3 ms·

I think keypaths also permit writing (but maybe the rvalue-reference does that?). And probably they allow using properties and not just fields, but I don't know

I think keypaths also permit writing (but maybe the rvalue-reference does that?). And probably they allow using properties and not just fields, but I don't know Swift tbh. But this touches on one thing I think would be interesting in a language. On the one hand you can define a function like you did, which is a black box that does something. On the other hand, you can talk about "the action of doing something", and assign that description to a variable, introspect it, change the action. Maybe convert from imperative to command-pattern, so you can undo, and so on. I think one of the few languages that allows something like this is Lisp. Keypaths are somewhat declarative, but don't allow introspection and are limited to getting/setting it seems.

gpderetta 4y ago
During return type inference, references decay to value types by default, so you have to use some awkward syntax if you want to use them to set values: auto item_user = [](auto&& item) -> decltype(auto) { return (item.user); }; item_user(item) = my_user; Even more awkward if you want some slightly better error messages with concepts: auto item_user = [](auto&& item) -> decltype(auto) requires requires { item.user;} { return (item.user); }; (yes, the double requires is not a mistake). At least is very macroable, which would also allow to wrap it in a type with more meaning than a opaque lambda (which I agree is a good thing). edit: Full on nonsense just for laughs: auto user_age = [](auto&& user) noexcept (noexcept((user.age))) -> decltype(auto) requires requires { user.age;} { return (user.age); };