Unit::Lift<T>

Summary: Lift void into the unit monad and pass other types through unscathed.

Test Plan: new unit tests

Reviewed By: yfeldblum@fb.com

Subscribers: exa, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2029785

Signature: t1:2029785:1430333928:ef2fbb2e3d94518a732f6818a06c32481120bd4f

folly/futures/Unit.h

@@ -16,7 +16,16 @@
 #pragma once
 namespace folly {
 
-struct Unit {};
+struct Unit {
+  template <class T> struct Lift : public std::false_type {
+    using type = T;
+  };
+};
+
+template <>
+struct Unit::Lift<void> : public std::true_type {
+  using type = Unit;
+};
 
 template <class T>
 struct is_void_or_unit : public std::conditional<

folly/futures/test/UnitTest.cpp

@@ -33,3 +33,17 @@ TEST(Unit, PromiseSetValue) {
   Promise<Unit> p;
   p.setValue();
 }
+
+TEST(Unit, LiftInt) {
+  using Lifted = Unit::Lift<int>;
+  EXPECT_FALSE(Lifted::value);
+  auto v = std::is_same<int, Lifted::type>::value;
+  EXPECT_TRUE(v);
+}
+
+TEST(Unit, LiftVoid) {
+  using Lifted = Unit::Lift<void>;
+  EXPECT_TRUE(Lifted::value);
+  auto v = std::is_same<Unit, Lifted::type>::value;
+  EXPECT_TRUE(v);
+}