Default methods for interfaces

[erickt]

I'm fleshing out the json library to add support for a to_json iface, among other things, and I'm finding a lot of code duplication between my instances. Here's an example:

iface to_json {
    fn to_json_writer(wr: io::writer);
    fn to_json_str() -> str;
}

impl of to_json for float {
    fn to_json_writer(wr: io::writer) {
        wr.write_str(float::to_str(self, 6u));
    }

    fn to_json_str() -> str {
        io::with_str_writer(self.to_json_writer)
    }
}

impl of to_json for int {
    fn to_json_writer(wr: io::writer) {
        wr.write_str(int::str(self));
    }

    fn to_json_str() -> str {
        io::with_str_writer(self.to_json_writer)
    }
}
...

Every implementation's to_json_str is just a straight copy of the others. It would be much nicer if we could provide default versions of functions like to_json_str and just have the implementations inherit it.

iface to_json {
    fn to_json_writer(wr: io::writer);
    fn to_json_str() -> str {
        io::with_str_writer(self.to_json_writer)
    }
}

[marijnh]

There are plans to make traits apply to ifaces and somehow wire that up to provide something similar to default implementations.

For now, just write a top-level parameterized function that does what you want, rather than duplicating the same method in every impl, or if you really want it to look like a method call, write an impl that takes an <A: to_json> type parameter and use it for static dispatch.

[catamorphism]

Seems like @marijnh 's solution would work, and traits are on the way. Closing, but reopen if you find that doesn't work for you.