FormatInto object safety - or how can I make a collection of Token Streams?

[rrichardson]

I am creating a custom HTTP handler class from a schema specification which has about 100 routes.

Because of reasons, my main dispatcher is basically a match from a usize to an function invocation:
e.g.

match index {
     0 => {  
         let input = FromRequest::from_request(req).await;
         let res = handler_0(input).await; 
         IntoResponse::into_response(res).await
      }, 
      1  =>   {  
         let input = FromRequest::from_request(req).await;
         let res = handler_0(input).await; 
         IntoResponse::into_response(res).await
      }, 
      N => {
          ... 
      }
}

I want to create a Vec<Box<dyn FormatInto<Rust>> of handlers and then iterate through them and create each match block.
However, I can't, because format_into takes self

It would be nice if there was an alternative to format_into that didn't consume self. Or maybe there is a better overall approach?

What is the recommended way to create a vector of token streams that I can inject into here?

[udoprog]

Interesting.

Have you tried to write your code so that you use a Vec<impl FormatInto> instead? It would require uniform types, but this can be accomplished by using say enums and matching over them.

If you want to use it multiple times you could also do: Vec<impl FormatInto + Copy>. I usually find it to be easy enough to have it be Copy since it just requires that you only use references in the corresponding quote_fn!. Or even Clone.

I appreciate you reporting on your experience and please continue doing so! This might feed into future choices on API design. I can definitely see how support for trait objects might be legitimately useful.

[udoprog]

Actually looking over the docs of FromFn it seems to be missing a forwarding Clone and Copy impl. I was sure it had one but I'm clearly misremembering. That makes the pattern I described a bit harder to implement since it can only be used for types for which you impl FormatInto manually.

I'd suggest adding those impls ASAP. I'll look closer into this tomorrow.

[udoprog]

Also of you have the time: posting a representative example of how you are trying to use genco would also be helpful. I might be able to suggest a different way to write it.

Looking at this repo and how it uses genco might also be helpful!

[rrichardson]

Adding the + Clone constraint to the trait object was the first thing that I tried. However, I think it'd need to be + Sized as well, otherwise the compiler wouldn't know how big to make the self to move out of. Fundamentally, I'm actually not sure it would be possible. I think making a FormatAs that works like FormatInto, but copies the internal bits instead of moving them, might be the most flexible approach.

I was able to get my code working by using a for loop calling quote_in in succession on a mut Token. It works, but it doesn't have the beautiful, functional flavor that my first attempt had. :)

Overall my experience with genco has been great. No surprises, it is well documented and does what it says.

[udoprog]

Cheers!

I was able to get my code working by using a for loop calling quote_in in succession on a mut Token. It works, but it doesn't have the beautiful, functional flavor that my first attempt had. :)

So maybe this is helpful. The typical way I'd do that pattern is instead of doing something like this:

let mut out = Tokens::new();

for thing in things {
    match thing {
        Thing::First => quote_in!(out => /* do first*/),
        Thing::Second => quote_in!(out => /* do second */),
    }
}

I'd do:

let mut out = Tokens::new();

quote_in! { out =>
    $(for thing in things join ($['\n']) => match thing {
        Thing::First => /* do first */,
        Thing::Second => /* do second */,
    })
}

And if one of them was a bit more involved, I'd break it out into a function:

fn do_third() -> impl FormatInto<Rust> {
    quote_fn! {
        /* do third */
    }
}

quote_in! { out =>
    $(for thing in things => match thing {
        Thing::First => /* do first */,
        Thing::Second => /* do second */,
        Thing::Third => $(do_third()),
    })
}

Note that due to the design of the FormatInto trait it doesn't require additional allocations to do so, since it will interact with the same Tokens collection.

I hope this helps!

[rrichardson]

I don't know what the match cases are at compile time, only after I parse them out of the schema. So I have this:

       let mut fn_matches = rust::Tokens::new();
       for (idx, op) in ops.iter().enumerate() { 
           let fun = to_snake_case(op.name.as_str());
           let input = &op.input; 
           if input == "()" {
               quote_in!{ fn_matches =>
                   $idx => { 
                       $fun(bucket, key, req)
                           .and_then($into_resp::into_response).await 
                   },$['\n']
               };
           }else {
               quote_in!{ fn_matches =>
                   $idx => { 
                       $shapes::$input::from_request(req)
                           .and_then(|req| $fun(bucket, key, req))
                           .and_then($into_resp::into_response).await 
                   },$['\n']
               }; 
           }
       }
       ```