTry(Duration dur) && {
+  wait(dur);
+  if (this->isReady()) {
+    return std::move(this->result());
+  } else {
+    throwTimedOut();
+  }
+}
+
+template <class T>
+T SemiFuture<T>::getVia(DrivableExecutor* e) && {
+  return std::move(waitVia(e)).value();
+}
+
+template <class T>
+Try<T> SemiFuture<T>::getTryVia(DrivableExecutor* e) && {
+  return std::move(waitVia(e)).result();
+}
 
 template <class T>
 Future<T>& Future<T>::wait() & {
-  detail::waitImpl(*this);
+  futures::detail::waitImpl(*this);
   return *this;
 }
 
 template <class T>
 Future<T>&& Future<T>::wait() && {
-  detail::waitImpl(*this);
+  futures::detail::waitImpl(*this);
   return std::move(*this);
 }
 
 template <class T>
 Future<T>& Future<T>::wait(Duration dur) & {
-  detail::waitImpl(*this, dur);
+  futures::detail::waitImpl(*this, dur);
   return *this;
 }
 
 template <class T>
 Future<T>&& Future<T>::wait(Duration dur) && {
-  detail::waitImpl(*this, dur);
+  futures::detail::waitImpl(*this, dur);
   return std::move(*this);
 }
 
 template <class T>
 Future<T>& Future<T>::waitVia(DrivableExecutor* e) & {
-  detail::waitViaImpl(*this, e);
+  futures::detail::waitViaImpl(*this, e);
   return *this;
 }
 
 template <class T>
 Future<T>&& Future<T>::waitVia(DrivableExecutor* e) && {
-  detail::waitViaImpl(*this, e);
+  futures::detail::waitViaImpl(*this, e);
   return std::move(*this);
 }
 
@@ -1084,32 +1544,45 @@ T Future<T>::get() {
 template <class T>
 T Future<T>::get(Duration dur) {
   wait(dur);
-  if (isReady()) {
-    return std::move(value());
+  if (this->isReady()) {
+    return std::move(this->value());
   } else {
-    throw TimedOut();
+    throwTimedOut();
   }
 }
 
+template <class T>
+Try<T>& Future<T>::getTry() {
+  return result();
+}
+
 template <class T>
 T Future<T>::getVia(DrivableExecutor* e) {
   return std::move(waitVia(e).value());
 }
 
-namespace detail {
-  template <class T>
-  struct TryEquals {
-    static bool equals(const Try<T>& t1, const Try<T>& t2) {
-      return t1.value() == t2.value();
-    }
-  };
+template <class T>
+Try<T>& Future<T>::getTryVia(DrivableExecutor* e) {
+  return waitVia(e).getTry();
 }
 
+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 collectAll(*this, f).then([](const std::tuple<Try<T>, Try<T>>& t) {
     if (std::get<0>(t).hasValue() && std::get<1>(t).hasValue()) {
-      return detail::TryEquals<T>::equals(std::get<0>(t), std::get<1>(t));
+      return futures::detail::TryEquals<T>::equals(
+          std::get<0>(t), std::get<1>(t));
     } else {
       return false;
       }
@@ -1122,52 +1595,12 @@ Future<T> Future<T>::filter(F&& predicate) {
   return this->then([p = std::forward<F>(predicate)](T val) {
     T const& valConstRef = val;
     if (!p(valConstRef)) {
-      throw PredicateDoesNotObtain();
+      throwPredicateDoesNotObtain();
     }
     return val;
   });
 }
 
-template <class T>
-template <class Callback>
-auto Future<T>::thenMulti(Callback&& fn)
-    -> decltype(this->then(std::forward<Callback>(fn))) {
-  // thenMulti with one callback is just a then
-  return then(std::forward<Callback>(fn));
-}
-
-template <class T>
-template <class Callback, class... Callbacks>
-auto Future<T>::thenMulti(Callback&& fn, Callbacks&&... fns)
-    -> decltype(this->then(std::forward<Callback>(fn)).
-                      thenMulti(std::forward<Callbacks>(fns)...)) {
-  // thenMulti with two callbacks is just then(a).thenMulti(b, ...)
-  return then(std::forward<Callback>(fn)).
-         thenMulti(std::forward<Callbacks>(fns)...);
-}
-
-template <class T>
-template <class Callback, class... Callbacks>
-auto Future<T>::thenMultiWithExecutor(Executor* x, Callback&& fn,
-                                      Callbacks&&... fns)
-    -> decltype(this->then(std::forward<Callback>(fn)).
-                      thenMulti(std::forward<Callbacks>(fns)...)) {
-  // thenMultiExecutor with two callbacks is
-  // via(x).then(a).thenMulti(b, ...).via(oldX)
-  auto oldX = getExecutor();
-  setExecutor(x);
-  return then(std::forward<Callback>(fn)).
-         thenMulti(std::forward<Callbacks>(fns)...).via(oldX);
-}
-
-template <class T>
-template <class Callback>
-auto Future<T>::thenMultiWithExecutor(Executor* x, Callback&& fn)
-    -> decltype(this->then(std::forward<Callback>(fn))) {
-  // thenMulti with one callback is just a then with an executor
-  return then(x, std::forward<Callback>(fn));
-}
-
 template <class F>
 inline Future<Unit> when(bool p, F&& thunk) {
   return p ? std::forward<F>(thunk)().unit() : makeFuture();
@@ -1197,241 +1630,15 @@ Future<Unit> times(const int n, F&& thunk) {
 }
 
 namespace futures {
-  template <class It, class F, class ItT, class Result>
-  std::vector<Future<Result>> map(It first, It last, F func) {
-    std::vector<Future<Result>> results;
-    for (auto it = first; it != last; it++) {
-      results.push_back(it->then(func));
-    }
-    return results;
+template <class It, class F, class ItT, class Result>
+std::vector<Future<Result>> map(It first, It last, F func) {
+  std::vector<Future<Result>> results;
+  for (auto it = first; it != last; it++) {
+    results.push_back(it->then(func));
   }
+  return results;
 }
-
-namespace futures {
-
-namespace detail {
-
-struct retrying_policy_raw_tag {};
-struct retrying_policy_fut_tag {};
-
-template <class Policy>
-struct retrying_policy_traits {
-  using ew = exception_wrapper;
-  FOLLY_CREATE_HAS_MEMBER_FN_TRAITS(has_op_call, operator());
-  template <class Ret>
-  using has_op = typename std::integral_constant<bool,
-        has_op_call<Policy, Ret(size_t, const ew&)>::value ||
-        has_op_call<Policy, Ret(size_t, const ew&) const>::value>;
-  using is_raw = has_op<bool>;
-  using is_fut = has_op<Future<bool>>;
-  using tag = typename std::conditional<
-        is_raw::value, retrying_policy_raw_tag, typename std::conditional<
-        is_fut::value, retrying_policy_fut_tag, void>::type>::type;
-};
-
-template <class Policy, class FF, class Prom>
-void retryingImpl(size_t k, Policy&& p, FF&& ff, Prom prom) {
-  using F = typename std::result_of<FF(size_t)>::type;
-  using T = typename F::value_type;
-  auto f = makeFutureWith([&] { return ff(k++); });
-  f.then([
-    k,
-    prom = std::move(prom),
-    pm = std::forward<Policy>(p),
-    ffm = std::forward<FF>(ff)
-  ](Try<T> && t) mutable {
-    if (t.hasValue()) {
-      prom.setValue(std::move(t).value());
-      return;
-    }
-    auto& x = t.exception();
-    auto q = pm(k, x);
-    q.then([
-      k,
-      prom = std::move(prom),
-      xm = std::move(x),
-      pm = std::move(pm),
-      ffm = std::move(ffm)
-    ](bool shouldRetry) mutable {
-      if (shouldRetry) {
-        retryingImpl(k, std::move(pm), std::move(ffm), std::move(prom));
-      } else {
-        prom.setException(std::move(xm));
-      };
-    });
-  });
-}
-
-template <class Policy, class FF>
-typename std::result_of<FF(size_t)>::type
-retrying(size_t k, Policy&& p, FF&& ff) {
-  using F = typename std::result_of<FF(size_t)>::type;
-  using T = typename F::value_type;
-  auto prom = Promise<T>();
-  auto f = prom.getFuture();
-  retryingImpl(
-      k, std::forward<Policy>(p), std::forward<FF>(ff), std::move(prom));
-  return f;
-}
-
-template <class Policy, class FF>
-typename std::result_of<FF(size_t)>::type
-retrying(Policy&& p, FF&& ff, retrying_policy_raw_tag) {
-  auto q = [pm = std::forward<Policy>(p)](size_t k, exception_wrapper x) {
-    return makeFuture<bool>(pm(k, x));
-  };
-  return retrying(0, std::move(q), std::forward<FF>(ff));
-}
-
-template <class Policy, class FF>
-typename std::result_of<FF(size_t)>::type
-retrying(Policy&& p, FF&& ff, retrying_policy_fut_tag) {
-  return retrying(0, std::forward<Policy>(p), std::forward<FF>(ff));
-}
-
-//  jittered exponential backoff, clamped to [backoff_min, backoff_max]
-template <class URNG>
-Duration retryingJitteredExponentialBackoffDur(
-    size_t n,
-    Duration backoff_min,
-    Duration backoff_max,
-    double jitter_param,
-    URNG& rng) {
-  using d = Duration;
-  auto dist = std::normal_distribution<double>(0.0, jitter_param);
-  auto jitter = std::exp(dist(rng));
-  auto backoff = d(d::rep(jitter * backoff_min.count() * std::pow(2, n - 1)));
-  return std::max(backoff_min, std::min(backoff_max, backoff));
-}
-
-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) {
-  return [
-    pm = std::forward<Policy>(p),
-    max_tries,
-    backoff_min,
-    backoff_max,
-    jitter_param,
-    rngp = std::forward<URNG>(rng)
-  ](size_t n, const exception_wrapper& ex) mutable {
-    if (n == max_tries) {
-      return makeFuture(false);
-    }
-    return pm(n, ex).then(
-        [ n, backoff_min, backoff_max, jitter_param, rngp = std::move(rngp) ](
-            bool v) mutable {
-          if (!v) {
-            return makeFuture(false);
-          }
-          auto backoff = detail::retryingJitteredExponentialBackoffDur(
-              n, backoff_min, backoff_max, jitter_param, rngp);
-          return futures::sleep(backoff).then([] { return true; });
-        });
-  };
-}
-
-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,
-    retrying_policy_raw_tag) {
-  auto q = [pm = std::forward<Policy>(p)](
-      size_t n, const exception_wrapper& e) {
-    return makeFuture(pm(n, e));
-  };
-  return retryingPolicyCappedJitteredExponentialBackoff(
-      max_tries,
-      backoff_min,
-      backoff_max,
-      jitter_param,
-      std::forward<URNG>(rng),
-      std::move(q));
-}
-
-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,
-    retrying_policy_fut_tag) {
-  return retryingPolicyCappedJitteredExponentialBackoff(
-      max_tries,
-      backoff_min,
-      backoff_max,
-      jitter_param,
-      std::forward<URNG>(rng),
-      std::forward<Policy>(p));
-}
-}
-
-template <class Policy, class FF>
-typename std::result_of<FF(size_t)>::type
-retrying(Policy&& p, FF&& ff) {
-  using tag = typename detail::retrying_policy_traits<Policy>::tag;
-  return detail::retrying(std::forward<Policy>(p), std::forward<FF>(ff), tag());
-}
-
-inline
-std::function<bool(size_t, const exception_wrapper&)>
-retryingPolicyBasic(
-    size_t max_tries) {
-  return [=](size_t n, const exception_wrapper&) { return n < 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) {
-  using tag = typename detail::retrying_policy_traits<Policy>::tag;
-  return detail::retryingPolicyCappedJitteredExponentialBackoff(
-      max_tries,
-      backoff_min,
-      backoff_max,
-      jitter_param,
-      std::forward<URNG>(rng),
-      std::forward<Policy>(p),
-      tag());
-}
-
-inline
-std::function<Future<bool>(size_t, const exception_wrapper&)>
-retryingPolicyCappedJitteredExponentialBackoff(
-    size_t max_tries,
-    Duration backoff_min,
-    Duration backoff_max,
-    double jitter_param) {
-  auto p = [](size_t, const exception_wrapper&) { return true; };
-  return retryingPolicyCappedJitteredExponentialBackoff(
-      max_tries,
-      backoff_min,
-      backoff_max,
-      jitter_param,
-      ThreadLocalPRNG(),
-      std::move(p));
-}
-
-}
+} // namespace futures
 
 // Instantiate the most common Future types to save compile time
 extern template class Future<Unit>;
@@ -1440,5 +1647,4 @@ extern template class Future<int>;
 extern template class Future<int64_t>;
 extern template class Future<std::string>;
 extern template class Future<double>;
-
 } // namespace folly