Proposal for unifying traits and interfaces

Unifying traits and interfaces

A real example of how we could use traits

In the middle::typeck::infer module (henceforth "infer"), there's a combine interface, and implementations of that interface for the three "type combiners" lub, sub, and glb. All three impls are 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 type combiners. Right now, "infer" deals with this by defining an out-of-line method super_foo for each method foo for which there are multiple identical implementations.

For example, here's what it looks like for the modes method. In fact, there are nine methods in infer that are this way -- modes is just 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
}