entrait

Experimental proc macro to ease development using Inversion of Control patterns in Rust.

entrait is used to generate a trait from the definition of a regular function. The main use case for this is that other functions may depend upon the trait instead of the concrete implementation, enabling better test isolation.

The macro looks like this:

#[entrait(MyFunction)]
fn my_function<A>(a: &A) {
    ...
}

which generates the trait MyFunction:

trait MyFunction {
    fn my_function(&self);
}

my_function's first and only parameter is a which is generic over some unknown type A. This would correspond to the self parameter in the trait. But what is this type supposed to be? We can generate an implementation in the same go, using for Type:

struct App;

#[entrait(MyFunction for App)]
fn my_function<A>(app: &A) {
    ...
}

// Generated:
// trait MyFunction {
//     fn my_function(&self);
// }
//
// impl MyFunction for App {
//     fn my_function(&self) {
//         my_function(self)
//     }
// }

fn main() {
    let app = App;
    app.my_function();
}

The advantage of this pattern comes into play when a function declares its dependencies, as trait bounds:

#[entrait(Foo for App)]
fn foo<A>(a: &A)
where
    A: Bar
{
    a.bar();
}

#[entrait(Bar for App)]
fn bar<A>(a: &A) {
    ...
}

The functions may take any number of parameters, but the first one is always considered specially as the "dependency parameter".

Functions may also be non-generic, depending directly on the App:

#[entrait(ExtractSomething for App)]
fn extract_something(app: &App) -> SomeType {
    app.some_thing
}

These kinds of functions may be considered "leaves" of a dependency tree.

Plans

The goal of this project is to explore ideas around how to architect larger applications in Rust. The core idea is to architect around one shared "App object" which represents the actual runtime dependencies of an application (various database connection pools etc).

Concrete things to explore

• generate mockall code (at least this would be needed when there are multiple trait bounds)