MOPA: My Own Personal Any

A macro to implement all the Any methods on your own trait.

You like Any—its ability to store any 'static type as a trait object and then downcast it back to the original type is very convenient, and in fact you need it for whatever misguided reason. But it’s not enough. What you really want is your own trait object type with Any’s functionality glued onto it. Maybe you have a Person trait and you want your people to be able to do various things, but you also want to be able to conveniently downcast the person to its original type, right? Alas, you can’t write a type like Box<Person + Any> (at present, anyway). So what do you do instead? Do you give up? No, no! No, no! Enter MOPA.

There once was a quite friendly trait
Called Person, with much on its plate.
    “I need to be Any
    To downcast to Benny—
But I’m not, so I guess I’ll just wait.”

A pitiful tale, isn’t it? Especially given that there was a bear chasing it with intent to eat it. Fortunately now you can mopafy Person in three simple steps:

1. Add the mopa crate to your Cargo.toml as usual and your crate root like so:

   #[macro_use]
   extern crate mopa;

2. Make Any (mopa::Any, not std::any::Any) a supertrait of Person;

3. mopafy!(Person);.

And lo, you can now write person.is::<Benny>() and person.downcast_ref::<Benny>() and so on to your heart’s content. Simple, huh?

Oh, by the way, it was actually the person on the bear’s plate. There wasn’t really anything on Person’s plate after all.

#[macro_use]
extern crate mopa;

struct Bear {
    // This might be a pretty fat bear.
    fatness: u16,
}

impl Bear {
    fn eat(&mut self, person: Box<Person>) {
        self.fatness = (self.fatness as i16 + person.weight()) as u16;
    }
}

trait Person: mopa::Any {
    fn panic(&self);
    fn yell(&self) { println!("Argh!"); }
    fn sleep(&self);
    fn weight(&self) -> i16;
}

mopafy!(Person);

struct Benny {
    // (Benny is not a superhero. He can’t carry more than 256kg of food at once.)
    kilograms_of_food: u8,
}

impl Person for Benny {
    fn panic(&self) { self.yell() }
    fn sleep(&self) { /* ... */ }
    fn weight(&self) -> i16 {
        // Who’s trying to find out? I’m scared!
        self.yell();
        self.kilograms_of_food as i16 + 60
    }
}

struct Chris;

impl Chris {
    // Normal people wouldn’t be brave enough to hit a bear but Chris might.
    fn hit(&self, bear: &mut Bear) {
        println!("Chris hits the bear! How brave! (Or maybe stupid?)");
        // Meh, boundary conditions, what use are they in examples?
        // Chris clearly hits quite hard. Poor bear.
        bear.fatness -= 1;
    }
}

impl Person for Chris {
    fn panic(&self) { /* ... */ }
    fn sleep(&self) { /* ... */ }
    fn weight(&self) -> i16 { -5 /* antigravity device! cool! */ }
}

fn simulate_simulation(person: Box<Person>, bear: &mut Bear) {
    if person.is::<Benny>() {
        // None of the others do, but Benny knows this particular
        // bear by reputation and he’s *really* going to be worried.
        person.yell()
    }
    // If it happens to be Chris, he’ll hit the bear.
    person.downcast_ref::<Chris>().map(|chris| chris.hit(bear));
    bear.eat(person);
}

fn main() {
    let mut bear = Bear { fatness: 10 };
    simulate_simulation(Box::new(Benny { kilograms_of_food: 5 }), &mut bear);
    simulate_simulation(Box::new(Chris), &mut bear);
}

Now should you do something like this? Probably not. Enums are probably a better solution for this particular case as written; frankly I believe that almost the only time you should downcast an Any trait object (or a mopafied trait object) is with a generic parameter, when producing something like AnyMap, for example. If you control all the code, Any trait objects are probably not the right solution; they’re good for cases with user-defined types across a variety of libraries. But the question of purpose and suitability is open, and I don’t have a really good example of such a use case here at present. TODO.

Usage

Cargo all the way. http://crates.io/crates/mopa

Author

Chris Morgan (chris-morgan) is the primary author and maintainer of this library.

License

This library is distributed under similar terms to Rust: dual licensed under the MIT license and the Apache license (version 2.0).

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.