Type-Safe, Lazily Evaluated, Plugins for Extensible Types

Plugins provide a consistent interface for mixin methods. You can use a plugin anywhere you would use a "mixin" trait and an implementation.

Example Usage

// Define a struct.
struct IntPlugin;

// Map it onto an `i32` value.
impl typemap::Key for IntPlugin { type Value = i32; }

// Define the plugin evaluation function.
// `Extended` is a type that implements `Extensible`.
impl Plugin<Extended> for IntPlugin {
    type Error = ();

    fn eval(_: &mut Extended) -> Result<i32, ()> {
        Ok(0i32)
    }
}
assert_eq!(extended.get::<IntPlugin>().unwrap(), 0i32);

To do the same thing with a trait, one could do:

trait IntProducer {
    fn get_int_value(&self) -> Option<i32>;
}

impl IntProducer for Extended {
    fn get_int_value(&self) -> Option<i32> {
        Some(0i32)
    }
}

Although using a raw trait is less code, plugins provide the following advantages:

• Automatic caching of values. Calling a method again is a constant time operation! This is particularly useful in pipeline structures where only the extensible object is passed around.
• A consistent interface, which also allows for neater name clash resolution. Two modules that provide PluginX can be differentiated using a module prefix.

e.get::<mod1::PluginX>();
e.get::<mod2::PluginX>();