how to invoke a generic function in rust

December 17, 2021

Now I am define a generic function in rust like this:

pub fn fav_music_query<T>() -> String {
    use crate::model::diesel::rhythm::rhythm_schema::favorites::dsl::*;
    let connection = config::establish_music_connection();
    let query = favorites.filter(like_status.eq(1)).paginate(1).per_page(10);
    let query_result = query.load_and_count_pages::<Favorites>(&connection).unwrap();

    return "ok".to_string();
}

now I want to invoke this function in the outer function, this is my outer function code which to invoke the generic function:

let dashboards = fav_music_query();

and shows error:

error[E0282]: type annotations needed
  --> src/biz/home/home_controller.rs:12:22
   |
12 |     let dashboards = fav_music_query();
   |                      ^^^^^^^^^^^^^^^ cannot infer type for type parameter `T` declared on the function `fav_music_query`

let dashboards = fav_music_query<>();

what should I do to invoke this function? I tried this way:

let dashboards = fav_music_query<Favorites>();

it seems did not work.

>Solution :

for that you can use the turbofish operator ::<>:

let dashboards = fav_music_query::<Favorites>();

But you don’t even use the generic in your function, I guess you forgot to replace query.load_and_count_pages::<Favorites>(&connection) with query.load_and_count_pages::<T>(&connection)