FromRequest for Option<MyType>

[shotor]

I'm trying to implement a cookie parser using the FromRequest trait on a custom Cookies struct. It works fine but I would like to implement the trait for Option<Cookies> instead of Cookies. But as soon as I do this I get the error:

11 | impl FromRequest for Option<Cookies> {
   | ^^^^^^^^^^^^^^^^^^^^^---------------
   | |                    |
   | |                    `Option` is not defined in the current crate
   | impl doesn't use only types from inside the current crate

If I leave out Option it works fine. But I want 3 states: Ok (has cookies and valid), None (doesn't have cookies at all), Error (something went wrong).

use actix_web::{dev, Error, FromRequest, HttpRequest};
use futures_util::future::{ok, Ready};
use std::option::Option;

#[derive(Debug)]
pub struct Cookies {
  pub session: String,
  pub refresh: String,
}

impl FromRequest for Cookies { // works
// impl FromRequest for Option<Cookies> { // doesn't work
  type Error = Error;
  type Future = Ready<Result<Self, Self::Error>>;
  type Config = ();

  fn from_request(
    req: &HttpRequest,
    payload: &mut dev::Payload,
  ) -> Self::Future {
    ...
  }
}

Any advice how to do this?

[robjtede]

It's an overlapping impl. Actix Web already implements FromRequest for Option<T> where T: FromRequest

[fakeshadow]

I'm just explaining how trait impl work. You error is because your trait want Result<T, E> as output when you actually trying to return Result<Option<T>, E>.

impl<T: FromRequest> FromRequest for Result<T, T::Error> { }
Notice T is bound to FromRequest trait so it include Option<T as FromRequest>.

[shotor]

I'm just explaining how trait impl work. You error is because your trait want Result<T, E> as output when you actually trying to return Result<Option, E>.

This part is clear

impl<T: FromRequest> FromRequest for Result<T, T::Error> { }
Notice T is bound to FromRequest trait so it include Option.

This is still confusing. Need to look at simpler examples.

In any case, I'm skipping this and wrapping the None case in an error. And handling it where I need the actual cookies.

[fakeshadow]

In fact if you want you can extract something like this. It's not like it has any meaning but it explains how the default impl works.

#[get("/")]
async fn index(foo: Result<Result<Option<Option<HttpRequest>>, Error>, Error>) -> &'static str {
    "Hello World!"
}

[shotor]

I'm guessing that's because you can substitute the generic T in the trait implementations with Option<T> and Result<T, T::Error> indefinitely.

Result<T, T::Error>
Result<Option<T>, T::Error>
Result<Result<Option<T>, T::Error>, T::Error>>
...etc

So the compiler will happily apply them. Even the one I want: Result<Option<context::Cookies>, actix_web::Error>,

But when implementing FromRequest to actually return a result option you're blocked because there's no implementation for Result<Option, E>.

[fakeshadow]

That's the rule of Rust. You can look up "orphan rules".