I find it too magical to be necessary, but I can see how it might be useful. This can be achieved with a wrapper, but will then require you to wrap every future, which is not too convenient
It’s not about magic, it’s just not how the abstraction works.
Let’s have a concrete example, poem::web::Data utilizes opentelemetry::context::FutureExt which has the trait methods with_context() and with_current_context(). But where is the data/context actually stored? In the WithContext struct of course. Because there is no such a thing as pinning (meta)data to a trait.
Other languages have ended up introducing it out of practical necessity, e.g. Go’s contexts, JS execution contexts. Pick your poison, although I expect Rust’s general minimal approach will leave it as extra parameters, Go-style.
Stating the obvious: a trait is type class you implement for types. It’s not something you can “pin metadata” (or any data) to.
It’s an API, if you add methods to it then the implementations will support that. That is in fact the entire point of a trait.
struct SomeFuture; impl Future for SomeFuture { // .... }where are you going to “pin metadata”?
I think, they mean:
pub trait Future { // .... fn put_metadata(...); fn get_metadata(...); }I find it too magical to be necessary, but I can see how it might be useful. This can be achieved with a wrapper, but will then require you to wrap every future, which is not too convenient
It’s not about magic, it’s just not how the abstraction works.
Let’s have a concrete example, poem::web::Data utilizes opentelemetry::context::FutureExt which has the trait methods
with_context()andwith_current_context(). But where is the data/context actually stored? In theWithContextstruct of course. Because there is no such a thing as pinning (meta)data to a trait.@[email protected] ^^
Other languages have ended up introducing it out of practical necessity, e.g. Go’s contexts, JS execution contexts. Pick your poison, although I expect Rust’s general minimal approach will leave it as extra parameters, Go-style.