On Sun, Sep 13, 2020 at 4:37 AM Dvir Yitzchaki via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

On Sun, 13 Sep 2020 at 11:49, Jorg Brown via Std-Proposals <std-proposals@lists.isocpp.org> wrote:

OK, so if I only use local control structures, and eschew goto and return, then how do I write the typical multiple-error-return flow, without ending up with code that is increasingly indented, right up until the end? What's the right way, according to your model, of writing:

std::optional<std::string> FindUsersCity(bool non_default) {
std::optional<std::string> city = std::nullopt;
std::optional<ContactsServer> contacts = GetOrOpenContactsServerConnection();
if (contacts) {
std::optional<UserId> uid = contacts->GetUserId();
if (uid) {
std::optional<GeoServer> geo = GetOrOpenGeoServerConnection();
if (geo) {
std::optional<Location> uloc = geo->GetLocation(*uid);
if (uloc) {
city = uloc->GetCityName();
}
}
}
}
return city;
}

with [P0798] it can be something like:

std::optional<std::string> FindUsersCity(bool non_default) {
return GetOrOpenContactsServerConnection()
.transform([](auto&& contacts) { return contacts->GetUserId(); })
.and_then([](auto&& uid) {
return GetOrOpenGeoServerConnection().transform(
[&uid](auto&& geo) { return geo->GetLocation(*uid); });
})
.transform([](auto&& uloc) { return uloc->GetCityName(); });
}

Just a correction. Transform takes an optional<T> and a function T->U and gives you an optional<U>. GetUserId() returns an optional<UserId>, so with your first transform here we'd end up with an optional<optional<UserId>>. Also the function takes a T, not an optional<T>, so the body should be using . not ->. I'm not sure if there should be any uses of transform here at all, it depends on what Location::GetCityName() actually returns -- from the code it could be either a string or an optional<string>. Since everything else returns an optional, I'm assuming that one does too, so that should also be and_then():

std::optional<std::string> FindUsersCity(bool non_default) {
return GetOrOpenContatsServerConnection()
.and_then([](ContactsServer cs){ return cs.GetUserId(); })
.and_then([](UserId uid){
return GetOrOpenGeoServerConnection()
.and_then([&](GeoServer gs){ return gs.GetLocationId(uid); })
.and_then([](Location uloc){ return uloc.GetCityName(); });
})
}

If GetCityName() returns just a string, then the last one should be a transform. Or use coroutines (remains to be seen how well this gets optimized relative to the previous):

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();
}

Barry