Commit 004d6c17 authored by Yedidya Feldblum's avatar Yedidya Feldblum Committed by Facebook Github Bot 2

Flesh out the comments about Unit

Summary:
[Folly] Flesh out the comments about `Unit`.

* Remove mention of `Future`.
* Compare `unit` to `void`.

Reviewed By: djwatson

Differential Revision: D3316114

fbshipit-source-id: d30cf36cc029025734bb16c5bdbd640a289cad79
parent a3323738
......@@ -21,11 +21,18 @@
namespace folly {
/// In functional programming, the degenerate case is often called "unit". In
/// C++, "void" is often the best analogue, however because of the syntactic
/// special-casing required for void it is a liability for template
/// metaprogramming. So, instead of e.g. Future<void>, we have Future<Unit>.
/// You can ignore the actual value, and we port some of the syntactic
/// niceties like setValue() instead of setValue(Unit{}).
/// C++, "void" is often the best analogue. However, because of the syntactic
/// special-casing required for void, it is frequently a liability for template
/// metaprogramming. So, instead of writing specializations to handle cases like
/// SomeContainer<void>, a library author may instead rule that out and simply
/// have library users use SomeContainer<Unit>. Contained values may be ignored.
/// Much easier.
///
/// "void" is the type that admits of no values at all. It is not possible to
/// construct a value of this type.
/// "unit" is the type that admits of precisely one unique value. It is
/// possible to construct a value of this type, but it is always the same value
/// every time, so it is uninteresting.
struct Unit {
template <typename T>
using Lift = std::conditional<std::is_same<T, void>::value, Unit, T>;
......
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