Proposal for unifying traits and interfaces

Unifying traits and interfaces

A real example of how we could use traits

In middle::typeck::infer (henceforth "infer"), there's a combine interface, and impls of that interface for the three "type combiners" lub, sub, and glb. Each of these impls is required to implement all of the methods in the combine interface, even though some of the implementations are identical in two (or in all three!) of the . The way that infer deals with this now is that for each method foo that has identical implementations, there's an out-of-line method super_foo.

For example, here's how it works for the modes method. This is all code taken directly from infer. In fact, there are nine methods in infer that are this way. I'm just picking modes as a representative example.

iface combine {
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode>;
    ...
}

impl of combine for sub {
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }

    fn args(a: ty::arg, b: ty::arg) -> cres<ty::arg> {
        super_args(self, a, b)
    }
    ...
}

impl of combine for sub {
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }
    ...
}

impl of combine for glb {
    ...
    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        super_modes(self, a, b)
    }
    ...
}

// Out-of-line method
fn super_modes<C:combine>(
    self: C, a: ast::mode, b: ast::mode)
    -> cres<ast::mode> {

    let tcx = self.infcx().tcx;
    ty::unify_mode(tcx, a, b)
}

With traits, we could put the default implementation in the interface, so, instead of all of the above, we could just write:

trait combine {
    ...

    fn modes(a: ast::mode, b: ast::mode) -> cres<ast::mode> {
        let tcx = self.infcx().tcx;
    	ty::unify_mode(tcx, a, b)
    }
    ...
}

impl of combine for sub {
    ... // only methods for which the default impl isn't enough
}

impl of combine for sub {
    ... // only methods for which the default impl isn't enough
}

impl of combine for glb {
    ... // only methods for which the default impl isn't enough
}