System impl macro for specs

system_macro.rs

use specs::System;

// Tired of writing out a bunch of giant tuples by hand?
// I have the macro for you!

struct BananaSystem;

system! {
    BananaSystem;

    fn run(&mut self,
           comp_a: ReadStorage<ComponentA>,
           mut comp_b: WriteStorage<ComponentB>,
           mut comp_c: WriteStorage<ComponentC>) {
        for (a, mut b, mut c) in (comp_a, comp_b, comp_c).join() {
            // etc.
        }
    }
}
// will generate what you'd expect (appropriate SystemData, etc.)

// Here's the macro definition
macro_rules! system {
    (
        $name:path;

        fn run(&mut self, $(
            $($bind:ident)+ :
                $type_root:ident < $( $type_args:ty ),* >
        ),+ )
        $contents:block

    ) => {

        impl<'a> System<'a> for $name {
            type SystemData = ( $(
                $type_root <'a, $( $type_args ),* >
            ),* );

            fn run(&mut self, args: Self::SystemData) {
                system_maybe_tuple!( args, $($($bind)+),+ );
                $contents
            }
        }
    };
}
macro_rules! system_maybe_tuple {
    ($args:expr, $($bind:ident)+) => (
        let ($($bind)+,) = $args;
    );
    ($args:expr, $($($bind:ident)+),+) => (
        let ($($($bind)+),+) = $args;
    );
}