Add unit() method to SemiFuture

Reviewed By: yfeldblum

Differential Revision: D17086382

fbshipit-source-id: 318834a1d82025bc0c34bcf7a95fedfffdb02ddb

folly/futures/Future-inl.h

@@ -868,6 +868,11 @@ SemiFuture<T> SemiFuture<T>::deferError(F&& func) && {
       });
 }
 
+template <class T>
+SemiFuture<Unit> SemiFuture<T>::unit() && {
+  return std::move(*this).deferValue([](T&&) {});
+}
+
 template <typename T>
 SemiFuture<T> SemiFuture<T>::delayed(Duration dur, Timekeeper* tk) && {
   return collectAllSemiFuture(*this, futures::sleep(dur, tk))

folly/futures/Future.h

@@ -791,6 +791,20 @@ class SemiFuture : private futures::detail::FutureBase<T> {
     return std::move(*this).deferError(&func);
   }
 
+  /// Convenience method for ignoring the value and creating a Future<Unit>.
+  /// Exceptions still propagate.
+  ///
+  /// Preconditions:
+  ///
+  /// - `valid() == true` (else throws FutureInvalid)
+  ///
+  /// Postconditions:
+  ///
+  /// - Calling code should act as if `valid() == false`,
+  ///   i.e., as if `*this` was moved into RESULT.
+  /// - `RESULT.valid() == true`
+  SemiFuture<Unit> unit() &&;
+
   SemiFuture<T> within(Duration dur, Timekeeper* tk = nullptr) && {
     return std::move(*this).within(dur, FutureTimeout(), tk);
   }

folly/futures/test/SemiFutureTest.cpp

@@ -50,6 +50,12 @@ TEST(SemiFuture, futureDefaultCtor) {
   SemiFuture<Unit>();
 }
 
+TEST(SemiFuture, semiFutureToUnit) {
+  SemiFuture<Unit> fu = makeSemiFuture(42).unit();
+  std::move(fu).get();
+  EXPECT_THROW(makeSemiFuture<int>(eggs).unit().get(), eggs_t);
+}
+
 TEST(SemiFuture, makeSemiFutureWithUnit) {
   int count = 0;
   SemiFuture<Unit> fu = makeSemiFutureWith([&] { count++; });