Question about generics and traits in Rust

Question

I have the DoSomethingProvider trait that expects a parameter of one of its functions to be of trait type DoSomethingListener.

I have a concrete struct DoSomethingManager that has a member of type DoSomethingProvider, and will implement the DoSomethingListener trait and pass it self as a listener to DoSomethingProvider.

Hopefully the code will illustrate what I'm trying to do:

pub trait DoSomethingListener {
    fn something_was_done(msg: &str);
}

pub trait DoSomethingProvider<'a, T>
where
    T: DoSomethingListener,
{
    fn add_do_something_listener(listener: T);
}

/* note:  The struct below will implement DoSomethingListener, and has
 * a DoSomethingProvider field.  It will pass itself as a listener to
 * DoSomethingProvider (which listens to a message queue and notifies
 * listeners of certain events)
 */

//this doesn't work.  Compiler complains about unused type T
pub struct DoSomethingManager<'a, B, T>
where
    T: DoSomethingListener,
    B: DoSomethingProvider<'a, T>,
{
    provider: Box<B>,
    // doesn't have any member of type T
}

// ...

/* So I tried this:
 * this doesn't work.  Compiler complains that DoSomethingProvider
 * expects one type parameter
 */
pub struct DoSomethingManager<'a, B>
where
    B: DoSomethingProvider<'a>,
{
    provider: Box<B>,
    // doesn't have any member of type T
}

//...

/* this compiles, but its a hack */
pub struct DoSomethingManager<'a, B, T>
where
    T: DoSomethingListener,
    B: DoSomethingProvider<'a, T>,
{
    provider: Box<B>,
    dummy: Box<T>,
    // added unused dummy member of type T
}

I am an experienced Python developer but I'm new to Rust. What is the proper way to implement this polymorphic code in Rust?

Francis Gagné Francis Gagné · Accepted Answer · 2019-05-05T19:46:46

Change DoSomethingProvider to use an associated type instead of a type parameter for the listener type.

pub trait DoSomethingListener {
    fn something_was_done(msg: &str);
}

pub trait DoSomethingProvider {
    type Listener: DoSomethingListener;

    fn add_do_something_listener(listener: Self::Listener);
}

pub struct DoSomethingManager<B>
where
    B: DoSomethingProvider,
{
    provider: Box<B>,
}

Note that by using either an associated type or a type parameter, a particular instance of DoSomethingProvider will only be able to accept listeners of a single specific type. If you want to be able to register listeners of various types, you will need to use dynamic dispatch via trait objects.

Question about generics and traits in Rust

2 Answers