> As I said in https://lists.isocpp.org/std-proposals/2020/09/1811.php,
> this already exists in C++20 in the form of coroutines... you just need to
> opt in support for optional<T> in the only sensible way and then you can
> write:
>
> std::optional<std::string> FindUsersCity(bool non_default) {
> ConstactsSeriver cs = co_await GetOrOpenContatsServerConnection();
> UserId uid = co_await cs.GetUserId();
> GeoServer gs = co_await GetOrOpenGeoServerConnection();
> Location uloc = co_await gw.GetLocationId(uid);
> return uloc.GetCityName();
> }
>
> co_await here is effectively syntax sugar for and_then() (in the same way
> in Haskell do-notation is syntax sugar for bind). It's just that the
> continuation gets written as the next statement in the function rather than
> the body of a lambda that gets passed as an argument.
>

That is really nice, but can you elaborate more?
Am I right that the trick is that expression of co_await can return
an awaitable, await_ready() of which can return:

   - either false (in which case we effectively return from FindUsersCity
   and it corresponds to some error),
   - or it can return true (in which case FindUsersCity continues, and this
   corresponds to execution without errors)?

-- 
Dmitry
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